Telecommunications Engineering

Gill Sans Bold

Engineering Studies HSC Course Stage 6

Telecommunications engineering

ES/S6 – HSC 41098

P0022162

Acknowledgments This publication is copyright Learning Materials Production, Open Training and Education Network – Distance Education, NSW Department of Education and Training, however it may contain material from other sources which is not owned by Learning Materials Production. Learning Materials Production would like to acknowledge the following people and organisations whose material has been used. Board of Studies, NSW

All reasonable efforts have been made to obtain copyright permissions. All claims will be settled in good faith. Materials devlopment:

Paul Soares, Harry Taylor, Ian Webster

Coordination:

Jeff Appleby

Edit:

John Cook, Josephine Wilms, Stephen Russell, Steve Cavanagh (BOS) electricity/electronics component

Illustrations:

Tom Brown, Barbara Buining

DTP:

Nick Loutkovsky, Carolina Barbieri

Copyright in this material is reserved to the Crown in the right of the State of New South Wales. Reproduction or transmittal in whole, or in part, other than in accordance with provisions of the Copyright Act, is prohibited without the written authority of Learning Materials Production. © Learning Materials Production, Open Training and Education Network – Distance Education, NSW Department of Education and Training, 2000. 51 Wentworth Rd. Strathfield NSW 2135. Revised 2002

Module contents

Subject overview .......................................................................................... iii Module overview .........................................................................................vii Module components .................................................................................... vii Module outcomes ......................................................................................... ix Indicative time ...............................................................................................x Resource requirements................................................................................. xi

Icons

...................................................................................................xiii

Glossary ................................................................................................... xv Directive terms..........................................................................................xxiii Part 1:

Telecommunications engineering – scope of the profession & engineering report................ 1–45

Part 2:

Telecommunications engineering – history of telecommunications.......................................... 1–45

Part 3:

Telecommunications engineering – materials .............................................................................. 1–37

Part 4:

Telecommunications engineering – mechanics and hydraulics ................................................ 1–33

Part 5:

Telecommunications engineering – electricity/electronics ......................................................... 1–65

Part 6:

Telecommunications engineering – communications ................................................................. 1–44

i

Bibliography.................................................................................................39 Module evaluation ......................................................................................43

ii

Subject overview

Engineering Studies Preliminary Course Household appliances examines common appliances found in the home. Simple appliances are analysed to identify materials and their applications. Electrical principles, researching methods and techniques to communicate technical information are introduced. The first student engineering report is completed undertaking an investigation of materials used in a household appliance. Landscape products investigates engineering principles by focusing on common products, such as lawnmowers and clothes hoists. The historical development of these types of products demonstrates the effect materials development and technological advancements have on the design of products. Engineering techniques of force analysis are described. Orthogonal drawing methods are explained. An engineering report is completed that analyses lawnmower components. Braking systems uses braking components and systems to describe engineering principles. The historical changes in materials and design are investigated. The relationship between internal structure of iron and steel and the resulting engineering properties of those materials is detailed. Hydraulic principles are described and examples provided in braking systems. Orthogonal drawing techniques are further developed. An engineering report is completed that requires an analysis of a braking system component.

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Bio-engineering both engineering principles and also the scope of the bio-engineering profession. Careers and current issues in this field are explored. Engineers as managers and ethical issues confronted by the bio engineer are considered. An engineering report is completed that investigates a current bioengineered product and describes the related issues that the bio-engineer would need to consider before, during and after this product development. Irrigation systems is the elective topic for the preliminary modules. The historical development of irrigation systems is described and the impact of these systems on society discussed. Hydraulic analysis of irrigation systems is explained. The effect on irrigation product range that has occurred with the introduction of is detailed. An engineering report on an irrigation system is completed.

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HSC Engineering Studies modules Civil structures examines engineering principles as they relate to civil structures, such as bridges and buildings. The historical influences of engineering, the impact of engineering innovation, and environmental implications are discussed with reference to bridges. Mechanical analysis of bridges is used to introduce concepts of truss analysis and stress/strain. Material properties and application are explained with reference to a variety of civil structures. Technical communication skills described in this module include assembly drawing. The engineering report requires a comparison of two engineering solutions to solve the same engineering situation. Personal and public transport uses bicycles, motor vehicles and trains as examples to explain engineering concepts. The historical development of cars is used to demonstrate the developing material list available for the engineer. The impact on society of these developments is discussed. The mechanical analysis of mechanisms involves the effect of friction. Energy and power relationships are explained. Methods of testing materials, and modifying material properties are examined. A series of industrial manufacturing processes is described. Electrical concepts, such as power distribution, are detailed are introduced. The use of freehand technical sketches. Lifting devices investigates the social impact that devices raging from complex cranes to simple car jacks, have had on our society. The mechanical concepts are explained, including the hydraulic concepts often used in lifting apparatus. The industrial processes used to form metals and the methods used to control physical properties are explained. Electrical requirements for many devices are detailed. The technical rules for sectioned orthogonal drawings are demonstrated. The engineering report is based on a comparison of two lifting devices.

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Aeronautical engineering explores the scope of the aeronautical engineering profession. Career opportunities are considered, as well as ethical issues related to the profession. Technologies unique to this engineering field are described. Mechanical analysis includes aeronautical flight principles and fluid mechanics. Materials and material processes concentrate on their application to aeronautics. The corrosion process is explained and preventative techniques listed. Communicating technical information using both freehand and computer-aided drawing is required. The engineering report is based on the aeronautical profession, current projects and issues. Telecommunications engineering examines the history and impact on society of this field. Ethical issues and current technologies are described. The materials section concentrates on specialised testing, copper and its alloys, semiconductors and fibre optics. Electronic systems such as analogue and digital are explained and an overview of a variety of other technologies in this field is presented. Analysis, related to telecommunication products, is used to reinforce mechanical concepts. Communicating technical information using both freehand and computer-aided drawing is required. The engineering report is based on the telecommunication profession, current projects and issues. Figure 0.1 Modules

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Module overview

Telecommunications engineering is the final focus module in the HSC course. This field of engineering, its history and impact on society are discussed. Ethical issues and current technologies are described. The materials section concentrates on specialised testing, copper and its alloys, semiconductors and fibre optics. Electronic systems such as analogue and digital are explained and an overview of a variety of other technologies in this field are described. Analysis, related to telecommunication products, is used to reinforce mechanical concepts. Communicating technical information using both freehand and computer aided drawing is required. The engineering report is based on the telecommunication profession, current projects and issues.

Module components Each module contains three components, the preliminary pages, the teaching/learning section and additional resources. •

The preliminary pages include: –

module contents

–

subject overview

–

module overview

–

icons

–

glossary

–

directive terms.

Figure 0.2 Preliminary pages

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•

The teaching/learning parts may include: –

part contents

–

introduction

–

teaching/learning text and tasks

–

exercises

–

check list.

Figure 0.3 Teaching/learning section

•

The additional information may include: –

module appendix

–

bibliography

–

module evaluation.

Additional  resources

Figure 0.4 Additional materials

Support materials such as audiotapes, video cassettes and computer disks will sometimes accompany a module.

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Module outcomes At the end of this module, you should be working towards being able to: •

describe the scope of engineering and critically analyse current innovations (H1.1)

•

differentiate between properties of materials and justify the selection of materials, components and processes in engineering (H1.2)

•

analyse and synthesise engineering applications in specific fields and report on the importance of these to society (H2.2)

•

use appropriate written, oral and presentation skills in the preparation of detailed engineering reports (H3.2)

•

investigate the extent of technological change in engineering (H4.1)

•

appreciate social, environmental and cultural implications of technological change in engineering and apply them to the analysis of specific problems (H4.3)

•

select and use appropriate management and planning skills related to engineering (H5.2)

•

demonstrate skills in analysis, synthesis and experimentation related to engineering (H6.2).

Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to for original and current documents.

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Indicative time The Preliminary course is 120 hours (indicative time) and the HSC course is 120 hours (indicative time). The following table shows the approximate amount of time you should spend on this module. Preliminary modules

Percentage of time

Approximate number of hours

Household appliances

20%

24 hr

Landscape products

20%

24 hr

Braking systems

20%

24 hr

Bio-engineering

20%

24 hr

Elective: Irrigation systems

20%

24 hr

HSC modules

Percentage of time

Approximate number of hours

Civil structures

20%

24 hr

Personal and public transport

20%

24 hr

Lifting devices

20%

24 hr

Aeronautical engineering

20%

24 hr

Telecommunications engineering

20%

24 hr

There are six parts in Telecommunications engineering. Each part will require about four hours of work. You should aim to complete the module within 20 to 25 hours.

x

Resource requirements During this module you will need to access a range of resources including: Part 3 Experiment • Multimeter • Test items – coffee mug, telephone body, metal knife, etc. OR • Small dry cell battery • 2 elastic bands • 3 pieces of wire or two paper clips • Torch globe • Test items – coffee mug, telephone body, metal knife, etc. Part 3 Activities • Old telephone OR • Your current telephone Part 3 Exercise • small dry cell battery • two elastic bands • 3 pieces of wire or 3 paper clips • torch globe Part 6 Exercise • tape measure • paper • sharp pencils.

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Icons

As you work through this module you will see symbols known as icons. The purpose of these icons is to gain your attention and to indicate particular types of tasks you need to complete in this module. The list below shows the icons and outlines the types of tasks for Stage 6 Engineering studies. Computer This icon indicates tasks such as researching using an electronic database or calculating using a spreadsheet. Danger This icon indicates tasks which may present a danger and to proceed with care. Discuss This icon indicates tasks such as discussing a point or debating an issue. Examine This icon indicates tasks such as reading an article or watching a video. Hands on This icon indicates tasks such as collecting data or conducting experiments. Respond This icon indicates the need to write a response or draw an object. Think This icon indicates tasks such as reflecting on your experience or picturing yourself in a situation.

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Return This icon indicates exercises for you to return to your teacher when you have completed the part. (OTEN OLP students will need to refer to their Learner's Guide for instructions on which exercises to return).

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Glossary

As you work through the module you will encounter a range of terms that have specific meanings. The first time a term occurs in the text it will appear in bold. The list below explains the terms you will encounter in this module. alternating current

current that varies with time

ampere (A)

the unit for current flow in a conductor

amplification

an increase in the energy of a signal

amplitude modulation

a modulation scheme in which the information to be transmitted is contained in the instantaneous variations in the amplitude of a modulated carrier wave

amplitude shift keying an amplitude modulation scheme in which the message signal is a digital signal analogue signal

a signal that is continuous in both amplitude and time

analogue transmission the use of a continuously varying signal to send information ASCII code

a coding scheme that uses a specific set of binary characters to represent alphanumeric and control characters for computing and telecommunications

attenuation

a decrease in the intensity of light travelling in a fibre; can also refer to other forms of energy, for example electrical signals and radio waves

bandwidth

the range of frequencies available in a particular band; the range of frequencies that can propagated by a channel; the range of frequencies that make up a signal

baseband

the frequencies at which a signal is generated

battery

electrical component used to create and store an electrical charge

binary signals

digital signals having only two possible levels of amplitude

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binary technology

where the numbers used are represented with two digits only, 1 and 0

broad-banding

being able to operate over a wide range of frequencies

capacitance

the ability to store electric charge

capacitor

electrical component used to store an electrical charge

carrier wave

a periodic signal at some desired transmission frequency used as the basis for a modulation scheme

channel

a link in a telecommunications network through which signals propagate

channel capacity

a measure of the amount of information that can be sent over a communications channel without error

coaxial cable

two concentric conductors separated and surrounded by an insulating material

coding

the process of modifying or adding to the representation of information (for security, error protection, compression, etc)

coherer

a device to detect the presence of radio frequency (RF) energy and thus receive a wireless pulse

conductor

a material that has low resistance to the flow of electricity

current

the rate of flow of electricity through a conductor

data compression

the process of reducing the amount of data in a message without significantly altering the information content of the message

data packets

short segments of a digital message that are combined with additional information (such as source and destination addresses, packet number and priority) to enable sending as individual submessages across a packet switched network

datagrams

see data packet

demodulation

the process of shifting the information content of a modulated signal back to baseband

demodulator

a circuit designed to implement a particular demodulation scheme

development

the drawing of the true flat shape of an object, often the flat shape of sheet metal ducts

digital signal

a signal that can take on only a finite number of possible amplitudes, and that only changes amplitude at discrete regular intervals in time

digital transmission

the use of discrete signal levels to send information

diode

an electrical component that only allows electrical flow in one direction

direct current (dc)

current that flows steadily in one direction

doping

adding an impurity to a semiconductor so it forms an n-type or p-type material

downlink

the communications channel from a satellite to earth

electromagnet

a piece of iron or steel that is made into a magnet by having an electric current passed through wires that are wrapped around it

electromagnetic spectrum

the range of frequencies at which electromagnetic signals may be transmitted and propagated

electromagnetic wave

an invisible form of radiation that consists of changing electric and magnetic fields – light, radio signals, microwaves are all electromagnetic waves

electromagnetism

the phenomena of the relationship between electric current and magnetism, for example, a magnetic field is produced by moving electrons (electric current)

electrostatics

the study of electrically charged particles

error correction

the process of discovering and rectifying errors made during transmission

error detection

the process of discovering whether an error has been made during transmission

facsimile

an exact copy; a method of transmitting pictures by radio telegraph

fibre optic

a clear, flexible ‘pipe’, commonly made in glass, that carries light pulses

freehand drawing

the drawing of engineering details without the use of instruments – all drawing standards should be applied where possible

frequency

the number of complete cycles of a signal in a fixed time, usually in one second a multiplexing scheme in which users are allocated their own frequency bands

frequency division multiplexing

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frequency modulation

a modulation scheme in which the information to be transmitted is contained in the instantaneous variations in the frequency of a modulated carrier wave

frequency shift keying a frequency modulation scheme in which the message signal is a digital signal

xviii

geostationary

a term used in conjunction with satellite technology to indicate an orbit around the earth above the equator that is the same as the rate at which the earth spins on its axis; a satellite so-positioned will appear stationary with respect to the earth

graded index fibre

type of fibre optic where the refractive index of the core changes from the centre outwards

guided medium

a medium for propagating electromagnetic signals that requires a physical connection between transmitting and receiving ends

hardwired

connected by a solid medium, for example cables, wires; not ‘wireless’ as with transmission through the air

information

a measure of the intellectual value of a message based on expected probabilities of the message being correct

infrared signal

electromagnetic signals at frequencies just below the visible light spectrum

infrastructure

buildings or permanent installations, for example, power poles, transmission wires, that are associated with an organisation or a system

insulation

an insulating layer normally applied over a conductor

insulation resistance

the resistance offered by insulation to an impressed voltage

insulator

a material that has very high resistance to the flow of electricity

integrated circuit

large numbers of transistors, diodes, capacitors and resistors formed and electrically joined on a single slice of semiconductor material

laser

stands for Light Amplification by the Stimulated Emission of Radiation and is a device for producing a high intensity beam of visible light; the light produced is of a single frequency and wavelength

light emitting diode

abbreviated to LED is a diode that emits light when electricity flows through the component, used instead of globes

mains wiring

electrical conductors used to distribute electrical power

megger testing

a non destructive test used to assess the insulation on a cable or electrical installation

message signal

a baseband signal containing information to be transmitted (usually in the context of modulation)

microwave signals

electromagnetic signals at frequencies in the 2–40 GHz range

modulated

the changed characteristics of a carrier wave due to the addition of a signal wave that creates a composite of the two waves

modulated carriers

set frequency waves used to carry information within signal waves broadcast with them

modulation

the process of shifting a baseband signal to another range of frequencies to facilitate transmission efficiency

modulator

a circuit designed to implement a particular modulation scheme

morse code

a code linking numbers and letters to sequences of dots and dashes for transmission by telegraph or other signal system

multimeter

a meter that can be used to measure voltage, current and resistance

multimode

type of fibre optic that allows the flow of many modes of light

multiplexing

the simultaneous transmission of several signals along a single path without any loss of identity of an individual signal

n type

a semiconductor in which current flow is caused by the movement of electrons

noise

a component of a signal that is undesirable and/or unavoidable

optical fibre

a filament of glass surrounded by mechanical protective materials

oscilloscope

a device for measuring and displaying the voltage of a signal as it varies with time

p type

a semiconductor in which current flow is caused by the movement of holes

packet switched network

a network in which data packets are sent via any suitable path from sender to receiver

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packetisation

the process of breaking a long digital message into smaller parts, each of which can be sent independently from other packets making up the total message

PCB

stands for Printed Circuit Board which has an insulating polymer base layer and copper tracks on the top surface

period

the time taken for a periodic signal to complete one cycle

periodic (signal)

a signal that is repetitive in time, such as a sinusoid or a triangular wave

phase modulation

a modulation scheme in which the information to be transmitted is contained in the instantaneous variations in the phase of a modulated carrier wave

phase shift keying

a phase modulation scheme in which the message signal is a digital signal

plain text

the body of a message prior to encryption, and recovered after decryption

p-n junction

the joining together of a p-type and an n-type semiconductor

protocols

the programming rules by which networks are able to connect into the Internet

quantisation

the process of rounding a measured amplitude to the nearest allowable ampitude in a finite set of allowable amplitudes digital signals having only four possible levels of amplitude

quaternary signals radio

the use of air and free space for propagation of unguided electromagnetic signals

regeneration

the process whereby a digital signal corrupted by noise can be reconstructed as a noise-free signal

repeater stations

installations that receive messages, then re-emit them at higher energy to ensure that signal strength is maintained and the signal does not fade out

resistivity

a measure of the ability of a material to resist the flow of current, for example, copper has a lower resistivity than aluminium; the resistance offered by a wire in a circuit is determined by the resistivity of the material of which it is made, its length and its thickness electrical component used to restrict electrical flow

resistor sampling

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the process of measuring the amplitude of a signal at regular intervals (in time or space)

semaphore

a signal system achieved with flag waving

semiconductor

a material between conductors and insulators that is used to control the flow of electrons in transistors, diodes, integrated circuits and similar electronic devices

signal to noise ratio

the ratio of amplitudes of a (desired) signal and noise

silicon

an element with four valence electrons that is commonly used as the basis for semiconductor devices

single mode

type of fibre optic that allows the flow of a single ‘mode’ of light

step-index fibre

type of fibre-optic where the refractive index is constant in the core and it steps to a different lower value in the cladding

ternary signals

digital signals having only three possible levels of amplitude a multiplexing scheme in which users are allocated their own time slots

time division multiplexing transistor

a semiconductor electronic device used to switch or amplify an electric signal

transition piece

a short section of sheet metal ducting used to join different shaped ducts

triangulation

a system of dividing a transition piece into triangular segments for the purpose of drawing the development of the piece

true length

the actual length of the line, rather than the apparent length

twisted pair

a pair of insulated wires that are twisted around each other so as to reduce the amount of noise that is induced into the conductors

twisted pair cable

a bundle of twisted pairs of wires in a common sheath

unguided medium

a medium for propagating electromagnetic signals that does not require a physical connection between transmitting and receiving ends

uplink

the communications channel from earth to a satellite electrons in the ‘outer shell’ of an atom that are generally involved in forming bonds between atoms – in metals they are relatively loosely held and can move from atom to atom

valence electrons

xxi

valve

a device using the passage of electrons across charged plates to produce the same electronic characteristics now achieved with semiconductors

virtual circuit network a network in which ordered data packets are sent via a specific path through a network

xxii

voltage

the potential difference between two points in a circuit – measured in volts

waveguide

a guiding medium used for microwave signals

wavelength

the distance in space between identical points of a periodic signal

Directive terms

The list below explains key words you will encounter in assessment tasks and examination questions. account

account for: state reasons for, report on; give an account of: narrate a series of events or transactions

analyse

identify components and the relationship between them, draw out and relate implications

apply

use, utilise, employ in a particular situation

appreciate

make a judgement about the value of

assess

make a judgement of value, quality, outcomes, results or size

calculate

ascertain/determine from given facts, figures or information

clarify

make clear or plain

classify

arrange or include in classes/categories

compare

show how things are similar or different

construct

make, build, put together items or arguments

contrast

show how things are different or opposite

critically (analyse/evaluate)

add a degree or level of accuracy depth, knowledge and understanding, logic, questioning, reflection and quality to (analysis/evaluation)

deduce

draw conclusions

define

state meaning and identify essential qualities

demonstrate

show by example

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describe

provide characteristics and features

discuss

identify issues and provide points for and/or against

distinguish

recognise or note/indicate as being distinct or different from; to note differences between

evaluate

make a judgement based on criteria; determine the value of

examine

inquire into

explain

relate cause and effect; make the relationships between things evident; provide why and/or how

extract

choose relevant and/or appropriate details

extrapolate

infer from what is known

identify

recognise and name

interpret

draw meaning from

investigate

plan, inquire into and draw conclusions about

justify

support an argument or conclusion

outline

sketch in general terms; indicate the main features of

predict

suggest what may happen based on available information

propose

put forward (for example a point of view, idea, argument, suggestion) for consideration or action

recall

present remembered ideas, facts or experiences

recommend

provide reasons in favour

recount

retell a series of events

summarise

express, concisely, the relevant details

synthesise

putting together various elements to make a whole

Extract from The New Higher School Certificate Assessment Support Document, © Board of Studies, NSW, 1999. Refer to for original and current documents.

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Telecommunications engineering

Part 1: Telecommunications engineering – scope of the profession & engineering report

Part 1 contents

Introduction..........................................................................................2 What will you learn?................................................................... 2

Scope of telecommunications engineering ....................................3 Current technology in the telephone network............................... 5 Current applications and innovations .........................................15 Health and safety issues ...........................................................22 Relations with the community ....................................................25 Legal and ethical implications....................................................27 Training and careers in the profession .......................................28

Engineering report............................................................................31 Structure of an engineering report..............................................31 Sample report ..........................................................................33

Exercise .............................................................................................41 Progress check .................................................................................43 Exercise cover sheet........................................................................45

Part 1: Telecommunication engineering – scope of the profession and engineering report

1

Introduction

Telecommunications is the transmission of voice, data and other information over extended distances. The telecommunications industry covers a wide range of systems and technologies. The industry includes among other areas; radio, television, telephone, satellite communications, microwave and computer networks. In this section on the scope of telecommunications and in the history section you will consider aspects of these technologies.

What will you learn? You will learn about: • nature and scope of telecommunications engineering • health and safety issues • training for the profession • career prospects • relations with the community • technologies unique to the profession • legal and ethical implications • engineers as managers • current applications and innovations. You will learn to: • define the responsibilities of the telecommunications engineer • describe the nature and range of work done in the profession • examine projects and innovations in the telecommunications profession • analyse the training and career prospects within telecommunications engineering. Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to for original and current documents.

2

Telecommunications

Scope of telecommunications engineering

Engineers involved with telecommunications work in one of the most rapidly expanding and complex fields of engineering. When you consider the range of telecommunications products available you will realise why engineers involved in this field tend to specialise in one general area. There is just too much to know. This is especially so when designing software that must conform to the many protocols and standards that are required by international conventions and agreements. One engineer who was interviewed when compiling this unit had spent 15 years as part of a university team that was developing and improving the methods for producing communications grade optical fibre. The complexity of this task is not readily apparent but he indicated that the work involved was both challenging and varied throughout his time on the project. Examples of general engineering areas that a telecommunications engineer could be involved in include: •

transmission media – the material or media in which the signals are carried or transmitted

•

transmission and receiving equipment – the equipment which actually converts and transmits the telecommunications signals

•

transmission technology – the method and protocols by which the signals are encoded and decoded

•

switching systems – the method of connecting and recording the connection of one piece of terminal equipment to another ( and calculating those terrific phone bills that arrive every three months).

Telecommunications engineers may be involved in projects which include: •

Research – both pure research and research aimed at developing commercial products. The principle of fibre optic communication was proposed more than 50 years before scientists at Corning glass developed a glass of high enough optical purity to make commercial optic fibre a reality.

Part 1: Telecommunication engineering – scope of the profession and engineering report

3

4

•

Design of equipment and systems – many engineers working in the telecommunications field are electrical or electronics engineers. Their specialist knowledge of electronics and power enable them to design, develop and test the equipment needed to transmit voice and data over long distances. Other engineers, specializing in software design and computing, develop the programs and code required to implement the protocols and encoding required for modern telecommunications systems.

•

Supervising the manufacture of equipment – this involves managing teams of skilled and unskilled workers, maintaining manufacturing standards and coordinating with component suppliers to maintain production schedules.

•

Installation and commissioning of equipment and systems – in this area the engineers with specialist knowledge are employed primarily for their skills as managers. They will be involved in coordinating large teams of people to install, test and commission complex equipment. Most often this has to be done to a tight time schedule. High levels of organizational and interpersonal skills are essential in this area.

•

Maintenance and upgrading of installed systems – again the engineer is primarily employed as a manager. Good maintenance requires strict adherence to scheduling and coordination of a wide range of personnel.

•

Sales, tender preparation and marketing of telecommunications systems – the complexity of telecommunications systems dictate that the personnel who sell and purchase equipment/systems in this field need to possess a high level of technical knowledge. Engineers and software designers who have worked on the technical aspects of telecommunications systems will have highly developed technical knowledge and expertise.

Telecommunications

Current technology in the telephone network In this section you will examine the current telephone network and the transmission media and technology that telecommunications professionals deal with.

Transmission media When you complete the history section you will realize that from the time of Alexander Graham Bell until the late 1970s the components used in the public telephone system have changed very little in their principle of operation and have evolved relatively slowly. The industry name for the public phone system is the public switched telephone network or PSTN. The earlier plain old telephone system (POTS) used solid copper wire twisted in pairs to run from the subscriber’s home to the closest telephone exchange. Many such pairs were present in a cable and it is technically known as unshielded twisted pair (UTP) cable. This is very similar to UTP cable that is currently used to connect computers together in local area networks (LANs). At the telephone exchange the wires were directly connected to an electro-mechanical switching system that then directly connected the subscriber to the desired line. This usually meant connecting to at least one other telephone exchange and then continuing on UTP to the telephone being dialed. A continuous, hardwired electrical circuit was formed between the two telephones involved. This is known as circuit switching and was in operation in Australia for over 100 years. Some components of this system are still in operation today. Unfortunately, twisted pair is subject to attenuation (loss) as the length of the wire is increased. Distances of over 3000 metres tend to attenuate the signal significantly. The shorter the copper line the higher the quality of the signal that reaches the exchange. For longer distances, amplification (boosting) of the signal was required at regular intervals to maintain the signal quality. Before we go on, list any other ways that you could transmit the signals between exchanges or over long distance without this loss of quality. __________________________________________________________ __________________________________________________________ __________________________________________________________

Part 1: Telecommunication engineering – scope of the profession and engineering report

5

Did you answer? Some of the ways you can transmit signals between exchanges over long distances without loss of quality include: • coaxial cable • radio transmitters • microwave • laser beams • satellites • optical fibre.

As it turns out, virtually any other media and transmission method works better than wire but unfortunately they were not available in the 1890s. Currently a number of systems are available and are used to transmit signals between exchanges. They include: •

coaxial cable

•

optical fibre

•

wireless – microwave, satellite and cellular

•

digital encoding rather than analogue.

Coaxial cable Coaxial cable consists of a single conductor running down the axis of the cable surrounded by a dielectric (or insulating) layer. A continuous conducting shield covers the cable. A protective insulating layer is placed over the shield. The shielding material prevents high frequency radiation from leaking from the cable. Coaxial cable is also used extensively in the cable television industry to carry dozens of TV channels on a single cable. This material is therefore said to have a high bandwidth compared to plain copper. Single solid conductor on the central axis

Insulating layer

Conducting material forming a continuous outer shield

Figure 1.1 Construction detail of coaxial cable

6

Telecommunications

Optical fibre Optical fibre is being used increasingly in telecommunications and computing networks. Optical fibre uses the internal reflection of light down a light guide to transmit signals. The light guide is made from a glass core enclosed in a glass cladding layer with a different refractive index. Optical fibre has a much higher bandwidth than either UTP or coaxial cable. In creating the first commercially viable optical fibre, scientists and engineers had to produce an incredibly pure glass. To put this in perspective, if you want to transmit light along a one kilometre long optical fibre then you have the equivalent of transmitting light through a pane of glass which is one kilometre thick. Even more mind blowing, a 100km transmission is the equivalent of having a glass window pane which is 100 km thick. Light beam reflected along the core

Protective acrylate coating

Core material manufactured from high purity glass

Cladding material with lower refractive index than the core

Figure 1.2 Light reflected along the glass core of an optical fibre.

The optical fibres developed in the 1970s only retained 1per cent of the light transmitted after traveling one kilometre. This equates to an attenuation of 20 decibels per km (dB/km). This was regarded as a great success. Today attenuation rates of 0.25 decibels per kilometer are common. It is now possible to transmit beyond 100 km in optical fibre without amplification. You will note the use of the term decibel which is abbreviated to dB. This term is often used to describe the gain or attenuation of signals in electronics. Decibel scale is logarithmic and as such yields a compressed measuring scale for values that can vary widely. The general formula for calculating decibel gain or loss is dB = 10 x log (gain or loss) In early commercial optic fibre only 1% of the signal traveled the full one kilometre. The attenuation or loss was therefore 100 times. The attenuation in decibels can be calculated by: Attenuation dB =

-10 x log 100 = - 20 dBs

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The bandwidth of optical fibre is phenomenal. This is because the carrier waveform is light and the frequency of light is much higher that radio waves. At the time of writing the transmission speeds for optical fibre were quoted as high as 9.9 Gbps. This equates to the entire 15 volumes of Encyclopedia Britannica in far less than one second. Optical cable is now commonly used to span great distances between exchanges and even between countries. Engineers in Australian companies such as Alcatel (formerly Standard Telephone and Cable) design and manufacture undersea optical fibre cable which carries telephone traffic world wide. Initially the telecommunications engineers and planners had envisaged a system where optical fibre was delivered right to the home. In the industry this is known as fibre-to-the-home (FTTH). This has recently been seen as overkill both in bandwidth capacity and in cost. The current planning involves hybrid systems that utilise fibre to the neighborhood (FTTN) then use cable or UTP to deliver the signal to the home. In the innovations and current technologies section you will learn about a specialised modem system that allows much higher speeds on standard UTP copper wire.

Wireless technology Wireless technology, as the name suggests, does not use cable or wires to connect between exchanges and or telephones but instead uses radio frequency broadcasting to transmit the signal. This can considerably reduce the need for costly hard wired infrastructure while simultaneously increasing the mobility of some users. Wireless broadcasts are commonly made in the microwave end of the electromagnetic spectrum. The application of wireless technology is varied. The common applications include ground based microwave networks, cellular phone systems, geo-stationary satellites and low earth orbit satellites. Microwave frequency transmissions travel in straight lines. Unlike lower frequency radio waves they do not refract in the earths atmosphere or bounce off the ionosphere to any significant degree. Microwave communication is essentially line of sight. If you cannot see the transmitter you cannot pick up the signal. The ground based microwave systems have a series of microwave towers spaced at regular intervals across the countryside. These towers have line of sight to each other. Each tower receives and then re-transmits signals from other towers that it can ‘see’. These towers are therefore called microwave repeaters. This system is used to transmit signals between

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Telecommunications

remote telephone exchanges and is commonly used in semi-remote areas. You will often see these towers with their small parabolic dishes on top of tall mountains and high ground. The NSW railways and electricity authorities have their own microwave communications networks. Why do you think these towers are placed on tall mountains?

Figure 1.3 Microwave towers

__________________________________________________________ __________________________________________________________ Did you answer? Placing towers on high points gives the longest possible straight line (line of sight) transmitting distance because there are no objects in the way.

Geo-stationary or geo-synchronous satellites Geo-stationary or geo-synchronous satellites hold a set position above the earth’s surface, rotating at the same angular velocity as the earth. A geo-stationary that was on station above say Dubbo would hold that position continuously and only vary if its navigation system was reprogrammed for a new location. These satellites are stationed approximately 37 200 kms from earth. Radio waves propagate at the speed of light. The combination of the satellite’s distance from earth and the finite speed of the radio signal introduces a very small delay between the transmission of the signal and the arrival of the signal back at earth. This delay is acceptable for data communications and some international voice communications but is not regarded as satisfactory for cell phone type communications. If you have seen the movie ‘Apollo 13’ you may recall that it took one or two seconds for the transmissions from the Apollo 13 module to reach earth. 37 200 km

Earth Dubbo Geo-stationary satellite Figure 1.4

Geo-stationary or geo-synchronous satellites have a time delay of around a quarter to half a second due to their distance from earth

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Telstra is a member of the INTELSAT and INMARSAT groups which operate a large number of geostationary satellites. Calculate the time delay that you would expect for a signal to reach a geo-stationary satellite then return to earth. Assume that the displacement of the satellite is 37 200 km from the surface of the earth and that the speed of light is 3 x 108 m/s.

Did you answer? velocity (V) = displacement (S) time (t) ... t = S V

S = 37 200 km 6

= 37.2 x 10 m 8

V = 3 x 10 m/s

= 37.2 x 106m 3 x 10 8 m/s = 0.124s t t

one way

= 0.124 sec

both ways = 2 x 0.124 sec = 0.248 sec ie about 1/4 sec out and back

Cellular phones Large numbers of people now own and operate mobile phones. These mobile phones are also known as cellular phones. Supposing that a single tower was placed in the center of Sydney to transmit and receive to and from all the phones owned by subscribers in Sydney. The tower would simply be overwhelmed by the number of subscribers trying to call. To eliminate this problem the metropolitan area is divided into ‘cells’ a few kilometres across, hence the name cellular phone. Each cell has a base station (mobile phone tower) operating at a different set of frequencies to nearby cells. The towers operate on low power outputs so that the operating frequencies can be re-used at other locations. When a subscriber moves from one cell to the next the call is simply transferred or ‘handed on’ to the base station associated with the next cell.

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Telecommunications

Base station

Cell

Figure 1.5

Hand on point

Four base stations co-ordinating calls from two mobile phone units in a cellular network

Originally Australia adopted an analogue transmission system. Analogue systems are fabulous for voice communication but fail to offer all the extra bells and whistles of digital systems. To be really cynical you could say that they don’t offer all the money making options that digital phones provide. As well, analogue transmissions require more of the available frequency bandwidth per channel and thereby make less efficient use of the narrow frequency band available. The analogue cellular telephone network was closed down in Australia in 2000. Digital mobile phones use the global system for mobiles or GSM transmission system. The digital nature of the transmission means that both data and voice are readily transmitted and that a wide range of features can be included. List four features available on a modern digital mobile phone, for example, personalised ring tone. 1

_______________________________________________________ _______________________________________________________

2

_______________________________________________________ _______________________________________________________

3

_______________________________________________________ _______________________________________________________

4

_______________________________________________________ _______________________________________________________

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Did you answer? Some of the features available on a modern digital phone include: • call forwarding • voice recognition dialing • infra red links • stored numbers • digital games • messaging systems.

When the analogue phone network was decommissioned in Australia, the coverage of GSM digital mobile phones was inadequate. This was especially so in rural Australia. The government allowed a new system to be introduced into the mobile network to supplement coverage in rural areas. This system is called code division multiple access, more commonly known as CDMA. Now you need two telephones, a CDMA to make calls in rural Australia and a GSM digital to make calls in metropolitan Australia. See what happens when politicians are asked to make a sensible decision!

Low earth orbit satellites Low earth orbit satellites are an emerging technology. Low earth orbit satellites orbit about 1500 kms above the earth’s surface, consequently they move quite rapidly across the sky and have no noticeable time delays. At this time, several companies are developing plans for LEO satellite systems. These companies include Teledesic, Globalstar, Odyssey and Motorola (Iridium). It is conceivable that not all these systems will be introduced or succeed. In a low earth orbit system the satellites are equivalent to mobile telephone towers in the sky. Instead of the telephone user moving from one cell to another, the cell associated with a particular satellite moves with the satellite across the surface of the earth. As one satellite passes below the horizon a new satellite is available so the telephone user can be handed on. The Globalstar system, for example, will require forty eight satellites to adequately cover the earth.

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Telecommunications

Low earth orbit satellite 1500 km above the surface Departing satellite

Earth

Figure 1.6

Low earth orbit satellites need to hand-off to other satellites before they move over the horizon

What do you think is the advantage of low earth orbit satellites over geostationary satellites? __________________________________________________________ __________________________________________________________ __________________________________________________________ Did you answer? The short distance, 1 500 km, means that there is no delay noticeable by the user. It is instant, therefore it is better for voice communications.

Transmission technology In the early 1980s Telecom Australia (now known as Telstra) began a revolutionary change in the technology used to transmit voice and data between telephone exchanges. Up until this time all the signals transmitted on the telephone network were analogue in nature. This was perfectly adequate for voice communications but the forward thinking engineers at Telecom were already planning for the digital revolution that was to descend on them. Telecom engineers and planners had decided to implement a fully digital switching and transmission system between telephone exchanges. This system is now able to be delivered right up to the subscriber in the form of ISDN but it is very costly. An analogue connection on single copper wire needs to be maintained continuously during a conversation and normally only one conversation is able to be carried out on that particular line. This is very wasteful use of expensive resources especially between exchanges. The situation can be improved by using a transmission media such as optic fibre where

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multiple channels can be carried on a single fibre. However, the connection must still be continuously maintained. With a digital telephone exchange, the connection between exchanges is a ‘virtual connection’. It is not a hard wired connection. The ‘connection’ is linked only when data is being sent or received. Also, all the data being transmitted is digital. This includes the voice communications components. Well how can this be so?

The fundamental process involved in digital transmission is a form of ‘packet switching’. Packet switching in telephone networks operates like this: •

you dial your number and the computer at the telephone exchange notes down where you want your voice to be sent to

•

you then speak into your telephone and your voice is transmitted via copper lines to the telephone exchange

•

the computer at the exchange then samples your voice at about 8 000 times a second and assigns it a digital number each time – that is, it is quantising your voice ready for digital transmission

•

the computer then bundles up the sampled conversations or data into an electronic ‘packet’, this packet is addressed and error correction codes are attached

•

the packet is then sent off to the destination exchange where it is digitally unpacked, converted back to analogue and then sent via virtual connection to the destination telephone line

•

the speed at which modern telecommunications computers operate is so fast that the user is totally unaware that this process is happening.

A simple analogy for this process is when someone moves house. First all the items in each room are packaged and labeled according to their destination rooms. The packages are placed into the moving van in any order that suits the person loading the van. The van drives off. At the destination the van is unloaded and the packages are taken to the rooms that match their labels and unpacked.

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Telecommunications

UT

P

DN

IS

Figure 1.7

DN

IS

Data packets in transit

Digital Exchange A

Digital Exchange B

UT

P

A digital network using packet switching between telephone exchanges (note that the UTP lines are analogue and the ISDN lines are digital between the telephone exchange and end user)

Two companies have supplied digital telephone exchanges for the current Telstra network. Ericsson have supplied the AXE digital exchanges and Alcatel have provided the System 12 digital exchanges. The engineer featured in the Engineering report at the end of this section was involved in the development and manufacture of the System 12 exchanges in Australia. Telecommunications engineers may be involved in any aspect of the above areas. They may specialise in designing wireless transmitting equipment, developing software to run digital mobile telephones, supervising the manufacture of telephone exchanges or any one of the multitude of tasks needed to keep the telephone network running and up to date. If you have access to the Internet and wish to find out more about telecommunications technology, both leading edge and historical information, then the International Engineering Consortium (IEC) have an excellent tutorial section at: <www.iec.org/tutorials> (accessed 04.12.01).

Current applications and innovations You will now learn about some of the emerging technologies in telecommunications engineering. Telecommunications technology is emerging at an incredible rate. To attempt to put down on paper and predict the direction of this technology is a risky business. In fact, by the time that you read this some of the technologies described here may have already been discarded. Currently two distinct but related concepts are driving telecommunications development. These concepts are broad-banding and convergence.

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Broad-banding Broad-banding is related to bandwidth and is primarily concerned with transferring as much data as possible through a transmission system in as short a time as is possible. The need for broad-banding comes from the ever increasing demands of industry and domestic users for high speed data transfers. This is often associated with computer networks such as wide area networks (WANs) and the internet. For example, internet web pages now often contain large amounts of photographic material, video clips, music clips and even full length movies. These graphics images require very high rates of data acquisition and currently take long periods of time to download. Two solutions suggest themselves. Compress the image data and/or increase the rate at which data can be transmitted. Compression solutions are constantly being refined by software designers in the internet industry. Compression formats now exist for photographs, video, music and other types of files. Compression reduces the file size required to store the image or sound. List one common compression format for each of the following file types: music, video and photographs. Photograph: ________________________________________________ Video:_____________________________________________________ Music:_____________________________________________________ Did you answer? • Photograph – JPEG, Compressed TIFF, GIF • Video – MPEG • Music – MP3.

The development of methods for increasing data transmission rates is the domain of the telecommunications engineer. Two areas currently being implemented are ATM and ADSL. ATM or Asynchronous Transfer Mode is a refinement of earlier packet switching. Unlike packet switching it is designed for high performance multimedia communications. ATM is essentially a protocol or an internationally agreed way of transferring data. It is interesting to note that another protocol that you may be familiar with is also out there competing with ATM. That protocol is the IP protocol that you may have associated with Internet communications. You will have most likely

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seen it written as TCP/IP. Currently, engineers with several large telecommunications companies are developing and optimizing ways of transmitting voice over these two protocols. Of more interest to the average home Internet user is the implementation of ADSL or asymmetric digital subscriber line. You will recall that the copper lines running from your house to the closest telephone exchange are not very good for transmitting data. In fact, if you currently had a standard 56k modem you would be extremely lucky to get anywhere near that Baud rate on a continuous basis. The copper UTP telephone line is the limiting factor. ADSL is not a new telephone line but in fact a pair of extremely sophisticated modems. One modem at the telephone exchange and another modem at the end users premises. ADSL is promising data transfer rates of up to 6 Mbps on 3 to 4 km of copper telephone line. This is quite adequate for real-time watching of movies and listening to music. Digital Telephone Exchange

UTP Home computer Figure 1.8

ADSL Modem

ADSL Modem

An ADSL modem pair set up to provide high speed data transmission to the home of a subscriber

Another innovation made possible by digital exchanges is the ‘intelligent network’. Intelligent networks use the computing technology now associated with telephone networks to make ‘intelligent’ decisions about subscriber calls. For example, a home delivered pizza company may have 25 outlets in Sydney. However, it has only one listed number in the telephone directory. When you phone for a pizza, to help you get through the hours of study that you are obviously putting in for your HSC, a marvellous thing happens. The pizza outlet that answers is the one that is closest to your home. The ‘intelligent network’ has taken your call and decided on the closest outlet then directed your call to that point. Intelligent networks provide an array of ‘value added’ services such as call forwarding, prepaid calling facilities and call waiting.

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Convergence This is a vast and expanding area. Until very recent times telephones were used to place telephone calls, television stations broadcasted movies, printing firms produced books, radio stations broadcast music and computers were stand-alone business tools. All these communications systems used different formats to store and transmit their content. Earlier in this section you learnt that many devices that were previously analogue are now available as digital devices: digital telephones, DVD players, digital televisions, CD players and even telephone exchanges. The data used by these digital devices can be shared, provided that a common communications protocol is used. At the same time a rather contrary change is occurring in transmission systems. Media that were once delivered by wireless technology are being delivered over wire cable and optic fibre, for instance cable television and some radio. At the same time, traditionally hard-wired technology, such as telephones, are rapidly moving into wireless technology in the form of mobile phones. The third factor that is coming into play here is the increasing speed of computers and development of the Internet. The Internet has provided a common protocol, that is TCP/IP. As you will recall IP (Internet protocol) is a transmission protocol that can be used to move data across telephone lines. Broadcast and digital media convergence is the coming together of telecommunications, television, computing, radio, music and the Internet. Convergence builds from technologies we already love to use: •

broadcast technology – television and radio

•

the computer and internet

•

the telephone.

Convergence is an emerging concept. It is currently available to a limited extent but its use should expand dramatically in the next five or so years. For instance you can currently receive television on your computer. You can also receive wireless internet by connecting your mobile phone to your laptop. You have to admit though, that the time to download and the overall quality is not that impressive. 1

List other examples of convergence that are currently available. _______________________________________________________ _______________________________________________________ _______________________________________________________

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Telecommunications

2

A current technology that attempts to address the demand for wireless internet is the WAP mobile phone. Indicate what the letters WAP stand for and what a WAP phone does. _______________________________________________________ _______________________________________________________ _______________________________________________________

Did you answer? 1 Interactive video on DVD and Web cams, for example @surfing beaches, available on the Internet are other examples of convergence. 2 Wireless Application Protocol, that is WAP, can access a range of modified Internet sites. However, the type and quality of the data is limited by the capacity of the phone.

In its present form, WAP is quite limited in what it can achieve. WAP will have to evolve into something more sophisticated and user friendly otherwise it will be overtaken by some other currently emerging technology. In Japan for instance, the DoCoMo company’s rival system ‘i-mode’ had signed up 10 million users between February 1999 to October 2000. ©

New Scientist, 21 October 2000.

You are now going to learn about two emerging and related technologies both of which are associated with convergence but also incorporate a large range of other systems. These technologies currently look like they are going to succeed but this is by no means certain. These emerging technologies are : •

3G – third generation mobile telephone

•

Bluetooth.

3G 3G mobile phones are the next generation of mobile phones. 3G will have data rates which are 150 times faster than a WAP phone. It is estimated that 3G will eventually achieve rates of 2 megabits per second. This will give 3G equipment speed and capacity at broadcast level. Applications can include direct Internet access, MP3 music on the move, video phones and phone shopping. 3G telephones will be the first telephone to fully exploit the Internet and broadband options.

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The significantly higher data rates available from 3G make another feature possible; tracking or position location. Using a known ‘listening station’ the telephone network can assess the time for signals to travel to and from a particular mobile phone and two or more known base stations. These signals can then be triangulated to give the location of the user within 5 metres. This could be very good for emergency services if you were making a ‘000’ call. Base Station 1

Base Station 2 Phone known displacement Listening Tower Base Station 3 Figure 1.9

The displacement of the 3G mobile telephone user is determined by the time taken for the signal to reach the listening tower compared to the time to reach the 3G mobile phone. The system then simply triangulates displacements from the 3 base stations.

Bluetooth Bluetooth is a communications standard and specification initially proposed by Ericcson but now taken up by a range of large telecommunications companies. It is a wireless technology designed to communicate with devices at ranges up to 10 metres. The bandwidth anticipated will be up to 720 kbps with a power consumption as low as one milliwatt (mW). Bluetooth has been backed by an interest group which includes Ericcson, Motorola, Intel, IBM, Nokia, Toshiba and Lucent Technologies. At this point you are probably thinking, what is the use of a communications system that can transmit only 10 metres. You can talk to someone at that range. The Bluetooth module is quite small. At present the Ericcson unit is less than 30 mm long. It is anticipated that the device will be placed inside a wide range of computing and microprocessor controlled devices. These could include standard telephones, 3G telephones, microwave ovens,

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Telecommunications

stereo systems, home computers, car stereo and vehicle and engine management systems. So what! Who wants to ring up their washing machine and discuss the quality of laundry detergent or the likelihood of rain today? Well as it happens very few people will want to do so. However, many people will be interested in some other possibilities. With a Bluetooth module installed in your new 3G mobile telephone, you are immediately identified to all other Bluetooth modules within 10metres. You leave for work in the morning and jump in the Bluetooth equipped family car. The car seats adjust to suit your preset requirements and the engine management system adjusts the engine and gearbox to suit your driving style. The Bluetooth enabled air-conditioning sets itself to 23.5 degrees. As you left the house the dishwasher and washing machine switched on. Their Bluetooth modules have been programmed to operate these machines when you are not at home because you cannot stand the sound of running water. Later in the day you buy some new shoes and need some cash out. The transaction is carried out using your mobile phone and PIN while standing at the cash register of the supermarket. Just as you leave the supermarket your telephone and the telephone of a person nearby beeps. Via Bluetooth interaction, your telephone has just found another person who loves restoring Datsun 200B’s. This scenario is already happening in places such as Japan where ‘i-mode’ devices are used to meet people of similar interests. i-mode has been especially popular amoung young people looking for like-minded friends. Outline any other uses that Bluetooth technology could be put to in everyday life. __________________________________________________________ __________________________________________________________ __________________________________________________________ Did you answer? Bluetooth technology could be used by business to market goods, for example, you walk past a shop and your phone beeps, it announces a special on hockey sticks in a nearby outlet – your phone had hockey programmed in as one of your interests.

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Health and safety issues In this section you will learn about some safety issues related to the telecommunications industry. In previous modules you have discussed health and safety issues associated with the manufacturing process and commercial activity. For instance, in Aeronautical engineering, you considered the effects of dust, noise and chemicals and the appropriate control measures for these hazards. In this module you will be investigating some hazards which telecommunications technology may present to both industry personnel and the end user. One of the most visible telecommunications technologies to emerge in recent times is the digital mobile telephone. These units emit electromagnetic radiation when transmitting. The emission levels from digital mobile telephones are very low but the frequencies are in the 800 to 900 megaHertz range. This is quite a high frequency. You may be aware of discussion in the media about the electromagnetic radiation emitted by mobile telephones. Outline any details that you can recall about these media reports. __________________________________________________________ __________________________________________________________ __________________________________________________________ Did you answer? There is currently debate about the level of risk associated with the use of mobile phone handsets and base station towers. These devices transmit signals in the 800/900 MHz range which may be harmful to human health.

If anything is agreed on about the dangers of radiation from mobile telephones it is that there is no agreement about the level of hazard posed by these devices. No study to date has proved a statistically valid link between mobile telephone emission and cancer or other diseases in the brains of telephone users. At the same time, some academics have expressed concern about young people being subjected to excessive mobile phone radiation and the possible effects on brain development. A domestic microwave oven cooks food by utilising high frequency electromagnetic radiation to excite the molecules in water, fats and sugars. The frequency required to do this is 2.4 GHz.

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Could you hazard a guess at the frequency used by Bluetooth modules?

You guessed it, 2.4 GHz. This is not the problem that it at first seems. The power output is at present so low, one milliwatt , that no danger is presented. However, if the manufacturers try to increase the range of Bluetooth much beyond 10 metres, then problems may well arise. Incidentally, there is some concern that ‘leaky’ microwave ovens may jam the Bluetooth signals in some houses. Ericcson have developed a frequency hopping mode for Bluetooth which the company believes will eliminate this possibility. Read the following extract from a newspaper article, New mobile networks could double radiation levels, then answer the questions below.

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New mobile networks could double radiation levels Julie Robotham Medical Writer New generation mobile phone networks operating on the radio spectrum sold off by the Federal Government last month could emit more than twice as much radiation as other digital mobile phone towers, if a new technical standard is adopted. The radio spectrum to be used by the socalled 3G networks raised $1.17 billion for the Government in a recent auction. Telstra, Vodafone and Optus were among six companies that bid successfully for the new frequencies. The 3G network towers would be allowed to generate electro-magnetic radiation of 10 watts per square metredouble the maximum allowed for most radio base stations in today’s digital networks- under new limits that could be formalised this year. An interim standard, which lapsed two years ago, allowed only 2 watts per square metre. The proposed standard would allow mobile phones based on either technology to operate at up to 2SAR- a measurement of radiation absorption by the head and body. This is an increase of about 70 per cent on previous limits. The proposed relaxation of restrictions on mobile phone radiation comes at a time of unprecedented community concern about its potential dangers. The National Health and Medical Research Council has committed more than $3 million to scientists studying the effects of radiation on brain function and cancer. The working group devising the new standard includes doctors, community groups and union representatives, under the aegis of the Australian Radiation Protection and Nuclear Safety Agency. Members concede its safety margins are very narrow. Dr David Black, an occupational and environmental physician at the University of Auckland Medical School, said mobile handsets were allowed to operate at levels

24

much closer to those associated with health effects than were television transmission towers. However, Dr Black was ‘happy with the numbers’ and believed the proposed standard would be safe for mobile phone users. He said it was essential the community was fully informed on how mobile phones might affect the human body, so that people could make an informed choice. One such issue was the blood-brain barrier, which usually prevented medications from acting on the central nervous system. Some medical conditions and high fever could make it more permeable, allowing drugs and chemicals to affect the brain more directly. Dr Black said that animal studies were inconclusive as to whether mobile phone radiation made the barrier more ‘leaky’. Another member of the working group, electrical engineer Mr John Lincoln, said he was unhappy with draft standard’s approach to the issue of differential heating of areas of the body. He said the standard was predicated on the idea that the body could efficiently cool heated areas through the circulation of the blood. Mr Lincoln said he did not believe the levels specified in the standard would be changed, but there was ‘no science at this point of time to suggest that these factors are safe’. Mr Keith Anderson, director of the Australian Mobile Telecommunications Association, which represents phone manufacturers and network operators, said it was a complicated issue. “We dispute the simplistic assertions that the proposed standard will result in large increases in exposure. The power of mobile phones will not be raised.” © Julie Robotham, Medical Writer, Sydney Morning Herald

Telecommunications

1

Why do you think that the government is proposing to increase the transmitting power allowed on base station towers? _______________________________________________________ _______________________________________________________

2

What is one medical concern associated with this level of electromagnetic radiation? _______________________________________________________ _______________________________________________________

Did you answer? 1 The government is proposing to increase the transmitting power allowed on base station towers as a result of pressure from phone companies who have paid very large sums for 3G spectrum. 2 Of medical concern is the break down of the blood/brain barrier by radiation allowing harmful chemicals to pass into the brain more readily.

The debate about electromagnetic radiation is not the only safety issue currently associated with the telecommunications industry. However, it is certainly the most controversial issue at this time.

Relations with the community In this section you will learn about relations between the telecommunications industry and the community. Relations between the telecommunications industry and some community interests could be characterized as somewhere between cynical and downright hostile. A number of issues have arisen in recent times. Some community relations issues that have caused comment and debate include: •

The rollout of coaxial cables for cable television. Cable television companies utilised the existing electric telegraph pole network to deploy their thick, black coaxial cables throughout the suburbs in metropolitan Australia. Many communities objected to this extra and unsightly use of these poles and some local government councils attempted to ban the placement of these cables. People claimed that the cables affected the visual amenity of their streetscape.

•

The placement of mobile telephone towers in residential areas. These units are both unsightly and pose a possible health risk. The towers have been placed close to many residential areas including schools. This has often been done without adequate community

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consultation. Many communities have protested vigorously about the placement of mobile telephone towers. •

The decommissioning of the analogue telephone network in the year 2000. This was a significant problem in rural areas where the low powered GSM digital phones lack suitable range. People in rural areas have had to throw away their analogue handsets and then purchase CDMA handsets. Subscribers in semi-rural areas often have to purchase both a GSM digital and a CDMA to obtain adequate coverage.

•

The high rate of theft of digital mobile telephones combined with the telecommunications industry’s reluctance to develop systems to decommission stolen telephones. From a telecommunications engineering perspective, this can be achieved by utilizing the handset’s IMEI number.

•

The quality of telephone services and internet provision in rural Australia. The ability to access internet services at similar quality and cost to city subscribers is an ongoing concern of country subscribers. These people often have to pay higher prices for lower quality services.

Figure 1.10

A mobile telephone tower

Would you like one of these put at the end of your backyard?

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Legal and ethical implications In this section you will learn about the legal and ethical considerations of telecommunications engineers and the products that they develop. A large proportion of the engineers engaged in telecommunications engineering are electrical engineers specialising in telecommunications. Professional engineers usually belong to a professional association. The Institute of Electrical and Electronic Engineers or IEEE is an international association for professional electrical engineers. The IEEE organisation has a professional ethics committee which has developed a code of ethics which the IEEE expect all engineers affiliated with the institute to follow. Listed below are some selected items from IEEE’s code of professional behaviour: •

‘To accept responsibility in making engineering decisions consistent with the safety, health and welfare of the public, and to disclose promptly factors that might endanger the public or the environment’

•

‘To avoid real or perceived conflicts of interest whenever possible, and to disclose them to affected parties when they do exist’

•

‘To improve the understanding of technology, its application , and potential consequences’

•

‘To be honest and realistic in stating claims or estimates based on available data’

•

‘To reject bribery in all its forms’.

The expectations listed above indicate that engineers are expected to display a great deal of integrity in their day to day dealings and decisions. The emerging technologies associated with mobile phones present a number of legal and ethical problems. Currently the Finnish government is investigating the use of mobile telephone SIM cards to act as personal identification. The phone and card could then be used for cashless transactions and for situations where personal identification is required. However the high rate of theft and simple loss of the telephone handsets will not assist the acceptance of such a system by the general population. ©

New Scientist, October 21, 2000

The Finnish government also envisages a large amount of other personal data being linked to this system via a database. This brings into play the larger issue of privacy and security of data. The general public is not likely to feel comfortable with such a system.

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Another issue associated with the emergence of 3G mobile telephones is the ability of the phone company to track the current user. This will be of benefit to the emergency services such as ambulance and police when a ‘000’ call is made but no address is supplied. It will also be useful in tracking down lost and stolen telephones. ©

New Scientist, October 21, 2000

Ethical questions that arise from this development include: •

Who should have access to the tracking capabilities; ambulance, police, private investigators, parents or debt collectors?

•

What legal process should someone wanting to track the phone have to follow?

Clearly, as sophistication and speed of technology increases, engineers, telephone companies and government will need to closely examine the extent to which technology impinges on people’s lives. They must then make ethical decisions about the use of this technology.

Training and careers in the profession As stated previously, most engineers employed in the telecommunications industry have a background in electrical engineering or software engineering. These types of engineering and computing courses are offered by most of the large Australian universities. Engineering courses are offered as undergraduate courses. In some institutions, post-graduate extension courses such as diplomas and masters degrees can be used to enhance engineering and science degrees. Universities such as the University of NSW, Sydney University and the University of Technology Sydney offer engineering degrees leading to employment in the telecommunications industry. Sydney University offers Bachelors of Engineering in Telecommunications Engineering, Software Engineering and Electrical Engineering. The University of NSW offers a Bachelor of Electrical Engineering with the ability to specialise in ‘communications’. In relation to specialising in communications, the UNSW’s website states that: ‘the activities of this department relate to all aspects of theory and applications for a broad range of systems such as telephone and data networks, radio and television broadcasting, satellite and deep space applications.

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The department carries out advanced research in digital communications, microwaves and antennas, optical communications (including the design and manufacture of lasers and optical fibres), signal and information processing and satellite mobile communications.’

In describing the possible roles of an electrical engineer the site indicates that: ‘an electrical engineer may be responsible for research, design, manufacture and operation of: •

Communication systems; satellite, microwave, optical

•

Telecommunications

•

Broadcasting; television and radio’

©

<www.eet.unsw.edu.au/programs/programs.html>

As was the case with Aeronautical Engineering, the courses have a very high level of mathematics. Electrical engineers need to be able to carry out difficult mathematical analyses of circuits and make predictions based on complex mathematical models. The mathematical subjects to be studied include: Vector calculus and complex variables, Fourier series and differential equations and Matrix applications. Any student contemplating these courses should be competent in high level mathematics. The rapidly expanding nature of telecommunications engineering means that career prospects are very good and starting salaries for recent graduates are very competitive. If you have access to the Internet visit the following sites for further information on telecommunications: <www.voicendata.com/aug98/milestone.html> (accessed 04.12.01) <www.uow.edu.au/informatics/ > (accessed 04.12.01) <www.uws.edu.au/seid/programs/engineering/engineering.html > <www.cit.uws.edu.au > (accessed 04.12.01) <www.ee.newcastle.edu.au/undergraddesc.html > (accessed 04.12.01).

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Engineering report

In the engineering profession, an engineering report: •

outlines the area under investigation

•

analyses available data

•

draws conclusions and/or proposes recommendations

•

acknowledges contributions from individuals or groups

•

documents sources of information

•

includes any additional support material.

Structure of the engineering report This engineering report varies from the previous reports in that you will be compiling a report about an individual involved in one area of the telecommunications engineering industry. Previously you have reported on a product, innovation or current project. For this section you will need to interview a person involved in the telecommunications industry at a technical level. It would be preferable that this person be a professional engineer. However, this may be difficult in some rural or isolated communities. The engineering report will include the following sections: Title The title page gives the title of the report, identifies its author/s and gives the date when the report was completed. The abstract The abstract is a concise summary of the report. The purpose of the abstract is to allow a reader to decide if the report contains information that is relevant to their needs. The abstract should be no more than two or three paragraphs, and shorter if possible.

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The introduction The introduction outlines the subject, purpose and scope of the report. It may contain background information regarding the topic. Scope and nature of the profession This section contains a description of the nature and range of past and current work carried out by the person that you have chosen in the telecommunications engineering industry. It outlines the typical tasks carried out by this engineer or technical person. Training for the profession This section outlines the training that this person has undertaken to work in their chosen sector of the telecommunications profession. This section should also define and examine the skills required for this area of the profession. Current projects, innovations and technologies in this profession This section examines current and emerging projects that the individual is involved with in their role in the profession. Where possible, this section describes the situation that led to the development of the projects. This section also describes any current or unique technologies used by this person that are associated with or unique to this profession. Management practices in the telecommunications industry This section outlines the management tasks and responsibilities associated with this person’s role in the industry. Health and safety issues This section examines any health and safety issues that the person has to deal with in the telecommunications industry. These may be associated with the design, development, manufacture and implementation of projects or with the processes that they deal with in their daily work. This section should then explain how these issues are dealt with in the industry. Conclusion This section draws to conclusion the elements outlined and developed in the preceding sections. It should summarise any major points or issues that have been detailed in the preceding sections.

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Acknowledgements The acknowledgement section provides the opportunity to credit the work or assistance of other people who contribute to the report. References To demonstrate that the report is well researched this section should include all references. The Harvard standard referencing system should be used. For example, Higgins, R.A. 1977, Properties of Engineering Materials, Edward Arnold, Sydney. Appendices Contains information separated from the main body of the report. The information may include drawings, diagrams, photographs and tables that may enhance the information presented in the main body of the report.

Sample report The engineer who was interviewed for the sample report is a senior engineer for a very large multinational telecommunications company. This engineer has extensive experience in developing leading edge technology. In compiling your engineering report you are not expected to find a person with this level of experience. Moreover, you are expected to interview people that are readily accessible from your location. The name of the engineer has been changed to protect their privacy.

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Telecommunications engineering Title Page Title:

The human face of telecommunications engineering

Module:

Telecommunications engineering

Author:

Anna Log

Date:

August 2001

Abstract This report examines the scope and nature of the work carried out by an electrical engineer, Nikki Tesla, working for a large multinational telecommunications company, Alcatel. Aspects of the engineers role including past and present projects, management issues, professional development, current technologies and health and safety issues are discussed and analysed.

Introduction Nikki Tesla is an engineering and technical manager with the Australian division of a large telecommunications manufacturing company called Alcatel. Similar multinational telephone companies include Ericcson, AT&T and Seimens. Nikki has been a practising engineer for approximately 20 years with experience in hardware and software design of telecommunications systems. Nikki worked briefly for another company which designed thermocouples and temperature control equipment.

Training for the profession Nikki had an early interest in electronics. She designed and constructed many electronics projects while still at school. Her initial engineering training was a Bachelor of Electrical Engineering from the University of NSW. She graduated in 1980. Her major undergraduate project was to design a computerised engine management and ignition control system.

As with all new engineers and staff at Alcatel, Nikki was given training in the company’s ethos and in the specialist areas relating to her work. For example, when she first joined Alcatel, Nikki completed an in-house, one week basic electronics soldering course. As a senior engineer Nikki is currently completing an international advanced management course. This requires her to attend two week training sessions in Paris every two months (its tough but someone has to do it). Alcatel is involved in installing and upgrading hardware and software connected to Australia’s telephone network and this often involves real-time programming. Real-time programming requires the engineer to make software upgrades and changes on a software program that is actually running and controlling active telephone exchanges. In this situation a small error could result in a whole town or major sections of a city being without a phone network. Alcatel provides comprehensive training for new staff in techniques for real time programming and training in the precautions that need to be taken when doing real time programming. From a legal and ethical view-point the company is directly accountable for any downtime it causes – this is especially so with the 000 emergency services.

Scope and nature of the profession Alcatel has been involved in a wide and varied range of telecommunications activities including telephone design, telephone exchange design and manufacture, manufacturing undersea optical fibre cables, mobile telephone technology provision, microwave transmission equipment and communication technology for satellites. Nikki has been primarily involved with telephone exchanges and transmission systems since joining Alcatel. The past 20 years with this company has seen a revolution in the equipment it designs, installs and maintains. When Nikki first joined Alcatel, exchanges were large, noisy electromechanical systems based on the principles developed nearly 90 years before by people such as Strowger. Now, all Australian exchanges are fully digital and systems such as ADSL and 3G mobile are ready to be trialled and implemented.

Current projects, innovations and unique technologies In the early 1980s, Telecom Australia decided to convert the Australian telephone system from analogue exchanges to fully digital exchanges. Soon after, Nikki became involved in the hardware and software design of the System 12 digital telephone exchange. This is one of the current exchanges used on the Australian telephone network and represents the current technology in telephony. Through her involvement in the System 12 project Nikki spent several years in Belgium and Austria working with Alcatel’s European branches. The overall project took approximately 15 years. A recent project that Nikki has had input into was the automation of ‘directory assistance’ for Telstra. This is where advanced voice recognition software is used to automatically recognise the names of commonly called companies and then automatically tell the subscriber the phone number requested. For instance, if you call directory assistance wanting to know the number for QANTAS, a computer will ask for the name of the company you need. Providing it can understand you, the computer then reads out the number requested or offers to connect you directly to that company. Nikki has also been involved in developing ‘Voice over IP’ communications ability for hardware supplied by her company. Associated with this Nikki has seen some of the development work being done on 3G telephony at present and may move into this area as it comes on-line. Nikki sometimes uses computerised management tools and technology to help organise her tasks. The management software QSM for instance, can use critical parameters such as resource profiles, size, lead-time and largest / smallest work component to help predict scenarios and completion times. Spreadsheets such as MS Excel are also used for planning tasks.

Management practices in the telecommunications industry As a technical manager Nikki is responsible for a large team of engineers, software analysts and technicians. The training for these professionals is available at most of Australia’s major universities. She is also legally responsible for monitoring the occupational health and safety of the team. Nikki divides her management responsibilities into three areas:

• Functional management – ensuring the infrastructure and performance of the department is sound. This includes oversight of the budget and purchase of equipment, recruiting and selection of staff, professional development and supervision of staff on her team and maintaining morale. One point that was made by Nikki was that team morale was high when there was a modest level of work overload. That is, people didn’t enjoy having too little to do. • Management of the quality of work – this includes monitoring and correcting the level of defects in the final product be it software or hardware, determining and setting the metrics for each project (the constraints and limiting factors), system level testing, acceptance testing for new installations, determining the teams level of compliance with quality standards such as ISO 9000 and monitoring the level of coding errors in software, (errors per 1000 lines of code). • Occupational health and safety – The team is concerned with software design and does not do any manufacturing. As a consequence the occupational health and safety issues are fairly easily identified and dealt with. Team members are regularly given the opportunity for OH&S training but this training is often directed toward safety issues in industrial and manufacturing settings. Machine guarding, chemical risk assessment and use of PPE is emphasised but the workers do not use this type of equipment.

Health and safety issues The work environment of the software developer is somewhat cloistered. The primary OH&S issues are concerned with adequate lighting, ergonomic furniture, radiation and glare from VDU screens and ensuring that the cabling and leads for computers are arranged so as to prevent a trip hazard. In this office the occupational health and safety issues are more mundane and include : •

providing ergonomic furniture for staff operating computers

• staff being aware of correct lifting techniques eg for carrying boxes • VDU units adjusted to the correct heights and glare reduction incorporated • computer cabling and other trip hazards eliminated by careful office planning •

lighting , temperature and noise maintained at comfortable levels

•

staff aware of emergency evacuation procedures

Staff are also given information on correct lifting and carrying techniques.

Conclusion Nikki has followed a typical career path for an electrical engineer. She began as a hands on junior engineer with a sound knowledge of electronics but with limited management skills. Nikki is now a technical manager, supervising a large team of communications specialists. Her skills base now includes her initial electronics as well as advanced software design skills and industrial management skills. It is conceivable that she will move into a senior management position in the future or another role in the company such as sales or contract negotiation. In her time with Alcatel, Nikki has seen and been part of a major revolution in telecommunications. Digital exchanges have replaced analogue technology and the Internet has changed forever the way in which people view and use their telephones.

Acknowledgements I would like to thank Nikki Tesla from Alcatel and Con Ductor from the UNSW for their assistance in preparing this report.

References Higgins, R.A. 1977, Properties of Engineering Materials, Edward Arnold, Sydney. Murray, J. 1995, Calling the world. The first hundred years of Alcatel in Australia, Focus Publishing, Double Bay.

Appendix

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Telecommunications

Exercise

Exercise 1.1 Interview an engineer or technician currently employed in the telecommunication industry and report on the following aspects of the interviewee’s professional employment. •

Training undertaken to practice and gain promotion in the industry.

•

The nature and scope of the work carried out.

•

Current projects, innovations and unique technologies used in day to day work and any emerging technologies that may affect future work.

•

Management practices used by the interviewee or employer.

•

Health and safety practices and issues associated with day to day work.

Present your report following the structure of the sample engineering report.

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Telecommunications

Progress check

During this part you examined the scope of the telecommunications profession and explored current and emerging technologies in the industry.

✓ ❏

Disagree – revise your work

✓ ❏

Uncertain – contact your teacher

Uncertain

Agree – well done Disagree

✓ ❏

Agree

Take a few moments to reflect on your learning then tick the box which best represents your level of achievement.

I have learnt about: • • • • • • • • •

nature and scope of telecommunications engineering health and safety issues training for the profession career prospects relations with the community technologies unique to the profession legal and ethical implications engineers as managers current applications and innovations.

I have learnt to: • • • •

define the responsibilities of the telecommunications engineer describe the nature and range of work done in the profession examine projects and innovations in the telecommunications profession analyse the training and career prospects within telecommunications engineering.

Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to for original and current documents.

During the next part of the module you will trace the history of telecommunications.

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Telecommunications

Exercise cover sheet

Exercise 1.1

Name:

_____________________________

Check! Have you have completed the following exercise? ❐ Exercise 1.1 Locate and complete any outstanding exercises then attach your responses to this sheet. If you study Stage 6 Engineering Studies through a Distance Education Centre/School (DEC) you will need to return the exercise sheet and your responses as you complete each part of the module. If you study Stage 6 Engineering Studies through the OTEN Open Learning Program (OLP) refer to the Learner’s Guide to determine which exercises you need to return to your teacher along with the Mark Record Slip.

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Telecommunications engineering

Part 2: Telecommunications engineering – history of telecommunications

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Part 2 contents

Introduction ..........................................................................................2 What will you learn?...................................................................2

History of telecommunication............................................................3 Harnessing electricity .................................................................7 The telegraph .......................................................................... 10 The telephone ......................................................................... 15 Wireless.................................................................................. 25 Television................................................................................ 29 Digital telecommunication......................................................... 30 The World Wide Web ............................................................... 33 Societal influences................................................................... 34

Exercises............................................................................................37 Progress check .................................................................................43 Exercise cover sheet........................................................................45

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Introduction

Effective communication is one of the most taken for granted forms of modern technology. We simply expect our radios, television receivers, telephones and computers to work. If they do not work it is no longer difficult to replace them, often less difficult and even less expensive than repairing them. The change in lifestyle created by modern telecommunications and consumerism has significant implications for the responsible use and disposal of natural resources as well as issues of privacy and ethics. As you investigate the development and use of telecommunication systems you should keep in mind the following questions: • Did a change in materials and an understanding of physical sciences lead to a change in design? • Was a new and innovative design developed using existing materials and knowledge? • What was the influence of new construction and processing methods? • In what ways did developments in related technologies influence change in telecommunications? • How have these changes impacted on society and the environment?

What will you learn? You will learn about: • historical developments in telecommunications • the effects of innovations in telecommunication on peoples lives and living standards • environmental implications of telecommunication systems. You will learn to: • research the history of telecommunication in Australia and understand the way it has impacted on peoples lives • examine safety issues related to the use of telecommunication systems. Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to for original and current documents.

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History of telecommunications

All communication was once by word of mouth and picture writing. As the world population increased and people became more mobile it was necessary to improve communication. The gradual changes from isolated tribe to scattered village centres and then to major international cities created two needs – better communication and consequently better literacy. Early forms of communication required the sender to know the position of the receiver. Messengers on foot or horseback were vital to early communication networks. Where some form of semaphore was used then either the sender and receiver had to be in line of sight with each other, or some form of repeating station had to be created. It was not uncommon for the message to be changed in some way as it was transferred through these repeating stations, the person responsible for relaying the message either misunderstanding or misinterpreting the content of the message. Sending messages at night or in bad weather created even more difficulties. Where animals were used to relay written messages it was never quite clear whether a lost message was due to poor training, natural enemies of the animal, or the animal being intercepted by unknown people. These forms of communication were often unreliable and, by the standards we now take for granted, very slow. Messages could take weeks to be delivered. Modern telecommunication relies on the ability to use and control electricity. Consequently telecommunication was revolutionized with early developments in the understanding and use of electricity. Subsequent telecommunication evolution was achieved when the ability to manipulate electricity was extended. The development and use of electrostatics for transmission by modulated airwaves provided the final revolution for the creation of telecommunication systems as we know them today, linked by invisible wireless waves across the earth and beyond. The earliest forms of telecommunication by electricity relied on hand operated electrical switching machines such as Morse code senders. With no electrical amplification available, these systems continued to require repeater stations and the security of the electrical wires needed

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to conduct the electricity was difficult to maintain through isolated areas. The use of undersea telecommunication cables to link continents created many practical difficulties that could not be solved until 1866 when the first successful trans Atlantic cable was laid. The age of international telecommunications had begun. Although these early forms of telecommunication suffered some of the disadvantages of the communication systems they replaced, they did have other great advantages – particularly their ability to deliver messages very much more quickly and over greater distances than any previous forms of communication. As you read through this section you will discover the link between telecommunications and developments in the knowledge and use of electricity and electronics. To provide a framework and a summary around which to build on your knowledge, read through the following time line.

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1200 BC Homer in his work ‘The Illiad’, discusses signal fire being used for communication. 700 BC

600 BC

to 300 AD Carrier pigeons used by the Greeks in association with the Olympic games. Thales of Miletus is reputed to have rubbed amber on cat fur to produce static electricity.

1600 AD William Gilbert wrote about magnets and magnetic effects. He proved that attraction due to static electricity was not a magnetic effect.

1861

America is connected coast to coast by the electric telegraph.

1865

The first Atlantic telegraph cable was laid.

1873

Heinrich Hertz confirms the existence of electromagnetic waves.

1874

Thomas Edison invents multiplex telegraphy.

1876

Alexander Graham Bell patents the first telephone.

1877

Western Union has the first telephone line in operation between Boston and Sommerville.

1675

Robert Boyle realized that electrostatic force could be transmitted through a vacuum.

1880

1729

Stephen Gray distinguishes between conductors and nonconductors (insulators).

American Bell Telephone Company is founded. There are 30 000 phones in use.

1882

1746

Benjamin Franklin concludes that electricity is a fluid. Another scientist Henry Cavendish, experimenting with current, includes himself in the circuit to estimate current flow.

Bell obtains a controlling interest in Western Electric, a former telegraph company. This company had earlier rejected Bell’s offer to sell them his telephone patent.

1891

A. B. Strowger, an undertaker, invented the automatic dial system for telephones to eliminate the operator from the system.

1895

Marconi demonstrates voice radio transmission.

1897

Thomson discovered the electron, adding significantly to the understanding of electricity.

1904

John Fleming invents the vacuum tube.

1907

Lee De Forest added a third electrode to the diode and created the first electronic amplifier, the triode.

1913

Robert Milikan measured the charge on a single electron.

1910

Peter De Bye, a Dutchman, develops a theory about optical wave guides. The practical application of this theory is ‘optical fibre’. It was many more years before it became possible to produce this as a viable product.

1915

Valve amplifiers are first used in coast to coast telephone circuits.

1786

1793

1827

Luigi Galvani noticed the effect on a frogs leg when electricity was discharged through it. Later, Alessandro Volta invents the battery. The Chappe brothers, two young Frenchmen, established the first commercial semaphore signaling system near Paris. The signaling rate was about 15 characters per minute. The semaphore’s use spread across Europe and to parts of the USA and employed thousands of workers over a 40 to 50 year period. G.S. Ohm discovers the relationship between voltage, current and restistance V = I x R.

1837

Charles Wheatstone patents an ‘electric telegraph’.

1844

Samuel Morse further develops and demonstrates the electric telegraph.

1851

51 telegraph companies are in operation.

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1926

Baird demonstrates an electromechanical TV using spinning discs and neon bulbs. The idea worked but a more versatile and wholly electronic system was soon to follow.

1970

Intel releases the 4004 microprocessor chip. The chip is only a 4 bit processor but improvements in its design led to the 8086, 80286, 80386 and 80486 series of chips.

1928

Zworykin patents files for an electronic scanning television system.

1971

Xerox patents the first laser printer.

1974

1935

The first telephone call made around the world.

1937

Bell Telephone Labs (BTL) introduces the Model 300 telephone handset

The first domestic ( non-military) satellites launched. The concept of an Internet is proposed by Vint Cerf and Bob Khan.

1975

The use of optical fibres for telecommunications is trialed in the USA. The 5.25 floppy disk drive was produced by Alan Shugart at IBM.

1976

ISD (International Subscriber Dialing) introduced in Australia, allowing direct dialing to 13 countries.

1979

Visicalc , the first computerised spreadsheet was invented by Dan Brinklin. Dan is still at college at that time.

1981

IBM releases the IBM PC (Personal Computer) computer and thus begins the 80xxx and Pentium series of computers that we are so familiar with today.

1987

Sony releases the first 3.5 floppy drive and IBM introduce the first hard drive suitable for use in an IBM PC. It held a maximum of 10Mbytes.

1992

Tim Bernes-Lee a physicist, develops the World Wide Web (WWW).

1993

The MOSAIC internet browser is introduced, the Netscape browser comes on line the next year, 1994.

1996

A 56 kbps modem chipset announced by Rockwell. However most phone lines are capable of only 42 to 44 kbps using this technology.

1997

ISDN lines capable of 128 kbps are announced.

1998

The modem standard V.90 56K was approved.

1939

1947

John Atanasoff and Clifford Berry of Iowa State University invent the first electronic computer. BTL introduces a germanium point contact transistor. This is the beginning of the solid state era and the decline of the use of vacuum tube technology.

1954

Sony releases the first transistor radio.

1955

A.W. Morten and H.E. Vaughan release a paper called “The Transmission of Digital Information over Telephone Circuits”. They were in fact describing the first real modem. IBM develops the first disk drive.

1957

Sputnik-1 was launched by the Russians. It is the first satellite.

1958

A big year – Texas Instruments create the first integrated circuit and introduce the first silicon transistor. These two developments form the basis of modern solid state electronics. Seymour Cray produces the first computer to ultilise transistor technology.

1964

The concept of a “mouse” is patented by Douglas Englebart at Stanford Research Institute ( SRI) and George Heilmeier invents the liquid crystal display ( LCD).

1969

The US Department of Defense initiates the ARPANet program which eventually leads to the development of the present day Internet. It is initially only for military and academic purposes.

Now you have an overview, let’s go through it in a little more detail.

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Harnessing electricity The effects of electricity were first noticed in natural occurrences. The attraction between amber stone that had been rubbed with cat fur and small bits of straw was noticed by the Greek philosopher Thales around 600BC. Another Greek philosopher, Plato, made a similar observation around 300BC. Almost 2000 years later in 1551 an Italian mathematician, Jerome Cardan, examined the attraction between loadstone and iron and compared these observations with similar occurrences around amber. By 1600 William Gilbert, an English physician, had noticed similar properties in diamond, glass, sulfur and wax. Gilbert classified these materials as ‘electrics’ from the Latin word ‘electrum’ for amber. The English physician Sir Thomas Browne first used the term electricity in 1646. Wow! we’ve just covered 2200 years in a paragraph. However, things were starting to hot up. Let’s continue.... With continued investigation into electricity various discoveries followed. In 1729 Stephen Gray, an English scientist, found that some materials were electrical conductors, and others were not. Charles Du Fay, a French scientist, found that some objects repelled each other, while others showed attraction. In 1746 Benjamin Franklin, an American inventor, scientist and diplomat, concluded that electricity was a fluid and positive objects had an excess of this fluid while negative objects had less of this fluid than they required. Franklin proved that lightning is electricity. His theory explained the attraction between oppositely charged objects, repulsion between like charged objects and the neutralizing of charge when this fluid could ‘flow’ between oppositely charged objects after they come into contact. Later work by other scientists would refine these concepts, introducing the electron and determining that Franklin’s positive objects actually had a deficiency of electrons while his negative objects had an excess of electrons. In 1785 Coulomb, a French physicist, formulated the laws of attraction and repulsion between charged bodies. How would you explain the attraction and then the neutralising of charge using a ‘flow’ between oppositely charged objects? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________

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Did you answer? A negatively charged object has an excess of electrons. A positively charged object has a shortage of electrons. These objects will attract each other because they have opposite charges.

If they touch, the electrons can flow (move) from one to the other. If there are enough electrons, the positively charged object can be neutralised by replacing its ‘missing’ electrons.

In 1786 the first observations that were to lead to the invention of the battery were made. A freshly killed frog was made to twitch. It was supported on a copper hook and brought into contact with an iron railing. A flow of electricity through the body of the frog resulted from the reaction between the dissimilar metals that were electrically connected by the still moist frog. This observation was wrongly thought to have indicated that the frog contained ‘animal electricity’. Some 15 years later, in the late 1790s, Count Allessandro Volta, an Italian physics professor, discovered that two different metals connected by a conducting liquid could produce electricity. He built the voltaic pile – the first battery. His battery consisted of a stack of silver and zinc discs in pairs separated by a sheet of paper that had been soaked in salt solution. This invention was the first source of steady electric current. Without this discovery, the laws of electricity could not have been derived and modern telecommunications systems would not have been developed. About 20 years later, in 1820, Hans Oersted, a Danish physicist, noted that a strong current passing through a wire would move the needle of a compass held near it. The magnetic field around a flowing current had been discovered, and electromagnetism was born. The French physicist André Ampère then immediately formulated detailed laws concerning the forces of attraction and repulsion between current-carrying wires. By 1831 Michael Faraday, an English physicist, and Joseph Henry, an American physicist, had separately reasoned that if a moving current could produce magnetism, then moving magnetism could produce a current. The invention of electric generators and transformers followed.

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An early capacitor

An early battery

Generator

Figure 2.1 Early developments in electricity ©

World Book Encyclopaedia, 1985, Vol.6, p.152

The concept that electricity could produce electromagnetic waves that would travel without a conductor at the speed of light was first suggested in equations for the laws of electricity and magnetism formulated by James Maxwell, a Scottish scientist, in 1873. In the late 1880s Heinrich Hertz, a German physicist, produced these waves. Consider some of the advantages of sending ‘wireless’ messages (using electromagnetic waves). ___________________________________________________________ ___________________________________________________________ Did you answer? • • • • •

messages can be sent reliably any time of the day no cables needed to be installed no cables to be damaged and maintained messages can be sent over inaccessible terrain messages can be sent to any location in wireless range

The existence of the electron, and its function in the flow of electricity, was suggested by Stoney, an Irish physicist, in 1891. By 1897 Joseph Thompson, an English physicist, had confirmed this theory and further discovered that all atoms contained electrons. In 1913 Robert Millikan, an American physicist, was able to measure the exact charge on an electron. Electronics began when John Fleming, an English scientist, built the first vacuum tube or valve in 1904. These devices were tubes containing very

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little air, a near perfect vacuum, through which electrons could be made to flow in a controlled way. Fleming’s vacuum tube could detect radio waves, a type of electromagnetic wave. In 1907 Lee De Forest, an American inventor, produced a vacuum tube that would amplify, that is make stronger, radio waves or signals. Radio as we know it was made possible by these vacuum tubes. Vacuum tubes also led to the development of television and radar between the 1920s and 1930s and by the 1940s electronic computers had become a reality. The large size, unreliability, current demands and sensitivity of valves to heat and vibration led to the development of the transistor in 1947 by three American physicists Barden, Brattain and Shockley. Continuing development saw the integration of whole transistor circuits into single devices called integrated circuits by the early 1960s. In less than 20 years the transistor and associated semiconductor devices totally replaced valves in virtually all electronics industries. Continuing miniaturisation and other developments led to the microprocessor and personal computer. All electronic equipment continues to become smaller, more reliable, less costly and more useful as this development proceeds.

Triode (1907)

Transistor (1947)

Integrated circuit (1960s)

Figure 2.2 Developments in electronics (note the ‘miniaturisation’ with time) Source:

World Book Encyclopaedia, 1985, Vol.6, p.153

The telegraph Telegraphic or ‘distant writing’ (from Greek words) communication was the first form of messaging to use electricity. It was the dominant form of communication for over 100 years. At one stage the telephone was expected to replace the extensive telegraph infrastructure stretching around the world but this proved to be only partly correct. It was not until the development of the personal computer and the creation of the Internet that communication by telegraphy finally became outdated.

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Why was the telegraph unable to compete with the Internet? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Did you answer? The telegraph requires operators and specialist terminals or some other delivery system. This uses time and money resources. People are able to complete these tasks for themselves on an Internet–linked computer terminal. The Internet offers the computer owner a method of sending and receiving messages exactly when required and enables the immediate ‘posting’ and receiving of linked documents. All of this involves little additional cost over that of a computer that has usually been purchased for other tasks such as word processing.

Cooke and Wheatstone in England and Morse and Vail in the United States invented the telegraph simultaneously. Earlier work by Oersted and Volta and the invention of the electromagnet in 1825 by William Sturgeon (a British electrician) were combined to make the first messaging by telegraph a reality. The English telegraph used six wires to provide electrical energy to five needle pointers. Depending on the signal sent through the wires letters of the alphabet were pointed out by the needles. In this way a message could be received and assembled letter by letter. This system was used to improve efficiency and safety between railway stations in London around 1837. Likewise the telegraph invented by Morse and Vail in the United States used a punching system to show the transmitted message, in this case on a soft tape. The first public use of this system was between Washington and Baltimore in 1844. With use it quickly became apparent that the sound of the equipment being used to record the letter characters provided a much faster method for the operators to receive a message. By 1856 telegraph messages were received by sound recognition from redesigned sounding equipment. This replaced the registers first used to make a record of the message that could be seen. Just before WWII, an American submarine, the ‘Squalus’, transmitted its position using Morse code just before it commenced a practice emergency dive. Unfortunately, an air inductor valve did not fully close and the submarine partially flooded leaving 33 of the crew trapped on the bottom. In addition the assumed position of the submarine was out by more than 5 miles because one digit in the position was incorrectly transcribed during the Morse transmission. Fortunately a rescue vessel

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discovered the submarine’s true position by accident and the surviving crew were rescued. Use your library or the Internet to find the Morse code. Then write the morse code symbols for the word ‘MORSE’ in the space below.

Did you answer? ––

–––

.–.

...

.

The Morse telegraph using sound generated by closing metal contact points was simple and reliable, and became the preferred system. However, the code of dots and dashes originally developed by Morse for his telegraph proved unsuitable for languages other than English and so was replaced by International Morse Code in 1851. The dots and dashes that characterise telegraph codes soon proved impossible to transmit reliably over long distances through undersea cables. Without the technology to provide undersea repeater or amplifier stations the dots and dashes became distorted due to the electrical capacitance of very long cables. The solution to this problem was found in sending positive and negative impulses along the cable instead of dots and dashes. This development also provided the basis for later radio codes that would allow the transmission of more messages more quickly and simultaneously.

Figure 2.3 The main telegraphs receiving room at Darwin ©

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The first telegraph sending and receiving instruments could transmit one message at a time. In 1871 an American by the name of J. B. Stearnes developed a duplex transmission that allowed sending and receiving to occur simultaneously. This doubled the capacity of the telegraph lines.

Figure 2.4 Morse key and sounder, c1870 ©

Jensen. P. R, 2000, p4

The concept of time-division multiplexing, allowing even more transmissions to be operated along a single line, was first introduced by Émile Baudot in France in 1871. He was able to create a system for transmitting five messages simultaneously. Later technology would see variations of the code he developed which would be used well into the 20th century in binary technology. With the continuing development of the vacuum tube concept from 1904 came the next great improvement to the capacity of telegraph lines. In 1918 modulated carriers were used to pass messages along telegraph lines. By varying the frequency of these carriers, and having senders and receivers operating at selected carrier frequencies, it became possible to send, receive and separate many messages simultaneously. With each message being conveyed in a separate frequency band, the number of simultaneous messages was only restricted by the frequency bandwidth of each carrier and the limits of frequency transmission. In 1918 it was possible to separate, or multiplex, 24 separate signals simultaneously. Other improvements created by the vacuum tube included the electrical amplification of weak signals to allow more reliable message transmission over far greater distances. Improvements in magnetic materials increased transmission speeds and permitted duplex operation in very long submarine cables by 1928. Despite these improvements the first successful underwater vacuum tube repeater was not possible until 1950. Just 15 years later valve technology became obsolete. Turn to the exercise section and complete exercise 2.1.

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List as many advantages as you can for multiplexing information. __________________________________________________________ __________________________________________________________ __________________________________________________________ Did you answer? • • • • •

Less development of infrastructure required Reduced maintenance of infrastructure More efficient use of an existing resource Greater availability of telecommunication facility to more people Reduced user costs

Business requirements led to the introduction of teletype in 1924. Messages were sent and received immediately in printed form removing the need for a code operator. A special typewriter was used, greatly reducing the time needed to convert a message into and from its electrical form for the telegraph. At first, the five bit Baudot Code was adequate to operate this system but as transmission and hardware speeds increased with the introduction of computers, code limitations became the slowest part of messaging. In 1966 a seven bit transmission code was established and called the American Standard Code for Information Interchange, or simply ASCII code. Code speeds of 150 words per minute replaced the maximum 75 words per minute that had been possible using the Baudot Code. Telex, a switched teleprinter network, was introduced in 1932 and operated manually until after World War II. This system was devised to create the fastest possible messaging of news and commerce using electro-mechanical devices. The facsimile telegraph was perfected in the 1930s for transmitting graphic information such as photographs by analogue transmission. With the advent of digital transmission, which has now virtually replaced analogue transmission, the digitally operating fax machine evolved from facsimile telegraphs. Today the telegraph, which started in 1837, has been replaced by digital data transmission systems based on computer technology. New electronics technology including transistors, integrated circuits now containing thousands of components, and other micro electronic inventions, have revolutionized the transmission of information and led to the virtual universal adoption of digital communication. During this time there have been profound changes in society and commerce.

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Efficient supervision by fast long distance telegraphy has led to the replacement of many small separate businesses with large corporate organizations, ultimately resulting in the development of multi-national corporations. Public transport has been able to increase both in complexity and safety due to the ability to rapidly and accurately locate traveling trains and aeroplanes. At community level, people who have moved for work or who are traveling for pleasure, are easily able to keep in contact with family and friends, and so are more likely to move and travel. As these trends have developed, businesses have banded together to make the most efficient use of telecommunication systems. Associated Press was formed in 1848 by six New York newspapers and in 1849 Paul Julius Reuters established a telegraphic press service that used pigeons to cover areas not linked by telegraph line. The rapid distribution of news and information has changed the way people view and react to events in other parts of the world. Turn to the Exercise section and complete exercises 2.2 and 2.3.

The telephone Telephones allow people to talk with one another even though they may be a long way apart. The telephone developed with the telegraph. Alexander Graham Bell, a Scottish born American inventor, applied for a patent for his telephone on February 14, 1876. Only 2 hours later another inventor, Elisha Grey, applied for a patent for a telephone based on very similar principles. Bell’s patent, No 174-465, was issued on March 7, 1876 and was the subject of many unsuccessful legal challenges. It is probably one of the most valuable patents ever issued. In the above paragraph you have read about patents. Outline what a patent is and discuss why it is of such importance to inventors and innovators such as Bell. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Did you answer? A patent is designed to protect the intellectual property of inventors. A patent prevents other people/companies from manufacturing a product that you have invented or developed. They are important because they allow you to make some money from your inventions.

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Compared to the telegraph, the advantages of the telephone included its ability to allow quick response in conversation and to convey some feeling through the ‘sound’ of someone’s voice. Unlike the telegraph, the telephone did require people to be in a certain place at a certain time. This, of course, is no longer the case. Today telephones can be totally mobile and may even transmit written words, drawings, photographs, video images and large amounts of data through connection to computer terminals. The original Bell telephones were sold in pairs and were directly connected together much like an intercom. This was fairly limiting. Soon, numbers of telephones were being connected together between local residences and businesses. To simplify this network, telephone exchanges were built so that a user, linked to the exchange could be connected to any business or other user that was also linked to the exchange. People were employed at the exchange to physically select the desired lines. Melbourne had the first Australian manual exchange in August 1880 followed by Brisbane two months later and Sydney in 1881. It is interesting to note that the last manual exchange in NSW was at Wanaaring. It was closed in 1991. In 1891 a Kansas undertaker, AB Strowger patented an automatic dialing and exchange system. You might wonder why an undertaker might undertake such a task! The story goes that the wife or girlfriend of a rival undertaker was employed at the local telephone exchange. Remarkably, the rival undertaker suddenly started to receive many of the business calls that Strowger should have received. Strowger then set about designing a system that cut out the need for a manual exchange.

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Figure 2.5 A manual telephone exchange from the 1880s ©

Soden et al, 1996, p28

Being an undertaker, Strowger was a formal dresser: black suits, top-hats and stiffened collars. The box that the stiff collars were purchased and stored in was a rigid cylinder. Strowger was to use this as the basic element of his automatic telephone switching system. The Strowger system was the first example of ‘step by step’ switching. His prototype as described here could select 100 subscriber lines. To dial the number ‘48’, a user would push the first button on the dialer four times. This was the first ‘step’ and it moved the selector to the fourth row. The user then pushed the second button eight times. This was the second ‘step’. This then selected the eighth contact on that row and directly connected the user to the desired number. To increase the number of lines to one thousand the size of the cylinder was considerably increased to allow the rows to be arranged in 10 groups of 10. While many refinements were made to his system over the next 60 to 80 years, Strowger pioneered ‘step by step’ switching and the concept of pulse type dialing. His system was also the first automatic ‘circuit switching’ system, where phones are physically (electrically) connected together without the need for operators. In 1993,in Australia, there were still 188 000 lines connected to step by step type exchanges but they were soon to be replaced by fully digital exchanges.

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Primary selector

Secondary selectors

Subscriber lines

Caller’s phone

Called telephone rings

Figure 2.6 The principle of a Strowger switching system

For the next forty years automatic exchanges based on Strowger’s original principles competed with large manual exchanges. Manual exchanges remained popular for a number of reasons. They offered a more personalized service, their initial installation costs were lower and the operators were mostly women - the wages for women were about half that of men in those times. Women continued to be paid less for the same work until relatively recently. Find out when women were legally entitled to receive equal pay and to vote in Australia. Write your answer in the space below __________________________________________________________ __________________________________________________________ __________________________________________________________ Did you answer? 1894 Women allowed to vote in South Australia (First) 1902 NSW – women given the vote 1908 Victoria (Last) – very slow to give women the vote 1972 Equal pay for equal work was granted to women

Improvements in telephone networks have developed through the introduction of many new products and technologies. Some of the developments and the problems from which they arose, are outlined below.

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Problem

New product / technology

A loss of signal in non-insulated steel or iron wires

1 Method of producing hard-drawn copper wire (a much better conductor) which could support itself between poles was developed. (1877) 2 Insulated covering put over wires

Every person you want to call must be connected directly to your phone

Exchanges were established so that only one line was required to each phone (Melbourne in 1880)

There was a loss of electrical signal and interference (1870s and 1880s) due to the wire needed to complete the circuit being a ground return (connected through the earth)

A two wire system was introduced (both wires connected into the system) (1881)

Connections between phone lines had to be done manually

Automatic exchanges were developed to replace manual exchanges (1891)

Signals were very weak and only able to be carried short distances

Invention of the vacuum tube to amplify signals (1904)

‘Cross talk’ – interference between parrallel conductors along a network.

Wires were twisted at set distances along the network. (reduced the effect of electromagnetic induction)

A loss of signal over long distances due to ‘capacitance’ in long wires

Inductance coils were placed along the telephone wires. (1904)

A large network of cables and poles was required

1 Cables were placed underground and under the sea. 2 Optical fibres are replacing metal cables. (from 1970s) 3 Radio and satellite transmission was developed which doesn’t require cables

Only single conductor cables were laid over very long distances due to cost.

1 Carrier multiplexing ie multiple messages were able to be transmitted through single pair of copper tubes in a coaxial cable 2

Fibre-optic cables were developed requiring less amplification (because less energy lost) and were able to carry much more information.

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Turn to the exercise section and complete exercise 2.4.

The telephone in Australia In Australia, the first telephones appeared in Melbourne in 1878 and the Melbourne Telephone Exchange Company was formed in August 1880. In 1880 some Bell telephones were brought to Sydney and installed in the warehouses and wharves of Darling Harbour (S.M.H. 7th August 1880). In 1887, the Victorian Government had taken over the Melbourne Telephone Exchange Company and had 1462 subscribers by 1888.

Figure 2.7 A set of telephones displayed at the 1880 Sydney Exhibition © Soden et al 1996, p27

Trunk services between the capital cities took a little longer. The Sydney to Melbourne trunk line was opened in 1907 and the Melbourne to Adelaide trunk line was opened in 1914. The development of radio communication enhanced the ability to make telephone calls over great distances. In 1927 overseas beam radio was introduced and in 1930 an Australia to United Kingdom radio link was established. Radio, while covering great distances, had many technical problems that limited its reliability and bandwidth. The 1950s onwards was to see massive radical changes in the transmission systems and bandwidth available. In 1953, Perth became the first capital city to have a fully automatic telephone network. By 1957 nearly all telephones in capital cities were connected to automatic exchanges.

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Laying cables to carry telecommunications signals is both time consuming and costly. Microwave radio technology allows broadband, high quality communications. In 1958 the first microwave trunk link was established between Melbourne and Bendigo. This technology now services thousands of kilometers of Australia. Microwave radio relies on line of sight transmitting and receiving and therefore uses tall towers on the top of the highest available terrain. You will often see these tall towers with their round receiving dishes as you travel around both metropolitan and rural Australia. The COMPAC transpacific undersea cable was opened in 1962, bringing a new era of reliability and quality to international telephone communications compared to that offered by the older radio system. Satellites are the ultimate microwave tower, offering direct line of sight communications to any ground station below it. In 1966 the first satellite broadcast between Australia and the United Kingdom occurred. By 1968 the Australian communications network was linked to the first global satellite communications system. This was facilitated via an earth station near Moree in northwest NSW. In 1976 International Subscriber Dialing was introduced giving Australians access to 13 countries. Previously the call would have been manually connected.

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How a telephone works A telephone handset consists of: 1 a dialing mechanism 2 a transmitter 3 a receiver. So that people can contact a particular telephone on a network containing millions of handsets it is necessary for each subscriber to have a particular electrical address on the network. The dialing mechanism enables contact with a particular subscriber. The first dial telephones were introduced in 1896. Prior to this press buttons or an operator were used to establish telephone connections. The ringing signal to gain the attention of the operator or receiver was first generated by the sender turning a crank on the sending telephone. At first dial telephones used a coded sequence of electrical pulses created on a rotary dialing mechanism, or later a push button keypad, to establish electrical connection with a particular telephone. We commonly refer to this coded sequence as the ‘telephone number’.

Figure 2.8 A rotary dialing mechanism

Today all telephones use a keypad to create a coded sequence of tones that, likewise, establish electrical connection with a particular telephone. The transmitter is a microphone located within the handset. The carbon transmitter was one of the earliest telephone transmitter designs and it remained in common use until no more than thirty years ago. The carbon transmitter produces a strong electrical output, very necessary before electrical amplification could be easily produced. It is mechanically simple to make and is very reliable. When speaking into a carbon microphone sound waves cause a thin round aluminium diaphragm, to vibrate. The diaphragm acts on a chamber containing many small grains of carbon. Electrical contacts on either side of the carbon chamber allow a low voltage current to pass through the carbon. When the grains of carbon are compressed by the vibration of the aluminium diaphragm more current is able to pass through them. With less compression, less current flows. In this way the sound pressure waves from a voice are transformed into a varying electric current.

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carbon block back contact

front contact

to current source

button diaphragm

Figure 2.9 Edison’s carbon transmitter

Today, foil electret microphones are in use. They are much smaller than carbon transmitters, more sensitive and able to reproduce a much greater audio frequency range. Speech is clearer and the person more recognizable through a foil electret microphone. The foil electret microphone consists of a diaphragm and a backing plate. An electric field is established between the diaphragm and the backing plate. Vibrations in the diaphragm change the strength of this field during speech, and these changes in field strength are used to create corresponding changes in an electric current for transmission along the telephone line. permanent magnet electromagnet diaphragm

Figure 2.10

The receiver

The receiver consists of an iron diaphragm with a permanent magnet around it. On the other side of the diaphragm an electro magnet receives a varying electric signal from a distant telephone. This varying signal creates a varying magnetic pull on the diaphragm from the electromagnet, and this varying force in turn causes the diaphragm to vibrate in the same pattern as the electric signal in the electromagnet. In this way a sound pattern very similar to the pattern that produced the varying electrical signal in the transmitter, is recreated in the receiver. Speech is then transferred from the sender to the receiver.

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Modern developments in the telephone As with the telegraph, modern developments in the telephone system have relied on the creation of multiplexing systems to allow many connections simultaneously on the one line, and also the use of radio technology to assist long distance communication. Radio systems have greatly expanded the number of subscribers able to use a single network. Radio relay systems using microwaves to conduct signals along the line of sight between relay stations are now extensively used. Satellites can be used to carry many conversations simultaneously but, because of the great distances involved in sending a signal to and from a satellite, a noticeable delay is introduced between the speech of the sender and the receiver. For this reason satellites are used for only one direction of a two-way conversation, the other direction being transmitted by landline or line of sight microwave link. Over the horizon radio relay systems have also been developed, enabling the distance between relay stations to be extended from 50 km to 320 km. This development reduces the number of relay stations required along a line and enables the transmission of signals across all but the very largest bodies of water without the need to direct those signals to a satellite. The cordless telephone and the mobile telephone also use radio signals to remove the need for a physical connection, metal wire or glass fibre optic cable, for at least part of the telephone network.

Figure 2.11 An early ‘mobile’ telephone

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The first mobile telephones appeared in the 1930s, although these units were more like mobile radios than mobile telephones. They were large and heavy and could not be conveniently carried with the user. Improvements in miniaturisation, most notably the development of semi conductor devices and battery technology, allowed the transformation of these units into the mobile telephones now in use. These changes have greatly improved the convenience of the telephone. The privatisation of Telstra in Australia has caused considerable debate. List some concerns that illustrate the importance of the telephone to the general community. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Did you answer? •

removal of untimed local calls

•

userpays increases the cost for people in country areas

•

concern over maintenance standards

•

increase in charges generally, because of the need to produce a profit for shareholders.

Now turn to the exercise section and complete exercises 2.5 to 2.7.

Wireless In the early 1800s, work with electromagnets by Joseph Henry and Michael Faraday indicated that a current traveling along one wire could produce a current in another wire even though the wires were not connected. James Clerk Maxwell explained this ‘induction’ effect in 1864 as being the result of electromagnetic waves that traveled between the wires at the speed of light – some 300 000 km per second. In the 1880s Heinrich Hertz proved the existence of these electromagnetic waves, using a loop of wire to act as an antenna. Shortly afterwards, experiments by Edward Hughes showed that a steel point placed onto a carbon block would not conduct a current unless electromagnetic waves traveled through the point of contact at the same time. In a similar way, electromagnetic waves produced by a spark

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transmitter could be used to switch a distant current by creating an attraction between zinc and silver filings in a glass test tube. The current to be switched was supplied to the filings by a battery – or voltaic cell as batteries were then known. Sir Oliver Joseph Lodge used this discovery to produce a coherer, a device which could detect the presence of radio waves. An improved coherer was needed to make better use of radio waves for improved reception and in 1895 Gugliemo Marconi, an Italian electrical engineer and inventor, achieved this. In 1897 he transmitted radio signals 29km from land to a ship at sea. In 1899 he was able to send a radio signal between England and France and by 1901 he achieved the transmission of a single letter across the Atlantic Ocean between England and Newfoundland. There was some thought that wireless communication would only work along a direct line of sight. In fact, due to the earth’s curvature, it was believed by some scientists that wireless communication systems would not be able to provide true long distance communication. This, obviously, was proven to be incorrect. In fact, longer radio waves are reflected by the ionosphere and literally bounce back to earth, thus increasing their transmission distance around the earth. ELEVATED WIRE ANTENNA

RF CHOKE

METAL FILINGS

BATTERY 1

BATTERY 2

TAPPER

COHERER

PAPER TAPE PRINTER SENSITIVE RELAY (NORMALLY OPEN) RF CHOKE

EARTH

Figure 2.12 Marconi coherer receiver 1895

By 1905 radiotelegraphy between ship and shore was becoming more common. The sinking of the Titanic in 1912 and the subsequent rescue of hundreds of passengers was an early instance that would help establish the necessity of radio telecommunication to a society that was becoming more demanding in its expectations of modern technology.

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Despite these advances, the use of radio communication was limited to signaling by code, and once again, it was only with the invention in 1907 of an electronic valve that could amplify radio signals that radio made its first real advances towards the system that we know today. In 1906 one of the first broadcasts of human speech was achieved between land and nearby ships at sea from Massachusetts in the USA. As with the telephone and telegraph, ongoing development of radio telecommunications would be dependent on developments in electricity and electronics. Valves were quickly improved to act as better detectors, amplifiers and oscillators. Wireless, or radio telephony became useful for sending long distance spoken messages as early as 1915 when voice communication was established between Virginia in the USA and Hawaii, and then Virginia and Paris in Europe. In 1918 radio transmission became truly effective and its great potential began to be fully realized with the development of the super heterodyne circuit by the American inventor Edwin H Armstrong. This circuit greatly increased selectivity and sensitivity in the reception of radio waves. In 1933 the same inventor developed FM broadcasting in response to community pressure for improved sound quality in commercial broadcasting. Drive motor

High-v

oltage

bus

1 2 3 4

Spark gap High-v

oltage

bus

Common shaft

Figure 2.13

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DRIVE MOTOR

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DIRECT CURRENT GENERATOR SG ALTERNATING CURRENT SOURCE

SG

1

2

EARTH

3

4 Figure 2.14

Creating ‘continuous’ Radio Frequency electromagnetic waves from sparks in 1918

After World War I amateur radio operators did much to develop the use and understanding of radio transmission. Transatlantic voice radio contact was made in 1921 and valuable voluntary assistance was regularly supplied in emergencies. Australia’s first commercial radio station was 2SB which began AM broadcasts in Sydney in 1923. Just two years later on Australia Day in 1925 2UE also began AM broadcasts in Sydney. Towards the end of the 1920s the super heterodyne radio receiver saw the demise of the piano as the chief source of home entertainment, replaced in most living rooms by the radio. Radio quickly became a medium for conducting government propaganda during war times. It was used to spread various political messages as well as providing true news and entertainment. As the methods used to create the radio frequency electromagnetic waves improved, from irregular mechanical sparking systems to electrical alternator methods and then to electronic methods, so the transmitter became more efficient and more powerful. The widespread acceptance of television from the 1950s was expected to end the radio era for commercial broadcasting. The ability of the organisations using radio to adjust to changing community wants such as ‘news talk’ radio, other special interest group radio stations and the spread of car radios has seen radio transmission maintained as a dominant form of commercial broadcasting.

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Although it has always been used for civil, military and commercial communication, radio has only become a significant medium for data transfer with miniaturisation and the development of digital computer processing. Radio is now being connected to lap top computers to improve their usefulness. True intercontinental radio telecommunication became possible with the development of the aerospace industry. Amateur satellites, usually called ‘Oscar’, for Orbiting Satellites Carrying Amateur Radio, provided the first direct satellite communications between the United States and the Soviet Union in 1965 when the cold war made it impossible for governments to achieve this milestone. These satellites were carried into space during regular government rocket launches. Governments and private industry make extensive use of satellite technology to create the radio telecommunications that we now simply expect. The use of satellites and digital technology enables uninterrupted radio telecommunication at any time of day over any distance around the earth. Radio telecommunication has been used to transmit speech between the moon and earth and to control satellites in deep space. Radio waves, emitted by all stars, are being used to explore the universe.

Television Television is simply the transmission of pictures as well as speech and data by radio waves. Its importance has largely been confined to the broadcasting industry. With the development of national and international television broadcasting, which relies on satellite technology, news and sport now reach most homes in the developed world through television receivers. However, personal and business communication has essentially continued with the transmission of speech and data. Teletext machines now provide access to the transmission of data through television broadcast signals and pictures are being sent to aid teleconferencing. The importance of these developments will be determined by future market needs and the cost of providing them. As with general radio development, advances in television broadcasting relied firstly on valve technology and then the invention of semiconductor devices. As early as the late nineteenth century efforts had been made to transmit pictures over long distances, but it would take valve technology to combine these pictures into a moving scene. Unlike radio, television broadcasting methods could not be researched and developed until valve technology was discovered. Early designs used an electromechanical system to capture still pictures and process them as moving images for radio transmission. The British Broadcasting Corporation (BBC) broadcast television signals in 1926. These signals were generated from a mechanical device that captured still images at

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121/2 frames per second and ran them into a moving image. By 1936 this system had greatly improved clarity and picture stability but was unable to compete with an all electric system developed conjointly by the Marconi Company and Electric and Musical Industries (EMI). This all electric ‘high definition’ 405 line scanned display system, repeating at 50 frames per second was so advanced in 1936 that it remained the standard for the BBC until 1985 when it was replaced with a 625 line system using the phase alternate line (PAL) mode. Australia, where television broadcasting was not introduced until 1956, commenced services with a 625 line system. The development of the Marconi – EMI electronic system, in competition with the electromechanical system developed by John Logie Baird, marked a turning point in the invention and design of scientific electronic products. Since that time the level of technology and expense involved in research and development has resulted in the disappearance of the isolated inventor, replaced by corporately or government funded teams of engineers and scientists. Modern television is a product of the same engineering and scientific developments that have provided the basis for all other telecommunications improvements since the 1930s. The use of semiconductor technology and improved circuit design to process ever increasing volumes of data at faster and faster rates has not only led to improvements in telecommunications but also improvements in broadcasting. Turn to the Exercise section and complete exercise 2.8.

Digital communication Telecommunications are now controlled by computer, or microprocessor, and are often dependent on the computer as both an input and output device. The development of the transistor, which can be used as a high-speed binary switch, revolutionised the telecommunications industry. The microprocessor is a direct descendent of the transistor. With the ability to register two states, on or off, the transistor is the heart of modern digital electronics. Clearly, to design complicated control systems to convert information and speech into a binary code for transmission, and to solve intricate mathematical problems, thousands of micro-switches are required. The integrated circuit, developed as a direct result of transistor technology and now containing thousands of transistors, diodes, and passive electronic devices, provides the core working system, or central processing unit CPU, of modern computers and microprocessor devices.

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Other integrated circuits act as memory devices and enable the reliable and accurate time switching that creates modern multiplexing units. The ability to send many messages along one conductor with a useful bandwidth for each message, as is now the case, has come as the result of several developments; improvements to semi conductors in transistors and integrated circuits, the use of light as a high frequency carrier, and the development of optical fibre cable as the travel path for the messages sent as multiplexed modulated light signals. The change from analogue transmission to digital transmission is proceeding now and offers the ability to send and receive far greater amounts of information more quickly and with greater transmission quality.

Figure 2.15 Small section of an integrated circuit greatly magnified

The computer as we now know it is an electronic device with input and output systems and the ability to calculate and store work for further calculation. Mechanical methods have been devised to attempt these same tasks. Before electronics, inventors designed large mechanical machines that, had they not been too expensive to build, could have provided accurate mathematical solutions for the scientific world. As late as the early 1800s inaccuracies in mathematical tables were affecting long distance navigation, resulting in ships losing their way and sometimes being destroyed on rocks and other objects that could not be accurately located from the charts of the day. Charles Babbage, a British inventor of independent wealth, worked on the problem of mechanical solutions to numerical computation. Over a period of some 30 years from 1822, he designed three mechanical devices to permit calculation to very fine accuracy of very large numbers. Principally due to lack of funding these devices were not able to realise their potential in his lifetime. In 1987 a replica of his last machine was commissioned by the Science Museum in London. Construction from Babbage’s original drawings proved that his concept would work and could have been made with the machinery available in the mid nineteenth century. Of course, Babbage’s machines

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were very complicated and their intricacy required hundreds of hours of construction. Although these mechanical devices could never have achieved the multitude of telecommunication tasks now completed by modern electronic devices they were capable of completing their design task – highly accurate, rapid computation. They also provided the systems concept by which electronic computers would develop. The first true electronic computer was developed for the British Secret Service as a code-breaking device in 1943 during World War II. Valves and other post office equipment of the time were used to build it. However every time one of its many valves burnt out, it failed. This machine combined the concept of a processing system with electronics but its unreliability and large size demonstrated the need for something better. Research into semiconductor materials and the development of the transistor and integrated circuit led to the availability of electronic components which were the solution to this problem. Continuing research in semiconductor materials and circuit design has resulted in dramatic increases in the volume and speed of computer processing over the last forty years. Machines that took vast amounts of time and money to develop, controlled power stations and took men to the moon, are now oversized technological relics when compared to current home computer systems. With the microprocessor now in control of telecommunication a variety of communication systems have developed within the basic transmission of messages by cable and by electromagnetic waves – radio and television. Whilst television has not, to this stage, been important in the data transfer industry its significance to commercial broadcasting cannot be overstated. However, personal and business communication has essentially remained with radio transmission and cable. The telephone, and machines such as faxes that use the telephone system, may be linked by combinations of metal and fibre optic cable and may even rely on radio transmission along part of the connection between sender and receiver. Similarly mobile telephone communications, that clearly rely on radio transmission, may also be carried along fixed cables for part of their transmission path. The switching that enables the direction of communication signals to the most efficient transmission path, and collects many separate signals for simultaneous transmission, is an unseen part of modern telecommunication systems.

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Caller

Microwaves

Repeater station Local exchange

London Telecom Tower

International exchange Earth station

Figure 2.16 Transmitting a long distance telephone call

The World Wide Web Through computers people are now able to operate their own telegraph by using the World Wide Web (WWW) and the Internet to search for news and information and to conduct conversation or written communication with any person or business anywhere around the world, all this quickly and for little cost. Having evolved from cable-linked telegraph to radio to television, we now find ourselves passing through the same cycle of developments on our computers, as the Internet grows before our eyes. In 1973, investigations began in the United States to design systems and transmission protocols that would allow separately networked computers to link with each other and thus transmit and share their information with each other. This was initially inspired by potential military advantages. By 1986 the major framework of the Internet had been developed by the National Science Foundation in the United States with the creation of the NSFNET. Today commercial network providers around the world are offering additional network facilities and access support, making the Internet more widely available at costs that are extremely low for the level of service and information available to the end user. As the Internet has developed it has become more able to accept different local network protocols and so a truly World Wide Web is developing. Open Systems Interconnection (OSI) protocols have greatly increased the availability of the Internet to providers and users since 1989. In the 1980s about 100 protocols were in use. By 1991 5000 networks in about 40 countries were linking about 700 000 computers used by an estimated 4 000 000 people.

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Although much of the support for early Internet development came from the United States Federal Government the development of OSI protocols opened the Internet to commercial development. Today the bulk of the system is made up of private networking facilities in business, educational and research institutions as well as government organisations around the world. The Coordinating Committee for Intercontinental Networks (CCIRN) attempts to maintain international cooperation in the Internet environment. The Internet Activities Board (IAB) was founded in 1983 and continues to provide various services that manage the day to day functioning of the Internet. Technical information for prospective and existing providers, records of user names and system identifiers, and ongoing research and development are coordinated through the IAB. Turn to the Exercise section and complete exercise 2.9.

Societal influences The combined effects of advances in transport and telecommunication have greatly reduced the time taken to travel between and communicate with places around the world that are great distances apart. This has effectively caused the world to ‘shrink’, encouraging people to travel for pleasure and to relocate for work. Families are now spread around the world instead of being concentrated in local villages and the concept of the ‘global village’ has resulted. The priorities that people hold for the use of their time continue to change and this has resulted in a shift from the family to the state as the provider of services for the aged, the sick and the young. The young now constantly question family values while the old often cannot understand the rate and direction of technological change that is taking place. In recent times education about technology as well as in technology has been provided in an attempt to help the wider community manage the sociological changes that have occurred with technological development. Some people have questioned the quality and intention of this information. As well, the ability of the computer and communication networks to receive, process, store and distribute vast amounts of information has led to privacy concerns for the telecommunications industry in general, and the use of computers in particular. Governments and people are currently facing these problems and attempting to establish an appropriate division between the ‘public good’ and the democratic ideals of freedom and privacy. Future developments in telecommunications will no doubt see ever increasing standards in the quality, volume and speed of communication systems with ever increasing vigilance on the type of information being collected, processed and provided for communication. With the ‘Web cam’ linking sight to other communication forms and increasing

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interactivity it seems just a matter of time before other senses, such as smell, become a part of the transfer of information by telecommunication. Turn to the Exercise section and complete exercises 2.10 and 2.11.

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Exercises

Exercise 2.1 Identify and describe four advances in telecommunications that followed the invention of the valve. 1

______________________________________________________ ______________________________________________________

2

______________________________________________________ ______________________________________________________

3

______________________________________________________ ______________________________________________________

4

______________________________________________________ ______________________________________________________

Exercise 2.2 Identify and describe four major improvements made to the telegraph system during its 100 years of international communications. 1

______________________________________________________ ______________________________________________________

2

______________________________________________________ ______________________________________________________

3

______________________________________________________ ______________________________________________________

4

______________________________________________________ ______________________________________________________

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Exercise 2.3 Explain how improvements in telecommunications have allowed small local business to market themselves nationally and internationally. __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ Exercise 2.4 Discuss how the development of hard drawn copper wire assisted the early development of the telephone and telegraph. __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ Exercise 2.5 Identify and discuss three effects or influences that the invention of the telephone has had on society and the community. 1

_______________________________________________________ _______________________________________________________

2

_______________________________________________________ _______________________________________________________

3

_______________________________________________________ _______________________________________________________

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Exercise 2.6 Discuss three possible reasons for the popularity of the mobile telephone. 1

______________________________________________________ ______________________________________________________

2

______________________________________________________ ______________________________________________________

3

______________________________________________________ ______________________________________________________

Exercise 2.7 Identify four different transmission media that your telephone message could take to get to someone on the other side of the world. 1

______________________________________________________ ______________________________________________________

2

______________________________________________________ ______________________________________________________

3

______________________________________________________ ______________________________________________________

4

______________________________________________________

Exercise 2.8 Identify and discuss two reasons why television will remain primarily a broadcast medium to the end of the twentieth century. 1

______________________________________________________ ______________________________________________________ ______________________________________________________ ______________________________________________________

2

______________________________________________________ ______________________________________________________ ______________________________________________________ ______________________________________________________

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Exercise 2.9 Describe three advances in telecommunications that followed the development of semiconductor materials. i

_______________________________________________________ _______________________________________________________

ii

_______________________________________________________ _______________________________________________________

iii

_______________________________________________________ _______________________________________________________

Exercise 2.10 Predict the possible outcomes for the inclusion of video as part of computer communication __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ Exercise 2.11 Select the alternative a, b, c, or d that best completes the statement. Circle the letter. 1

2

40

Semaphore is a communication system that: a

requires line of sight

b

was used on horseback

c

resulted in accurate message transfer

d

used electricity for the first time.

Ohm defined the relationship between: a

ohms, amps and resistance

b

amps, ohms, and current

c

voltage, ohms and amps

d

none of the above.

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3

4

5

6

7

Some advantage of wireless communication are: a

no cables need be installed

b

cable maintenance is eliminated

c

damage to cables does not occur

d

all of the above.

Which of the following is not an advantage of multiplexing? a

reduce the user cost

b

more efficient in the use of resources

c

uses more flexible cables

d

increase peoples access to telecommunication.

The electric telegraph which was the basis of early Australian telecommunications used the following system to send and receive messages a

Baudon code

b

Morse code

c

ASCII code

d

Common control.

The development of which device revolutionised the telecommunication industry a

telephone

b

resistor

c

valve

d

transistor.

A protocol is a

a first design

b

a transmitter

c

a rule

d

none of the above.

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Progress check

In this part, you have traced the developments that have led to the current telecommunication technologies and how they have affected our society.

✓ ❏

Disagree – revise your work

✓ ❏

Uncertain – contact your teacher

Uncertain

Agree – well done Disagree

✓ ❏

Agree

Take a few moments to reflect on your learning then tick the box which best represents your level of achievement.

I have learnt about: •

historical developments in telecommunications

•

the effects of innovations in telecommunication on peoples lives and living standards

•

environmental implications of telecommunication systems.

I have learnt to: •

research the history of telecommunication in Australia and understand the way it has impacted on peoples lives

•

examine safety issues related to the use of telecommunication systems.

Extract from Stage 6 Engineering Studies, © Board of Studies, NSW, 1999. Refer to for original and current documents.

In the next part you will examine materials used in telecommunication engineering.

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Exercise cover sheet

Exercises 2.1 to 2.11

Name: _____________________________

Check! Have you have completed the following exercises? ❐ Exercise 2.1 ❐ Exercise 2.2 ❐ Exercise 2.3 ❐ Exercise 2.4 ❐ Exercise 2.5 ❐ Exercise 2.6 ❐ Exercise 2.7 ❐ Exercise 2.8 ❐ Exercise 2.9 ❐ Exercise 2.10 ❐ Exercise 2.11 Locate and complete any outstanding exercises then attach your responses to this sheet. If you study Stage 6 Engineering Studies through a Distance Education Centre/School (DEC) you will need to return the exercise sheet and your responses as you complete each part of the module. If you study Stage 6 Engineering Studies through the OTEN Open Learning Program (OLP) refer to the Learner’s Guide to determine which exercises you need to return to your teacher along with the Mark Record Slip.

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Telecommunications engineering

Part 3: Telecommunications engineering – materials

Part 3 contents

Introduction ..........................................................................................2 What will you learn?................................................................... 2

Specialised testing..............................................................................3 Copper and other metals used in telecommunications.................. 8 Ceramics as insulation materials............................................... 12 Semiconductors....................................................................... 13 Polymers as insulation materials............................................... 15 Fibre-optics ............................................................................. 18

Exercises............................................................................................25 Progress check .................................................................................35 Exercise cover sheet........................................................................37

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Introduction

Engineers are interested in the development, properties and availability of materials and how this can affect design in telecommunication. In this, the second engineering focus module, you will be studying specific materials, examining structure/property relationships and investigating testing procedures as they relate to telecommunication engineering.

What you will learn? You will learn about: •

specialised testing relating to electrical properties

•

copper and its alloys used in telecommunications

•

ceramics as insulating materials

•

the types of semiconductors and their uses in telecommunication

•

polymers as insulating materials

•

the types and properties of fibre-optics.

You will learn to: •

analyse structure, properties, uses and appropriateness of materials in telecommunications engineering applications

•

select and justify materials and processes used in telecommunications engineering.

Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to for original and current documents.

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Specialised testing

Voltage testing The difference in electrical potential energy between two points is known as the voltage. The voltage between the ends of a conductor governs the size of the current flowing through the conductor. This is also known as the ‘electromotive force’ that drives electricity in a circuit. Voltage (V) is measured by a voltmeter and this meter is placed parallel to the component that it is measuring as shown in figure 3.1. The voltmeter must have a high resistance compared to the component being measured otherwise the total resistance of the circuit is reduced and this changes the value of the voltage being measured. The voltage in both Direct Current circuits and in Alternating Current circuits can be measured but different meters must be used. Resistor

Battery

Resistor

V

Voltmeter

Figure 3.1 Measuring the potential difference across a resistor (note that the voltmeter is in parallel with the resistor that it is measuring)

A cathode ray oscilloscope (CRO) is one of the most important measuring instruments used in electronics. Figure 3.2 shows the basic parts of the CRO. When a DC or AC voltage is applied to the Y-plate input, the voltage causes the electron beam to bend. This produces a line on the screen.

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Evacuated glass tube Y-plates X-plates

Electron gun Electrons in a beam Fluorescent screen Spot of light

Figure 3.2 The main parts of a CRO

Have you ever heard of a multimeter? How do think it is different to a simple voltmeter? __________________________________________________________ __________________________________________________________ Did you answer? Did you mention that a multimeter can be used to measure voltage as well as current and resistance? (‘multi’ means many, hence a meter that measures many things).

Current testing Current is basically the quantity of electrons moving from one point to another. Current is measured in amperes (A) and is carried by the valence electrons in conductors with the electrons flowing from negative to positive. 1k

Approximately 10 V

Ammeter reading A 0.01 A or 10 mA

Figure 3.3 Measuring the current flow in a circuit

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Current is measured by a simple ammeter and it is connected in series in the circuit. The current flowing through a simple torch globe is about 0.5A. About 10A flows through a fast-boil kettle element. In most low voltage circuits, like in telecommunication devices, the current flow in different parts of the circuits is very low and is normally measured in milliamperes (mA) or microamperes (mA). The amount of current that flows will vary with changes in voltage and resistance. This is represented by the formula that you learned during the preliminary course: I = V/R or V = IR As with voltage, current can also be measured by a multimeter. These are normally only able to measure direct current (current that flows continually in the one direction) and typically can only measure up to 500mA.

Insulation testing Suggest reasons why insulation and insulators may be important when dealing with electricity and electronics. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Did you answer? Insulation is important for: •

protection from electrocution

•

protection from damage to wires or cables

•

reduction of energy leakage

•

reduction of interference from external sources.

If you live in an area that has overhead power lines, you can see the glazed ceramic insulators that hold the cables at each pole. The pole is probably made from timber. Another insulating material! If you also look at the cables that run to the front of your house, you should notice that they are covered in a plastic polymer coating. Yet another insulator! All of this has been done to make the current go where it is needed and to protect us from electrocution. Insulators are also vital in low voltage circuits. The epoxy printed circuit boards and the outer bodies of various electronic components are all examples of the use of insulators.

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Megger testing Specialised non-destructive tests have been developed to assess the condition of electrical insulation. The megger test will measure the: •

relative amount of moisture in the insulation,

•

leakage current over the dirty or moist surface of the insulator

•

winding break-downs or faults as a measure of resistance vs time.

The megger test uses a dc voltage of 500 to 1000 volts applied to the insulator and current will flow in two ways. Small amounts of current will be conducted within the structure of the insulator and current may also flow along leakage paths on the surface. When the voltage is applied to the insulation, readings are taken of the insulation resistance and graphed against time. Data should be recorded at the 1 and 10 minute intervals and at other intermediate times. Only a person experienced with conducting megger tests can compare the test results with expected norms because factors like temperature, moisture and previous charge will all have a significant effect on the results. This type of test is normally used for large electrical equipment, like generators and transformers or high voltage cables.

Resistance testing The insulating qualities of a material can also be simply measured by the amount of resistance that is offered to the flow of current. A multimeter is most commonly used to test this resistance. The multimeter has an internal power source, normally a battery, and when the resistance setting is selected on the meter, the current is ready to flow through any object that is introduced to complete the electrical circuit. Simple resistance, on the multimeter, is used to measure the polarity of components like diodes and transistors, to check for faults in items like fuses and to identify the resistance in parts of complex circuits. For this experiment you will need either a multimeter, if you have access to one, or a continuity tester. We can make a simple continuity tester with the following items:

6

•

small dry cell battery

•

two elastic bands

•

three pieces of wire or three paper clips

•

torch globe

•

test items – coffee mug, telephone body, metal knife, etc.

Telecommunications

Fit the elastic bands to the battery and the globe in such a way that they will hold the ends of the bared wire or the straightened out paper clip onto the terminals of the battery and globe. Attach a wire from one end of the battery to the globe and another from the globe to act as a probe. Attach the third wire to the other end of the battery. This is the other probe. When the probes are connected the globe should glow brightly. Elastic bands

Probes

Lamp 1.5 V

Battery 1.5 V

Figure 3.4 Simple continuity tester

Use this continuity tester to complete the table below. Select a few other items from around the house and record whether the globe glows bright (B), dull (D) or not at all (N) for these items too. Item

Globe

Item

Globe

Coffee mug

B D N

Telephone body

B D N

Metal knife

B D N

B D N

B D N

B D N

Those you circled ‘B’ are good conductors. Conductors have very low resistance. If you circled ‘D’ the item offered some resistance. Insulators didn’t let the globe glow at all; they have a high resistance. Turn to the exercise sheet and complete exercise 3.1.

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Copper and other metals used in telecommunications List the mechanical and physical properties that would be important in telecommunications conductors. __________________________________________________________ __________________________________________________________ Did you list the following? Low resistivity, suitable strength, ductility and ease of joining.

The lower the resistivity of a material, the smaller the amount of material that is needed to carry current. It also means there are less insulating materials needed because the wires are thinner, sheathing costs are lower and transport costs are lower. To allow the material to be made into wire, it must exhibit ductility. The material must be able to withstand the tensile stresses applied during manufacture, extrusion of the insulation and the installation of the cable. Joining the conductors may be achieved through twisting, soldering or welding. Some materials are easier to join than others! In modules that you studied during the preliminary HSC course, you looked at the structure and atomic bonding of materials. Using this knowledge, explain why metals are normally conductors and why copper is an excellent conductor of electricity. __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ Did you answer? Did you discuss the metallic bond that has the valence electrons in a cloud surrounding the ions and that conduction is due to the migration of these electrons? Did you mention how these ‘free’ electrons easily transmit the ‘flow’ of current?

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Metal ion (positively charged) Electron (negatively charged)

Figure 3.5 Simple representation of the metallic bond

Of course this theory of current ‘flow’ is a bit too simplistic and further development in wave theories has allowed a much clearer understanding of conductivity. While the individual valence electrons are involved in the movement of a current, the current moves in the form of a wave and these waves will move much more easily through a regular arrangement of obstacles. The regular arrangement of ions in the crystal lattice structure of an annealed metal, such as the face centred cubic arrangement of copper, provides little resistance to the passage of the current waves. Any amount of cold working or the introduction of alloying elements that sit in the spaces between the ions will increase the random nature of the obstacles and will increase the resistance of the material. Heating will cause the ions to vibrate and will increase the possibility of the migrating electrons hitting an ion and thus being slowed down. This explains the increase in resistivity noticed when the temperature of a conductor is raised.

Copper Copper is the metal that has been traditionally used for communications wires and cables. It is ductile, has suitable tensile strength and is a very satisfactory conductor. As a conductor it is second only to silver and if the conductivity of silver is 100 units then pure copper would measure 97 units. Electrolytic tough pitch copper is used for wires and this grade of copper has a minimum copper content of 99.9 per cent with around 0.04 per cent of oxygen in the form of an oxide. This level of purity is essential as the introduction of some alloying elements or impurities can greatly reduce conductivity. For example only 0.04 per cent phosphorus will reduce the conductivity by 25 per cent. Other alloying elements, like cadmium, have little effect on the conductivity. The presence of cadmium, dissolved in the copper, increases both the strength and wear resistance of the transmission cable, so it is actually a favourable alloy in this application. The manufacturing process used to produce copper wires could easily induce stress and reduce the conductivity. To overcome this problem, the cables are cold drawn into wire then the roll of wire is fully annealed.

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Copper is also an essential part of coaxial cables that are still used for some applications in telecommunications. Copper braid Solid copper

Polymer layer Polymer skin Figure 3.6 The structure of coaxial cable

In previous modules you looked at some of the alloys of copper. Some of these alloys have properties that make them suitable for use in telecommunications devices. Name some of these alloys, state the alloying element/s and suggest at least one use for each. __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ Did you answer? Did you suggest sheet cartridge brass (copper with 30 per cent zinc) that could be used as contacts and cartridge brass cold formed screws and rivets. Even bronzes (copper with up to 11 per cent tin) could be used where extra strength is needed. Non-corroding nuts and bolts could be made in bronze.

Aluminium Aluminium has three advantages over copper when used as conducting wires. It is lighter, less expensive and more abundant in nature than copper. With a density of only 2.7g/cm3, compared to 9g/cm3 for copper, aluminium is specially suitable for aerial power transmission cables. Only half the quantity of aluminum, by weight, is needed for conductors with the same resistance. However, it does not conduct as well as copper (only about 60 per cent of the conductivity of copper) so larger diameter cables are needed. The larger amount of insulation sheathing needed offsets some of the savings made on the conductor material.

10

Telecommunications

On the other hand, aluminium has some inferior properties to those of copper. These include marginally poorer ductility, tensile strength, jointing properties and corrosion resistance. This fact has retarded aluminium’s general use in communication cables. Aluminium alloys are sometimes used for cables. A common alloy contains 0.5 per cent iron and 0.5 per cent cobalt. These alloying elements distort the normal aluminium structure and while this increases the strength of the cable, the conductivity is reduced.

Gold The conductivity of gold is around that of copper and it is used for the linkage ‘wires’ in some semiconductor devices. It is suitable for this application because while it is very expensive, only small quantities are used in these miniature circuits. The gold is ductile, doesn’t oxidise and bonds easily to other metals such as aluminium and copper.

Lead The outer layer on telecommunications cables is known as the sheath and is designed to create a stable environment for the cable core. Lead was once used extensively as it has good corrosion resistance, adequate strength and flexibility and is easy to join. It has been replaced with polymers because lead suffers from fatigue failures, is heavy and is relatively expensive. Lead alloys containing antimony and tin were used to reduce fatigue failures. Turn to the exercise sheet and complete exercise 3.2.

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Ceramics as insulation materials From the information available in previous modules, define a ceramic and explain why ceramics are often used as electrical insulators. __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ Did you answer? Did you mention that ceramics contain both metal and non-metal phases? Did you also discuss that they often contain both ionic and covalent bonds and that both these types of primary bonds do not have free valence electrons to allow for the ’flow’ of electrons?

In insulating materials, there is a large gap between the full valence band and the next electron energy level. For an electron to be free to transmit a current, it must move up to this next energy level. Under normal conditions, the gap is so large that electrons are unable to cross. At high temperatures there is a greater chance that an occasional electron will possess the energy needed to cross the gap and allow some conduction. In ionically bonded materials, ions may migrate, rather than electrons. This will provide a small degree of conductivity. At elevated temperatures, ions can become more mobile and conductivity may increase. Very high voltages may cause the break-down of some insulators. This occurs because the electric field is sufficient to raise the energy of some electrons and ‘free’ them across the gap allowing electron flow. Surface breakdown is more common and the presence of moisture or accumulation of dirt may allow conduction. The glazing of ceramic insulators helps eliminate moisture because water runs off easily. It also is less susceptible to dirt build up because it is smooth. The use of a corrugated design greatly increases the length that the current must travel.

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Semiconductors Some materials are known as semiconductors because the gap between the filled valence band and the empty conduction band is relatively small. Conduction can occur through two mechanisms. Heating for intrinsic semiconductors, and doping in extrinsic semiconductors.

Intrinsic semiconductors Silicon and germanium are semiconductors due solely to the distribution of electron energies within the pure material. When one valence electron is freed to cross the energy gap it will mean that one atom within the crystal lattice only has three bonds as shown in figure 3.7. This gap is known as an electron hole. The freed bonding electrons are constantly moving and can even switch from one atom to another. This movement of the electron in one direction means that the hole ‘moves’ in the opposite direction. This could be considered as a positively charged carrier. Both these movements allow the material to conduct. Heat may be used to provide the initial energy to free the electron. So, in contrast to metals, increasing the temperature of an intrinsic semiconductor will increase conductivity.

B A

An electron is freed from a covalent bond creating an electron hole at A.

B

C

A

Electron transfer to A from Electron transfer from C adjacent site,B. Hole to B. Hole moves to C. effectively moves to B.

Figure 3.7 Electrical conduction by the movement of ‘holes’

Extrinsic semiconductors Silicon and germanium have four outer shell electrons per atom but if an ‘impurity’ element, that only has three outer electrons is introduced, there will be electron holes left in the lattice structure. Conduction due to these holes can occur, and the majority carriers in this type of semiconductor, are these positive electron holes. Aluminium in silicon is an example of this type that is commonly known as a p-type semiconductor (p- for

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positive). These ‘dope’ atoms are introduced in the ratio of around one atom to a million base material atoms. Alternatively, if an element like phosphorus, that has five electrons in its outer shell, is added to the silicon structure there will be an extra electron for each phosphorus atom added. Only four of the electrons are bonded to both the phosphorus and the silicon so the fifth valence electron can easily move in the conduction energy band and allow conduction to take place. The electrons are the majority carriers in this type of semiconductor, so it is known as an n-type semiconductor. Turn to the exercise sheet and complete exercise 3.3.

The p-n junction When a piece of n-type semiconductor is joined to a piece of p-type semiconductor a type of ‘one way’ valve results. The normal method is to introduce p-type and n-type impurities into opposite ends of a crystal of silicon or germanium. At the junction of the two types of materials, the positive holes in the p-type are filled with electrons from the n-type. In this region the p-type atoms have gained an electron and are negatively charged and the n-type atoms have lost an electron and become a positive ion. This ‘depleted’ zone has a positive charge on one side and a negative charge on the other. When a voltage is applied across the component containing the p-n junction, it will either conduct or insulate. If the p-type end is made positive compared to the n-type end then the current will flow easily. If the voltage is reversed, the positive holes and electrons are attracted away from the depleted layer and it becomes very hard for charged particles to move across the junction. Depletion layer

Forward bias

Reverse bias

Figure 3.8 A p-n junction exposed to a voltage

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Telecommunications

This simple type of semiconductor device is known as a diode. When three layers of semiconductor material are combined, npn or pnp, a transistor is formed. Now you will have an idea of how they work. These semiconductor devices form the basis of the integrated circuits that ‘drive’ the modern telecommunications industry. These devices are made from wafer thin layers of pure silicon into which the many individual microelectronic circuits are formed. This ‘chip’ is then packaged so that it can be fitted into a printed circuit board and used in different electronic applications.

Polymers as insulation materials Drawing on knowledge and understanding that you gained in previous modules, briefly explain why polymers are insulators. You should refer to the type of primary bonds found in polymers. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Did you answer? Did you talk about the covalent bonds normally found in polymers and the fact that all the valence electrons are involved in the bond and are therefore not free to ‘transmit’ electrical ‘flow’? Nucleus

Electron

Cl + Cl = Cl2 Figure 3.9 Simple representation of the covalent bond

Many of the insulating materials in personal telecommunication devices are made from polymers. They are subject to low voltages and low temperatures and are therefore quite suitable for these applications.

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If you can find an old broken telephone, pull it apart! If you don’t have an old phone, look at the one in your home and answer the following activity. Suggest those parts of the telephone that are made from polymer. Indicate with an ‘I’ those parts that must be insulators. __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ Did you answer? The body, buttons/dial, receiver are all moulded in polymer. They could possibly be high impact polystyrene which is a copolymer of polystyrene and the rubbery polymer, polybutadiene. It doesn’t break when you drop it on the floor! Other tough polymers that would be used for telephone bodies are ABS (acrylonitrile butadiene styrene) and polycarbonate. The printed circuit boards (epoxy resin), wire insulation (polyethylene), integrated circuit bodies (polyurethane) and transistor bodies are all polymer so that they insulate.

In telecommunication cables, an insulating layer covers the surface of the conductor material. Traditionally, paper was used to insulate telecommunication cables and while it has high insulation resistance, if it gets wet, immediate and complete failure usually results. Paper contains a high proportion of the polymer, cellulose. Various polymers are currently used in place of the traditional paper.

Polyethylene Polyethylene has superior insulation resistance to paper, is suitable for high frequency cables, can be accurately made to size in a variety of colours, has good jointing properties and maintains good electrical properties under humid conditions. Its main disadvantages are cost and low softening temperature. When used as an outer sheathing on groups of cables, polyethylene allows water vapour to penetrate and is difficult to join. For these reasons it is only used for interior cables or as the outer layer on sheaths with a wound aluminium foil inner and polyethylene outer.

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Telecommunications

Polyvinyl chloride Polyvinyl chloride (PVC) has poorer electrical properties than either paper or polyethylene but is tougher, withstands higher temperatures and survives better in a fire. Under extreme temperatures and combustion, hydrogen chloride fumes are liberated and may cause corrosion problems. It is a suitable alternative to polyethyelene.

Polypropylene Polypropylene has similar electrical properties to polyethylene but is tougher and has a higher softening temperature. It is not as flexible and is more expensive than either PVC or polyethylene.

Nylon Nylon is often used as an insect resistant outer layer or sheath on cables that are used underground. The hard, smooth surface of the nylon makes it difficult for an insect or termite to grip the cable. Turn to the exercise sheet and complete exercise 3.4.

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17

Fibre-optics Light has been used throughout history to convey messages over long distances. Identify historical long-distance communication methods that have used light. __________________________________________________________ Did you answer? Did you suggest bonfires and mirrors (using the sun)? What about smoke signals?

History Up until the 1840s, both bonfires and mirrors were used to relay messages from one hilltop to the next. The electric telegraph quickly replaced these simple ‘light’ methods as the wires carried the message regardless of the weather or the terrain. Light travels very fast, around 300 000 kilometres per second, and it has long been known that the shorter the wavelength, the more information a wave could carry. Light waves are only millimetres to nanometres long and can carry a huge amount of information. Early experiments saw lasers being fired between towers but fog or rain blocked the message and it quickly became obvious that the light beam should be guided through a cable or pipe. Optical fibres were chosen for this purpose. Typical optical fibres are very fine fibres of glass – ‘hairs’ made of pure silica. The method of manufacturing optical fibres had been patented back in the 1930s ‘just in case someone ever finds a use for it.’ Initially it was difficult to keep the transmitted light inside the glass fibre but eventually the glass core was enclosed in a glass sleeve or cladding. The cladding has a different refractive index to the core and causes the light energy to be reflected back off the core-cladding interface. This total internal reflection means that all the light is reflected and continues to zig-zag along the core of the fibre. The optical fibres guide the light beam so wherever the fibre goes, the light follows. These fibres can be made to make the light bend around corners. Materials used for optical fibres must:

18

•

be able to be formed into long thin structures

•

be flexible enough to go around bends

Telecommunications

•

allow light to travel through them and so need to be transparent.

Only silica glass and some polymers have these properties. Buffer coating Core

Cladding Figure 3.10 The structure of fibre-optic cable

The light source The ‘light’ used in fibre-optic systems is either at or just beyond the red end of the visible light spectrum. This length of wave is less susceptible to attenuation in the glass. The light is generated by a little semiconductor laser (Light Amplification by the Stimulated Emission of Radiation) made from gallium, aluminium and arsenic. This device produces a stream of electromagnetic radiation, light, at a constant frequency. The pulses generated in the laser by the transmitter are sent down the glass fibre and converted back to electrical impulses by the receiver.

Movement of light in the fibre As light beams move down the core of the glass fibre they bounce from side to side. As long as they only hit the junction between the core and the cladding at a low angle the total energy of the light rays is reflected back into the core and none escapes into the cladding. The rays bounce to the other side and again, as long as the angle is low, bounce back and continue to be transmitted to the end of the fibre.

Part 3: Telecommunications engineering – materials

19

Total internal reflection Cladding Core

Higher refractive index (n1)

Lower refractive index (n2)

Figure 3.11 Movement of light in a fibre

Turn to the exercise sheet and complete exercise 3.5.

Attenuation Any decrease in the intensity of the light travelling in a fibre is known as attenuation. Attenuation occurs in glass fibres for three main reasons. •

atomic absorption of the light by the glass

•

the scattering of light by flaws and impurities

•

reflection of light by splices and connectors.

To overcome this attenuation, the signal is boosted at regular intervals. One of the advantages of glass fibres over copper conductors is that the signal in glass travels a lot further without needing boosting. In data networks, for example, this can be up to 2km without the use of repeaters.

Advantages of optical fibres One of the other advantages of using glass fibres is their light weight which means easier installation. For example, a copper coaxial cable can be replaced by a fibre conductor that is around one-ninth the mass. Optical fibres also have very wide band widths and this gives large transmission capacity. Unlike copper, glass fibres aren’t affected by electromagnetic interference and because glass doesn’t conduct there aren’t problems with earth loops. Glass is also suitable in dangerous environments as it doesn’t spark like metals. It is also more secure than coaxial cable as it can’t be spiked to tap off the data signals and certainly the data being carried is more secure than information transmitted in the atmosphere. Glass is also inert in corrosive environments, like sea water, and the raw materials, silica and polymers, are relatively inexpensive.

20

Telecommunications

Making glass fibres In Australia, in the early 1970s, the CSIRO experimented with glass fibres filled with a liquid that had a greater refractive index than that of the glass. While this worked, these proved hard to make and to handle, mainly because the liquid leaked out. Today, optical fibres are generally made by the process of modified chemical vapour deposition. A pure silicon tube, with a refractive index of 1.46, is filled with a special gas while being heated by an external heat source. The gas deposits an inner layer of SiO2 doped with 10 per cent germanium oxide (GeO2). This lining has a refractive index of 1.47. The composite tube is then heated to 2 400∞C and collapsed to achieve a solid cored fibre. Turn to the exercise sheet and complete exercise 3.6.

Heat source approximately 1600∞C

Fibre material formed by chemical reaction

Doped gas

Silica tube cladding

Deposited core material (solid)

Collapsed preform

Heat source approximately 2000∞C Figure 3.12

Core

Production of optical fibre by MCVD (Modified Chemical Vapour Deposition)

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21

Protecting glass fibres The simplest method of protecting fibres is to pass them through a bath of molten polymer to form a protective outer skin. To further isolate the fibre from external forces, a number of methods can be used. In a loose buffer cable, a loose polymer sleeve is fitted and the gap between the fibre and sleeve is filled with a gel material. Sometimes multiple fibres are combined inside a single gel-filled sleeve. This type of sleeve also provides the fibre with greater insulation from external heat sources. Tight buffer cables simply use an extra tight-fitting skin over the initial fibre coating. A refined form uses a nylon yarn coated with a PVC jacket. To protect cables from tensile stress during installation, internal strength members can be added when multiple fibre cables are constructed. These members keep the fibres free from stress by minimising elongation and contraction. Kevlar yarn, glass filled epoxy rods and steel wire can be used for this purpose. The steel reinforcing is favoured for extreme cold temperature applications. Slow feed

Furnace

Thickness monitor Molten polymer

Polymer bath

Curing oven

Take-up drum

Figure 3.13 Plastic coating of glass fibres

22

Telecommunications

Fibres in use today There are two main types of fibres.

Step-index (multimode, single mode) In a step-index fibre, the refractive index is constant within the core and it ‘steps’ to a different, lower value as you move into the cladding. This type of fibre is available with an 8–12mm core to allow a single light beam or with a 50–200mm core for carrying multiple beams. This latter type is known as a multimode carrier. Multimode carriers allow light to move along the fibre following many different paths. Some ‘modes’ take the direct route straight down the middle while others bounce from side to side all the way down. Unfortunately the rays from one pulse of light may reach the other end of the optical fibre at different times. This is known as Intermodal Dispersion. Cladding

Core Figure 3.14 Single mode step-index fibre

Cladding

Core Figure 3.15 Multimode step-index fibre

Graded index (multimode) This type of fibre was developed to address the problem of intermodal dispersion. In this type of fibre, the refractive index of the core changes from the centre outwards. It has a ‘quadratic’ profile meaning that the refractive index of the core is proportional to the square of the distance from the centre of the fibre. This graded difference in refractive index slows any modes that travel straight down the centre of the fibre and

Part 3: Telecommunications engineering – materials

23

allows those travelling at the edges to move more quickly. Both modes are more likely to arrive at the end at the same time. This reduces intermodal dispersion and improves the output signal. Cladding

Core Figure 3.16 Multimode graded-index fibre

Polymer fibres Certain clear polymers can also be formed into optical fibres but, because of the much greater attenuation than in glass fibres, all-polymer fibres are only used on short links up to 100 m in length. Polymer fibres are usually of the multimode step-index type and are less expensive, more flexible and easier to handle than glass fibres. Two common polymer optical fibre combinations are: 1

polystyrene core – refractive index of 1.6 polymethylmethacrylate cladding – refractive index of 1.49

2

polymethylmethacrylate core – refractive index of 1.49 fluoroalkyl methacrylate cladding – refractive index of 1.4.

Suggest places where optical fibres are used in modern communications networks. __________________________________________________________ __________________________________________________________ Did you answer? Did you suggest within heavy telecommunication traffic areas like large cities or over long distances such as between cities and major country centres? What about near electric rail networks where there is a need for a communication system that is not interfered with by electromagnetic radiation? How about the cabling in a media outlet where there is a need for wide bandwidth to cope with multimedia applications? All these applications now use fibre-optics and the role of fibre-optics in telecommunications is constantly growing.

Turn to the exercise sheet and complete exercises 3.7 and 3.8.

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Telecommunications

Exercises

Exercise 3.1 a

With the aid of a sketch, describe how the voltage across a resistor, in a circuit, would be measured.

______________________________________________________ ______________________________________________________ ______________________________________________________ b

What do the letters CRO stand for and what can you do with this device? C

___________________________________________________

R

___________________________________________________

O

___________________________________________________

______________________________________________________ ______________________________________________________ ______________________________________________________ c

What electrical properties can be measured with a multimeter? ______________________________________________________ ______________________________________________________

d

List three things about insulators that can be measured using a megger tester. i

___________________________________________________

ii

___________________________________________________

iii ___________________________________________________

Part 3: Telecommunications engineering – materials

25

Exercise 3.2 a

Complete the table below indicating if you think the item listed would show high or low resistance when tested by a multimeter. Item

b

c

Item

Resistance

Cordless phone aerial

H

L

Cover on a power cord

H

L

Circuit board base

H

L

Fibre-optic ‘cable’

H

L

Polymer radio body

H

L

Copper wire phone cord

H

L

In terms of the properties of copper, give three reasons why it is used extensively for electrical wires and cables. i

___________________________________________________

ii

___________________________________________________

iii

___________________________________________________

Aluminium is also used for electrical cables. Give some advantages and disadvantages of aluminium as compared to copper. Advantages

d

Resistance

Disadvantages

Why is gold often used for contacts in telecommunication devices? _______________________________________________________ _______________________________________________________

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Telecommunications

Exercise 3.3 a

Briefly explain in terms of structure, why some materials are insulators. ______________________________________________________ ______________________________________________________ ______________________________________________________ ______________________________________________________ ______________________________________________________ ______________________________________________________

b

Germanium and silicon can behave as intrinsic semiconductors. What is the effect of heat on this type of semiconductor? ______________________________________________________ ______________________________________________________ ______________________________________________________

c

Briefly explain how an extrinsic p-type semiconductor is formed. ______________________________________________________ ______________________________________________________ ______________________________________________________ ______________________________________________________ ______________________________________________________

d

What is formed when phosphorus is used to ‘dope’ silicon or germanium? ______________________________________________________

e

Describe, with the aid of sketches, what happens when a voltage is applied across a p-n junction in two situations. i

When p-end is made positive. ___________________________________________________ ___________________________________________________

ii

When the p-end is made negative ___________________________________________________ ___________________________________________________

Part 3: Telecommunications engineering – materials

27

Exercise 3.4 a

With the aid of a sketch, explain how the structure of polymers means that they are typically insulators.

_______________________________________________________ _______________________________________________________ b

complete the table below by suggesting a specific application for each of the polymers listed.

Polymer

Application

polyethylene epoxy ABS nylon PVC

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Telecommunications

Exercise 3.5 a

State three traditional methods that have used light to convey messages over long distances. i

___________________________________________________

ii

___________________________________________________

iii ___________________________________________________ b

Sketch and label the structure of an optical fibre in the space below.

c

Briefly explain, with the aid of a sketch, why light moves down an optical fibre from one end to the other and doesn’t ‘escape’ through the walls.

______________________________________________________ ______________________________________________________ ______________________________________________________ ______________________________________________________ d

What materials are used to make the type of laser that provides the light source in a fibre-optic network? ______________________________________________________

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29

Exercise 3.6 a

What is attenuation? _______________________________________________________ _______________________________________________________

b

c

Give three reasons why attenuation occurs in glass fibres: i

___________________________________________________

ii

___________________________________________________

iii

___________________________________________________

State three advantages of optical fibres over copper cables. i

___________________________________________________ ___________________________________________________

ii

___________________________________________________ ___________________________________________________

iii

___________________________________________________ ___________________________________________________

d

Discuss how early Australian-made optical fibres were different from those in use today and suggest a problem with these early fibres. _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________

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Telecommunications

Exercise 3.7 a

Explain one method of making modern glass fibres with the aid of a sketch/s. ______________________________________________________ ______________________________________________________ ______________________________________________________

b

Briefly outline a method that can be used to protect glass fibres when they are being installed or when they are in use. ______________________________________________________ ______________________________________________________ ______________________________________________________ ______________________________________________________

c

Explain the difference between step-index and graded index fibres with the aid of sketches. ______________________________________________________ ______________________________________________________ ______________________________________________________

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31

Exercise 3.8 Select the alternative a, b, c, or d that best completes the statement. Circle the letter. 1

2

3

4

5

32

Current is measured in: a

volts

b

watts

c

ohms

d

amps.

Megger testing is used to measure the: a

resistance in electronic components

b

insulation onlarge electrical equipment

c

current flowing through low voltage circuits

d

light intensity in fibre-optics.

Which of the following is not used as an insulator: a

mercury

b

glazed porcelain

c

paper

d

polyethylene.

Which statement best describes a metallic bond. a

a primary bond where all valence electrons are ‘locked in’

b

a secondary bond where electrons are free to ‘flow’

c

positive ions surrounded by a ‘cloud’ of valence electrons

d

attraction between positive and negative dipoles.

Comparing aluminium with copper, which statement is correct? a

aluminium is lighter

b

aluminium is a better conductor

c

aluminium has greater tensile strength

d

aluminium joins easily to other metals.

Telecommunications

6

7

8

9

Two elements commonly used as the basis for semiconductor devices are: a

gold and silicon

b

lead and silicon

c

germanium and carbon

d

germanium and silicon.

One of the disadvantages of using polyethylene for insulation on cables that are to be used outside is: a

water vapour can penetrate the polymer

b

it melts when in direct sunlight

c

the colour is washed off by rain

d

it reacts with the copper that it is protecting.

Laser is the abbreviation of: a

light activated switch with emission response

b

light amplification by the stimulated emission of radiation

c

light application of stimulated emissions of radar

d

light across sensitive extrinsic region.

Which of the following is not used to reinforce fibre-optic cables: a

kevlar yarn

b

glass-filled epoxy

c

steel wire

d

copper wire.

10 Graded index optical fibres have been developed in an attempt to: a

prevent intermodal dispersion

b

minimise attenuation

c

provide greater flexibility

d

increase resistance to tensile stress.

Part 3: Telecommunications engineering – materials

33

34

Telecommunications

Progress check In this part you investigated many of the materials used in telecommunications applications. You have also studied the properties that make these materials suitable for these applications.

✓ ❏

Disagree – revise your work

✓ ❏

Uncertain – contact your teacher

Uncertain

Agree – well done Disagree

✓ ❏

Agree

Take a few moments to reflect on your learning then tick the box which best represents your level of achievement.

I have learnt about: •

specialised testing relating to electrical properties

•

copper and its alloys used in telecommunications

•

ceramics as insulating materials

•

the types of semiconductors and their uses in telecommunication

•

polymers as insulating materials

•

the types and properties of fibre-optics

I have learnt to: •

analyse structure, properties, uses and appropriateness of materials in telecommunications engineering applications

•

select and justify materials and processes used in telecommunications engineering.

Extract from Stage 6 Engineering studies Syllabus, © Board of Studies, NSW, 1999. Refer to for original and current documents.

In the next part you will study the application of mechanics in telecommunications.

Part 3: Telecommunications engineering – materials

35

36

Telecommunications

Exercise cover sheet

Exercises 3.1 to 3.8

Name: ______________________________

Check! Have you have completed the following exercises? ❐ Exercise 3.1 ❐ Exercise 3.2 ❐ Exercise 3.3 ❐ Exercise 3.4 ❐ Exercise 3.5 ❐ Exercise 3.6 ❐ Exercise 3.7 ❐ Exercise 3.8 Locate and complete any outstanding exercises then attach your responses to this sheet. If you study Stage 6 Engineering Studies through a Distance Education Centre/School (DEC) you will need to return the exercise sheet and your responses as you complete each part of the module. If you study Stage 6 Engineering Studies through the OTEN Open Learning Program (OLP) refer to the Learner’s Guide to determine what you need to return to your teacher along with the Mark Record Slip.

Part 3: Telecommunications engineering – materials

37

Telecommunications engineering

Part 4:

Telecommunications engineering – mechanics and hydraulics

Part 4 contents

Introduction..........................................................................................2 What will you learn?................................................................... 2

Mechanics in telecommunications...................................................3 Forces and moments ................................................................. 3 Stress and strain ..................................................................... 13

Exercises ...........................................................................................15 Progress check .................................................................................31 Exercise cover sheet........................................................................33

Part 4: Telecommunications engineering – mechanics and hydraulics

1

Introduction

At any one time millions of people are talking to each other across the globe. At the same time massive quantities of data are being shipped from computer to computer. Underpinning all this communication is a fascinating technology that is the focus of the work of the telecommunications engineers. There is a network strung out across the globe called the Global Telecommunications Network. The nodes of this network are computers programmed to perform as switches. The links of the network are wires, optical fibres, cables, satellite links and radio channels. Telecommunication engineers are engaged in the business of designing, improving, extending, maintaining and operating this network. This rapidly growing section of the economy is transforming the way in which health, banking, education, retail, library and many other services are provided.

What you will learn? You will learn about: •

engineering mechanics and hydraulics as applied to telecommunications engineering.

You will learn to: •

2

apply mathematical and graphical methods to solve telecommunication related problems.

Telecommunications engineering

Mechanics in telecommunications

Telecommunications of all types require an intricate and complex network of cables, wires, optical fibres, repeater stations, transmission towers and receiving devices. Further, there are broadcasting stations, satellites and exchanges … the list goes on! This extensive infrastructure involves many types of structures that require the consideration of mechanics in their design. In this part we will investigate some ways in which mechanical principles can be used in the telecommunications industry.

Forces and moments The application module on landscape products covered: •

nature and types of forces

•

addition of vectors

•

space and freebody diagrams

•

resultants and equilibrants

•

transmissibility of forces

•

3 force rule for equilibrium

•

moments of a force

•

force/couple systems

•

equilibrium of concurrent forces.

Refer back to the Landscape products module to refresh your memory on these topics. Now consider the following: A power pole with lines attached is used to elevate telecommunication lines out of reach from the ground. The pole is supported by two stays as shown in figure 4.1.

Part 4: Telecommunications engineering – mechanics and hydraulics

3

Electrical wire Stay

Figure 4.1 Power pole supported with two stays

The single pole is commonly used in preference to a trussed section. 1

Can you suggest a reason for this? _______________________________________________________ _______________________________________________________

The stays and the lines act through a common point on the pole. 2

What is the word given to describe the fact that the lines of action act through a common point? _______________________________________________________

3

What is the nature of the force in: i

the wires____________________________________________

ii

the pole_____________________________________________

Did you answer? 1 cheaper and simpler construction and installation; less intrusive visually 2 concurrent 3

i

tension

ii

compression.

Worked example 1 If the tensions in the telecommunication lines are equal to 460 N each and the tensions in the stays are equal to 375 N each, find analytically the reaction offered by the ground if the pole and fittings have a combined mass of 400 kg. The lines make an angle of 10∞ to the horizontal and the stays make an angle of 20° to the vertical. Verify your answer by using a graphical method.

4

Telecommunications engineering

B 10∞ 460 N

460 N

10∞

375 N

375 N 20∞ 20∞

A Figure 4.2 Telecommunication lines supported by a pole

Solution The reaction at the ground will be equal and opposite in direction to the total of all the vertical components. There will be no horizontal component to the reaction as the horizontal components of both the lines and the stays will be balanced by each other. The vertical component created by the mass will equal the weight. W =

mg

=

400 x 10

=

4000 NØ

The vertical component created by the lines will equal: 460 sin 10∞

460 cos 10∞ 10∞ 460 N 375 N

Figure 4.3

V = =

2 x 460 sin 10∞ 159.8 N Ø

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5

The vertical component created by the stays will equal:

375 N 20∞ 375 cos 20∞

375 sin 20∞ Figure 4.4

V = = Total reaction at the ground = +
SFv = = Rg =

2 x 375cos 20∞ 704.8 N Sum of vertical components 0 - 4000 - 159.8 - 704.8 + Rg 4.865 kN


This answer can be verified graphically by adding all the vectors acting on the pole. You will recall that to add vectors, they must be drawn to scale, and drawn tip-to-tail.

6

Telecommunications engineering

460 N

Note that the forces are added, one after the other (in any order) tip-totail. The resultant force is found by drawing a line from where you started to where you finished.

375 N

The equilibriant is found by closing the force polygon resultant = 4865 N

4000 N

375 N 460 N Figure 4.5 Graphical solution using a force diagram to scale

Turn to the exercises sheet and complete exercise 4.1.

Worked example 2 A television antenna has three guy wires attached to it at point A as shown in figure 4.4. What tension is required in the third guy wire so that the resultant of the three forces will act vertically downwards? Determine the magnitude of the resultant force. As part of the data supplied, it is common to provide an additional diagram to indicate correct angles and positioning of the forces.

What is the diagram given as part of the data called?

………………… The answer is below the diagram.

Part 4: Telecommunications engineering – mechanics and hydraulics

7

A C 40∞

20∞

25∞

2.2 kN

1.3 kN

Figure 4.6 Space diagram of TV antenna on house

Before solving the problem, it is often convenient to summarise the forces that are acting. This diagram is shown as a sketch (not drawn to scale) showing the magnitudes of known forces as well as the directions of each of the forces acting. What is this diagram called?

………………… The answer is below the diagram. A

40∞

20∞

25∞

2.2 kN

C R

1.3 kN

Figure 4.7 Free body diagram of forces acting

A graphical solution is generally the easier method. This is a convenient method of adding vectors by drawing them ‘tip-to-tail’. Name the diagram that allows the addition of vectors by drawing them ‘tip-to-tail and to scale. __________________________________________________________ Did you answer? A force diagram.

8

Telecommunications engineering

Graphical solution 45∞ 2.2 kN

1

Draw the 2.2 kN force to scale and at the correct angle.

2

Draw the 1.3 kN force to scale and at the correct angle.

3

Draw the directions of the force C and the resultant, R.

4

The intersection of R and C will give you the size of these forces.

1.3 kN 20∞

R = 5.2 kN

40∞

C = 3.1 kN

Figure 4.8 Force diagram drawn to a scale of 10 mm = 0.5 kN

Mathematical solution If the resultant force is to act vertically downward then the sum of the horizontal forces must equal 0. As there are three forces with horizontal components:

45∞ 2.2 cos 45∞

1.3 cos 20∞ 20∞ 1.3 kN 2.2 kN

2.2 sin 45∞

1.3 sin 20∞

40∞ C

C cos 40∞

C sin 40∞

Figure 4.9 Three forces with Horizontal components

Part 4: Telecommunications engineering – mechanics and hydraulics

9

Horizontal Resultant (+ )

= ÂHorizontal components of individual forces = 0 O = 2.2 sin 45∞ + 1.3 sin 20∞ – C sin 40∞

1.56 + 0.44 \ C = sin 40 = 3.11 kN = S Vertical components

Vertical Resultant (+
)

= - 2.2 cos 45∞ - 1.3 cos 20∞ - 3.11 cos 40∞ = -1.56 - 1.22 - 2.38 = -5.16 kN \ the resultant force is 5.16 kN acting  Turn to the exercise sheet and complete exercise 4.2.

Worked example 3 An axial force is induced in a mast by two wire stays as shown in figure 4.10.

30∞

A = 1.5 kN

20∞

B

Figure 4.10 Telecommunication mast with two stay wires

10

i

Find the magnitude of the axial force acting along the mast

ii

What is the magnitude of the force acting in the stay wire B.

Telecommunications engineering

O

A = 1.5 kN

Graphical solution

30∞

B = 2.2 kN 20∞

1

Draw force A to scale.

2

Draw direction of axial force through the origin point O.

3

Draw direction of force B acting at the end of force A.

4

Add arrow heads, measure magnitude of B and axial force.

Figure 4.11 Graphical solution of force acting in mast

Mathematical solution If the resultant force is to act vertically then the sum of the horizontal forces must equal 0. Horizontal Resultant (+ ) = 0 = B sin 20 – 1.5 sin 30 B = 1.5 x 0.5 / 0.342 = 2.2 kN Vertical Resultant (+
) = -2.2 cos 20 - 1.5 cos 30 = -2.07 - 1.3 = -3.37 kN (this is the axial force in the mast) \ the resultant force is 3.37 kN acting  Turn to the exercise sheet and complete exercise 4.3.

Part 4: Telecommunications engineering – mechanics and hydraulics

11

Worked example 4 The radio tower shown in figure 4.10 is 3 metres square and 15 metres high. It has a mass of 7 tonnes. It is supported against horizontal wind loads by four guy wires attached 10 metres above the central base B. Its effective projected area of 12 m2 is subjected to a horizontal wind pressure of 650 Pa. When the wind blows from left to right, only one guy wire AC is active. Determine: i

tension in the guy wire AC.

ii

the reaction at B

A

30∞ Guy wire CG

C

B

Figure 4.12 Radio tower

Mathematical solution The unit of wind pressure is a pascal. (Remember that 1 Pa = 1 N/m2.) Pressure =

650 = = \ F =

Force Area F 12 650 x 12 7800 N

This force created by the wind of 7.8 kN can be considered to act through the centre of gravity. The centre of gravity for a uniform structure will be half way up the height. This will be 7.5 metres up for this problem.

12

Telecommunications engineering

The mass of the tower = 7 T or 7000 kg or 7 x 103 The weight of the tower equals mg = 7 x 103 x 10 N = 70 kN i

tension in the guy wire AC.

Let T be the tension in the cable AC For equilibrium, ÂMB

=

0

=

(Tsin 30 x 10) – 9.5 x 7.8 +(Tcos 30 x 1.5)

T =

9.3 kN

ii the reaction at B For equilibrium, (+
) ÂV = RBV - 70 – 9.3 cos 30 = RBV =

0 70 + 8.1 78.1 kN ≠

For equilibrium, (+ ) ÂH = = RBH = =

0 7.8 – 9.3 sin 30 + RBH –3.15 3.15 kN 

Part 4: Telecommunications engineering – mechanics and hydraulics

13

Rb is made up of two components RBH

RBV

RB

Figure 4.13 reaction Rb

Rb

=÷ (3.152 + 78.12) = 78.2 kN

tan q = Rbv/ Rbh = 78.1 / 3.1 q

= tan -1 31.65 = 88º

\ the reaction at is 78.22 kN at 88º 

Stress and strain The application module Civil structures dealt with: •

stress and strain

•

Young’s modulus.

Revise the theory on these topics for the following worked example. Worked example 10 A copper wire 3 metres long and of uniform circular cross-sectional area is stretched 6 mm by the application of a tensile load of 2.5 kN. Calculate the wire diameter if the modulus of elasticity of the wire material is known to be 120 GPa.

14

Telecommunications engineering

Solution Remember when dealing with stress/strain calculations, the units should all be converted to MPa, N or mm. Note 1MPa = 1 N / mm2 F = 2.5 kN = 2.5 x 103 N L = 3 m = 3 x 103 mm e = 6 mm E = 120 GPa = 120 x 103 MPa Modulus of Elasticity =

Young’s Modulus, E

E =

stress strain

E =

Fl eA

A =

Fl eE

= p d2 = 4 d2 = d2 = d = =

2.5 x 103 x 3 x 103 6 x 120 x 103 10.42 10.42 x 4 p 13.27 ÷13.27 3.64 mm

Turn to the exercise sheet and complete exercises 4.10 to 4.12.

Part 4: Telecommunications engineering – mechanics and hydraulics

15

16

Telecommunications engineering

Exercises Exercise 4.1 A power pole with power lines connected is supported by two stays as shown in figure 4.14. The power lines are tensioned so that they are at an angle of 15∞ to the horizontal when the tension in them equals 400 N. If the tension in each stay equals 380 N, determine mathematically the reaction at the bottom of the pole if the pole and associated fittings have a combined mass of 280 kg. Verify your answer by using a graphical solution. 15∞

15∞

400 N

400 N

380 N

380 N

25∞

25∞

Figure 4.14 Power pole

Part 4: Telecommunications engineering – mechanics and hydraulics

17

Exercise 4.2 A television antenna has three guy wires, P, R and S attached to it at point A. What tension is required in the second and third guy wires (R and S) so that the resultant of the three forces will act vertically downwards with a magnitude of 6.5 kN?

A P = 2.2 kN

S

45∞

20∞

25∞

R

Figure 4.15 TV Antenna

Mathematical solution:

Graphical solution:

.

0

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Telecommunications engineering

Exercise 4.3 An axial force is induced in a mast by two wire stays as shown on figure 2.31.

30∞

20∞

A

B = 1.5 kN

Figure 4.16 Telecommunication mast with stay wires

i

Find the magnitude of the axial force acting along the mast

ii

What is the magnitude of the force acting in the stay wire A?

Mathematical solution:

Graphical solution:

.

0

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Exercise 4.4 A radio tower shown in figure 4.17 is 2.6 metres square and 16 metres high. It has a mass of 6 tonnes. It is supported against horizontal wind loads by four guy wires attached 10 metres above the central base B. Its effective projected area of 11 m2 is subjected to a horizontal wind pressure of 550 Pa. When the wind blows from right to left, only one guy wire AC is active.

A Guy wire 30∞

CG

B

C

Figure 4.17 Radio tower

a

20

Discuss why only one, or at most two, of the four guy wires will be active.

Telecommunications engineering

b

Find: i

the tension in guy wire AC

ii

the reaction at B

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Exercise 4.5 A pay TV satellite dish is attached to the gutter fascia as shown in figure 4.36. The 700 mm diameter dish and single output LNB unit has a mass of 12 kg and acts at the centre of mass 250 mm from the support pole. The dish is inclined at an angle of 15∞.

Figure 4.18 Pay TV satellite dish

250

231 231

N

120 N 120 N

N

231 N

W = mg = 12 x 10 = 120 N

100 mm

15∞

Figure 4.19 Free body diagram

If a wind is blowing is with a pressure of 600 N/m2 perpendicular to the dish and the pole is secured with two bolts 100 mm apart, Determine: i

the force-couple reaction at the support pole

ii

the axial force acting on each bolt. (You may assume that the magnitude of the axial forces in each bolt will be equal).

Refer to previous work if needed

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Exercise 4.5 cont.

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Exercise 4.6 A simplified diagram of the top of a 50 metre telecommunication tower is shown in figure 4.20. 15 kN 4m

A

B

C

4m

15 kN

D

E

4m

50 m

15 kN

15 kN

F

G

I

H 80∞

Figure 4.21 Simplified free body diagram of telecommunication tower

If a wind of 15 kN is acting on radio control units located on joints A, B, D and F as shown, determine mathematically : i

the magnitudes and nature of the forces in members FG and GI

ii

the distance between the legs at the base

iii the total left and right reactions at the base supports, if no other forces are acting on the tower. Refer to previous work if needed

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Exercise 4.6 cont.

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Exercise 4.7 The relay units shown in figure 4.22 are common sights throughout the country in order to establish a mobile phone network. They are usually located either on buildings or on telecommunication towers. The units shown are mounted on round pipe and bolted to the sides of a building.

Figure 4.22 Telstra telephone exchange – Charlestown

a

Discuss the use of round pipe as a support. _______________________________________________________ _______________________________________________________ _______________________________________________________

b

What is the term used to describe a structural member that is supported at one end only? _______________________________________________________

c

Discuss the forces that would be acting on the bolts that secure the pipe support to the building. _______________________________________________________ _______________________________________________________ _______________________________________________________

d

How would the engineer decide on what size bolts to use? _______________________________________________________ _______________________________________________________ _______________________________________________________

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Telecommunications engineering

Exercise 4.8 An aluminium wire 3 metres long and of uniform circular cross-sectional area is stretched 6 mm by the application of a tensile load of 2.5 kN. Calculate the wire diameter if the modulus of elasticity of the wire material is known to be 70 GPa.

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Exercise 4.9 Select the alternative a, b, c, or d that best completes the statement. Circle the letter. 1

2

3

4

5

28

Telecommunication towers are often made out of a triangulated truss assembly. This type of bar assembly is known as: a

an unstable assembly

b

a stable assembly

c

an assembly containing redundant members

d

a structural column.

When discussing members in a structural frame, the: a

tie must be a stiff member

b

strut needs stiffness as well as strength to take tensile loads

c

frame may have some ties in tension and compression

d

ties will be in tension and the struts will be in compression.

Given that the modulus of elasticity of aluminium is 70 GPa and copper is 120 GPa, then for a wire to stretch the same amount under the same load, the diameter of the aluminium wire will be: a

equal to the copper wire diameter

b

1.7 times larger than the copper because Ecopper = 1.7 x Ealuminium

c

1.7 times smaller than the copper because Ecopper = 1.7 x Ealuminium

d

approximately 1.3 times larger than the copper wire diameter.

A bolt is tightened and used as a connection to join two structural members together. The bolt may experience: a

a tensile force

b

a compressive force

c

a shear force

d

both tensile and shear forces.

A 3 kg telephone, with four rubber feet, is sitting on a table. Each foot will set a reaction force with the table top equal to: a

3 kg

b

0.75 kg

c

30 N

d

7.5 N

Telecommunications engineering

6

7

8

A 68 cm television set, of mass 30 kg, has legs which are 600 mm apart in the front view of the television. It is placed centrally on a 1 metre shelf which is supported at each end at A and B. Neglecting the weight of the shelf, the reactions at supports A and B will be: a

RA = 300 N ≠ and RB = 300 N ≠

b

RA = 150 N ≠ and RB = 150 N ≠

c

RA = 15 N ≠ and RB = 15 N ≠

d

RA = 150 N Ø and RB = 150 N Ø

The shear force diagram for shelf and TV set will be: a

b

c

d

The bending moment diagram for the shelf and TV set will be:

a

b

c

d

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20 kN

A

B

C

D

G

F

9

E

The cross brace members BC, CD, DE and FE in the tower for the wind load of 20 kN as shown will be: a

redundant (carry no loading)

b

in tension only

c

in compression only

d

in either tension or compression.

10 A rectangular beam, 100 mm wide and 200 mm deep is subjected to a maximum bending moment of 240 kNm. If the second moment of area, IXX = 66 x 106 mm4, and the maximum bending stress is determined by the formula s = My. I For the given data, the maximum bending stress will equal:

30

a

120 MPa

b

180 MPa

c

360 MPa

d

720 MPa.

Telecommunications engineering

Progress check

In this part, you have investigated the mechanics associated with telecommunication structures.

✓ ❏

Disagree – revise your work

✓ ❏

Uncertain – contact your teacher

Uncertain

Agree – well done Disagree

✓ ❏

Agree

Take a few moments to reflect on your learning then tick the box which best represents your level of achievement.

I have learnt about: •

engineering mechanics and hydraulics as applied to telecommunications engineering.

I have learnt to: •

apply mathematical and graphical methods to solve telecommunication related problems.

In the next part you will examine the application of transmission media in telecommunications.

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Exercise cover sheet

Exercises 4.1 to 4.9

Name: _______________________________

Check! Have you have completed the following exercises? ❐ Exercise 4.1 ❐ Exercise 4.2 ❐ Exercise 4.3 ❐ Exercise 4.4 ❐ Exercise 4.5 ❐ Exercise 4.6 ❐ Exercise 4.7 ❐ Exercise 4.8 ❐ Exercise 4.9 Locate and complete any outstanding exercises then attach your responses to this sheet. If you study Stage 6 Engineering Studies through a Distance Education Centre/School (DEC) you will need to return the exercise sheet and your responses as you complete each part of the module. If you study Stage 6 Engineering Studies through the OTEN Open Learning Program (OLP) refer to the Learner’s Guide to determine which exercises you need to return to your teacher along with the Mark Record Slip.

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Telecommunications engineering

Part 5: Telecommunications engineering – electricity/electronics

Part 5 contents

Introduction..........................................................................................2 What will you learn?................................................................... 2

Principles of telecommunications ....................................................3 Signals and noise ...................................................................... 3 Transmission of images ............................................................17 Baseband transmission.............................................................19 Modulation of carriers ...............................................................20 Transmission media..................................................................30 Evaluating telecommunications systems ....................................46

Exercises ...........................................................................................55 Progress check .................................................................................63 Exercise cover sheet........................................................................65

Part 5: Electricity/electronics in telecommunications engineering

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Introduction

In this part we will look at the principles that underpin many of the telecommunications technologies currently in use, and which are likely to still be relevant in the context of the new telecommunications technologies of the future.

What will you learn? You will learn about: •

telecommunications –

•

analogue and digital systems, modulation, demodulation, radio transmission (AM, FM), television transmission (B/W, colour), telephony (fixed and mobile), transmission media (cable, microwave, fibre-optics)

satellite communication systems, geostations.

You will learn to: •

describe the basic concepts and applications of modulation and transmission systems in telecommunications

•

distinguish the communication bands in the electromagnetic spectrum

•

contrast the differences in transmission media

•

describe the basic principles of satellite communication systems.

Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to for original and current documents.

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Principles of telecommunications

Modern telecommunications is one of the fastest changing and most influential technologies in our society. There are many different formats and technologies currently in use. What technologies and applications lie ahead?

While we can do no more than guess what forms telecommunications will take in ten or twenty years time, history suggests that changes are inevitable. In ten years time we will most likely take for granted telecommunications systems that have not yet been invented. The diversity of current forms of telecommunications prohibits an exhaustive study of all of them. Furthermore, such a study would not necessarily give us insights into how newer technologies will work. Instead, we will look at the principles that have underpinned many of the technologies currently in use, and which are likely to still be relevant in the context of the new technologies of the future.

Signals and noise The performance of a telecommunications network is generally governed by three simple factors: •

the power of the signal being transmitted

•

the power of the electrical noise in the system

•

the bandwidth of the system, that is, the range of frequencies that can be carried.

In this section, we define these basic parameters. We also reinforce the ideas of analogue and digital signals, and make some simple comparisons between the two.

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Analogue signals An analogue signal is continuously variable in amplitude and time. Examples of analogue signals include: •

the variation of temperature

•

the amount of water in a dam

•

the weight of a person.

At each and every instant in time, we can measure an analogue quantity to any arbitrary accuracy.

Digital signals A digital signal is one that differs from an analogue signal in two important aspects: •

its amplitude can only be one of a set number of possible levels

•

its amplitude only changes at regular time intervals.

Digital signals can be obtained from analogue signals by the dual processes of sampling and quantisation. Sampling of an analogue signal means taking a series of measurements at regular instants in time. For example, we might check the temperature every hour, or measure our weight once a week. Quantisation is the process of approximating a measurement of amplitude by the nearest value from a set of possible values. For example, we might round the temperature to the nearest degree, or our weight to the nearest kilogram. The set of possible amplitude values is bounded. That is, there are minimum and maximum values that we cannot exceed. (This can lead to errors if our analogue variable exceeds these values.) Figure 5.1 shows an analogue signal together with a digital approximation. Note that there are many different possible digital approximations to a given analogue signal. By varying the sampling rate and the quantisation interval, we can obtain more or less accurate digital approximations to the analogue signal.

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Telecommunications engineering

Analogue signal

Sampling and quantisation

Digital signal

Figure 5.1 An analogue signal and a digital approximation to that signal obtained by sampling and quantisation

Theoretically, a digital signal can take on any number of possible values, as long as that number is finite. Often, however, the term ‘digital’ is (mis)used to describe a system that can take on only one of two possible values. A system that can only take two possible values should properly be called a ‘binary’ system. Similarly, ‘ternary’ system can take on three possible values, while a ‘quaternary’ system can take on four possible levels, and so on. Figure 5.2 illustrates binary signals, ternary signals and quaternary signals. While binary signaling is simpler to implement, multilevel signaling has the advantage of being able to send more information per pulse. A binary signal can indicate one of two possible values at each signaling interval. A quaternary signal, on the other hand, can represent one of four possible values at each interval. (An analogue signal has an infinite number of possible values.)

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As we shall see later, multilevel signaling can: •

send more information per pulse than a binary system

•

use less pulses to send a fixed amount of information than a similar binary system.

If our objective is to send information as quickly as possible, then multilevel schemes can be attractive. Indeed, they are used in most computer modems and fax machines! 5.0

0.0

5.0 2.5 0.0

5.0 3.3 1.6 0.0 Figure 5.2 Binary, ternary and quaternary signals having two, three and four possible levels respectively spread across the same 5 Volt range

Turn to the exercise section and complete exercise 5.1.

Time domain representation of a signal The time domain representation of a signal shows the amplitude of the signal as it varies with time. This is the type of waveform we would see if we examined the signal with an instrument called an ‘oscilloscope’. The horizontal axis represents time. Time scales used in telecommunications applications typically vary between milliseconds per division to nanoseconds per division depending on the type of signal being viewed. The vertical axis is amplitude. This is most often measured in Volts, or milliVolts.

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Telecommunications engineering

Figure 5.3 shows an analogue signal and a (binary) digital signal as viewed on an oscilloscope. (This particular oscilloscope allows us to view two signals simultaneously.) The horizontal scale is 50 msecs (m= 10-6) per division, and the vertical axis is 1 Volt per division for the upper trace, and 2 Volts per division for the lower trace.

Figure 5.3 Analogue and digital signals as viewed on an oscilloscope

Figure 5.4 shows the same two waveforms as in the previous figure, but this time the oscilloscope is set to have an expanded time scale. The scale is now 500 nsecs (n = 10-9) per division. At first glance we might not recognise that they are the same signals!

Figure 5.4 Analogue and digital signals as viewed on an oscilloscope with an expanded time base (note that the digital signal appears to be analogue)

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The lower signal – the digital signal – now appears to be an analogue signal! Strictly speaking, all signals are analogue signals. For this reason it is important to distinguish what we mean by the terms ‘analogue’ and ‘digital’. If we have a signal that we assume to be analogue, then we are free to examine and take measurements of the signal at any instant in time, and to make those measurements with any degree of accuracy. If, on the other hand, we assume a particular signal to be digital, then we should only examine the signal at the mid-point of each interval, and we should round the signal at that point to the nearest permissible amplitude level. Figure 5.5 illustrates how we should examine and interpret a digital signal. In this case the signal is binary having two possible values 0 and 1. 1

0

1

1

0

1

0

0

1

1

0

1

0

1

Figure 5.5 Interpreting a digital signal: we should examine the digital signal at the mid-point of each interval, shown in this case by the black dots

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Telecommunications engineering

Amplitude, period, frequency and wavelength of a sinusoid Figure 5.6 shows an analogue signal and a digital signal, with their amplitudes and periods illustrated. The amplitude of the analogue signal is normally specified as either a peak amplitude (that is, from zero to the maximum height) or as a peakto-peak value (the amplitude from the lowest point to the highest point). The units of amplitude are most often Volts. The amplitude of the digital signal is normally specified simply as a peak amplitude (that is, from zero to the maximum height). Again, the units of amplitude are most often Volts. The period of a wave can be measured via a time domain representation of the waveform as the time from one point on the waveform, to the same point on the next cycle of the wave. We usually measure from one peak to the next, or from one positive zero crossing to the next positive zero crossing. The units associated with the period are time units (such as seconds, milliseconds or microseconds). Period Vp Vpp

Period Vp Figure 5.6 Amplitude and period of a sinusoidal wave and a digital wave

Another parameter associated with sinusoidal waveforms is the wavelength. The wavelength is the distance in metres between equivalent points on successive cycles of a wave as it probigates through a medium. (Note the distinction between period and wavelength – period is an interval in time, wavelength is an interval in metres.) The size of a radio antenna is a function of the wavelength of the signal it is designed to transmit or receive. Low frequency signals have long wavelengths and thus require large antennae. In contrast, high frequency signals have short wavelengths and thus require smaller antennae.

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Figure 5.7 shows the transmission mast for a radio station with a broadcast frequency of 1 413 kHz and corresponding wavelength of 212 metres. The antenna height is just over 100 metres, corresponding to approximately one-half wavelength.

Figure 5.7 Transmission mast for a radio station with a broadcast frequency of 1 413 kHz – the height of the mast is just over 100 metres, corresponding to a one-half wavelength for the frequency used

Figure 5.8 shows an antenna designed to receive frequencies at 2.4 GHz, or a wavelength of 125 mm. The antenna in this case is approximately one wavelength long.

Figure 5.8 Antenna for reception of frequencies at 2.4 GHz – the antenna is approximately one wavelength long for that frequency

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Telecommunications engineering

The wavelength of a signal also determines its ability to propagate over and around objects in its path. In general, an object whose principal dimensions (length or height) are the same as, or larger than, the wavelength of the signal will effectively block the signal. Signals with long wavelengths (low frequencies) are only blocked by large objects (such as mountains). Signals with short wavelengths (high frequencies) are blocked by both large and small objects (buildings, cars or trees). In general terms, this means that higher frequency signals need to have a clearer path between transmitter and receiver than do signals operating at lower frequencies. Satellite and microwave links that operate at GHz frequencies need to have a line of sight between transmitter and receiver. Even then, they can be affected by atmospheric conditions such as rain. Lower frequency signals, such as those found in the AM radio band, have quite long wavelengths of around 200 to 300 metres and hence are able to find their way around most objects. This is the principle reason that AM radio offers good reception in most areas. Turn to the exercise section and complete exercise 5.2.

Noise in a communications link All telecommunications links contain ‘electrical noise’. The term ‘electrical noise’ is used to describe any electrical signals that are undesirable in the system. The sources of electrical noise are many and varied. Common sources of electrical noise include interference from other electrical appliances (such as fluorescent lights and electric drills), nearby lightning, radio signal interference and faulty connections. Electrical noise is most often measured in terms of its amplitude relative to the signals of interest. This is usually referred to as the ratio of the signal amplitude to the noise amplitude, or simply signal to noise ratio, or SNR. To understand the significance of the SNR, think about the situation in a classroom where you are trying to listen to the teacher. The teacher's voice, in this case, is the signal. Invariably, there will be other people in the same room also talking, or whispering: these people are sources of noise. Your ability to hear the teacher is dependent on the ratio of the volume of the teacher's voice, and the amount of other noise.

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If there is no noise, then you should be able to hear the teacher perfectly. If there is some noise in the room, the teacher will have to speak more loudly to overcome the background noise. In speaking more loudly, the teacher is increasing the signal to noise ratio, thus making it easier for you to listen. Good electronic design can help reduce the amount of noise that gets into a circuit. Shielded cables such as coaxial cables, for example, are specifically designed to reduce the amount of noise that enters a circuit. Electrical noise that does enter an electrical circuit can distort (or in extreme cases – obscure) the desired signal. You have probably tried to listen to a telephone call or radio broadcast that was affected by electrical noise. A small amount of noise is generally easy to live with. However, in some applications very large amounts of noise are inevitable: a spaceship transmitting back to earth from Jupiter can only produce a very weak signal. The ambient electrical noise is much (much) greater than the desired signal.

The effect of noise on analogue and digital signals We noted above that electrical noise can distort or corrupt an electrical signal. If the signal is an analogue signal, it is quite difficult to separate the signal from the noise. If we amplify the signal, we will also amplify the noise. Figure 5.9 shows three signals (or traces) on an oscilloscope. The top trace is noise induced into an analogue communications link by electrical interference. The second (middle) trace is the result of that noise corrupting a sinusoidal signal. By definition an analogue signal can be examined at any arbitrary instant, and its amplitude may be any value. Assuming that we do not know what signal was sent before being corrupted by noise, we cannot separate the noise from the desired signal. The third (bottom) trace shows the signal we have to live with.

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Telecommunications engineering

Figure 5.9 Effect of noise on an analogue signal –we cannot separate the desired signal from the noise

If the transmitted signal is digital, we can reconstruct the original digital signal from the noisy received signal without introducing errors (that is, we can separate the noise from the desired signal)! Let us consider the situation where a communications channel can transmit a signal of between 0 and +5 Volts. Let us also assume that some external interference introduces random electrical noise of up to +/1 Volt into the channel. This means that the received signal might be 1 Volt greater or less than the original received signal. Suppose we now send a binary digital signal over the same channel. We will use 0 Volts and +5 Volts for the two signal levels. We do not know at the receiving end what information was sent from the transmitter. However, we do know that the signal sent must have been either at 0 Volts or at 5 Volts. When the digital signal is affected by noise, a 0 Volt signal might be received as -1 Volt or as +1 Volt. Similarly, a 5 Volt signal might be received as 4 Volts or as 6 Volts. However, it is a trivial matter to correctly interpret the received noisy signal: we simply decide which of the two logic levels is closest to the noisy received signal, and replace the noisy signal with a fresh clean signal of the correct logic level! If the noise levels are too large, causing a logic 0 Volts to be received as, say +4 Volts, we will make the wrong decision at the receiving end (by assuming it was supposed to be +5 Volts) and an error can occur. However in practice we try to reconstruct the signal before this much noise accumulates in the signal.

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This process of reconstructing the original digital signal from a received noisy digital signal is called regeneration. Regeneration means that as long as we do not allow too much noise to affect the signal, we can always rebuild and retransmit digital signals without errors! Figure 5.10 shows the process of digital signal reconstruction. The upper trace shows the noise in the channel. The middle trace shows the signal corrupted by noise, and also the threshold level by which we determine whether the received signal should be a ‘0’ or a ‘1’. The lower trace shows the reconstructed signal. The ability to regenerate signals is one of the key advantages of digital signals, and makes their use worthwhile even though they require a greater bandwidth.

Figure 5.10 Reconstruction of a digital signal after corruption by electrical noise – at each sampling instant we simply decide whether the received signal should be a ‘1’ or a ‘0’

Turn to the exercise section and complete exercise 5.3.

Comparison of analogue and digital signals We have seen that digital signals allow regeneration, whereas analogue signals do not. Do digital signals have any other inherent advantages over analogue signals?

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Telecommunications engineering

The main benefits of digital signals over analogue signals are: •

Immunity to noise – As seen in figure 5.10, digital signals can be regenerated prior to retransmission. This prevents noise from accumulating in the signal as it propagates through the system, and thus allows long distance transmission without error. By comparison, noise that is induced into an analogue signal is accumulated as it propagates through the system.

•

Cost of digital equipment – The advent of mass produced digital electronic components has significantly reduced the cost per unit of digital devices. Evolving technologies have also allowed more complex functions to be constructed on a single integrated circuit, further reducing costs. By comparison, analogue devices and circuitry have not been able to offer similar cost reductions.

•

Channel capacity utilisation – As communications frequencies increase (into the GHz range), so do the bandwidths of the available channels. Multiplexing techniques allow us to take advantage of the increased bandwidths. However, time division multiplexing (as used in digital systems) is more easily and cheaply implemented than frequency division multiplexing (as used in analogue systems).

•

Security and privacy – Techniques for encrypting signals and data are more easily implemented in the digital domain than they are in the analogue domain. This ensures enhanced privacy for sensitive transactions such as financial transactions that are conducted electronically.

•

Integration of formats – Some sources of information are analogue, some are digital. If we wish to simplify our telecommunications systems, it is beneficial to send all types of data over a common format. It is generally easier to use digital signals rather than analogue signals as the common format.

Turn to the exercise section and complete exercise 5.4.

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The maximum capacity of a communications link We have discussed a number of important concepts so far: •

the bandwidth of signals

•

signal amplitudes

•

electrical noise.

How do these parameters combine to affect the capacity of a telecommunications system? Alternatively, is there a maximum limit on the amount of information that can be sent through a communication link? The bandwidth describes the range of frequencies that can be sent through the link. The bandwidth of a communications link is dependent on its electrical and electronic (and optical) properties. The signal power is simply the power of the signal that is to be sent across the link. The noise power is a measure of the electrical noise or interference that exists in the channel. The ratio of signal power to noise power is the Signal to Noise Ratio (SNR). In order to avoid errors in transmission across a channel we should: •

try to increase the power of our signals

•

try to decrease the electrical noise in the system

•

try to increase the bandwidth of the system.

We will see in the following sections how these key parameters affect the performance of communications links. Turn to the exercise section and complete exercise 5.5.

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Telecommunications engineering

Transmission of images Images, or graphical representations, are arguably the oldest forms of communication. Cave paintings were used by primitive civilisations to convey impressions and information long before any written language was used. Images remain a cornerstone of modern communications. The adage that ‘a picture is worth a thousand words’ still holds true for many consumers of information. We now commonly expect images in our newspapers to illustrate any story that we may read. How might we represent images so that they may be telecommunicated?

Most images these days (with the notable exception of photographic film) use a digital format. We have already seen how an analogue signal can be converted into a digital signal via the processes of sampling and quantisation: sampling takes ‘snapshots’ of the signal at regular time intervals, while quantisation assigns one of a finite number of amplitudes or levels to the signal at each sample. How does digitisation work for images that are not changing with time, but are two dimensional and often in colour? In digitising an image, we do not sample at regular instants in time, but rather we sample at regular intervals across (and down) the image. That is, we divide the image into a grid, and represent the whole of the image by a finite set of regularly spaced samples. Each of the samples is known as a ‘picture element’, which is often abbreviated to ‘pixel’. Figure 5.11 shows a digital image sufficiently enlarged to be able to see the individual pixels.

Figure 5.11 Enlargement of a digital image to illustrate individual pixels

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The sampling of the image is often measured in terms of the number of pixels used per unit of distance. The most common units are ‘dots per inch’ or dpi. (An inch is approximately 25 mm.) For example, an image represented on a video display is usually sampled at around 72 dpi. Modern low cost printers reproduce around 600 dpi. High quality commercial printers use 1 200 dpi. The greater the number of dots per inch, the greater the resolution of the reproduced image. Unfortunately, the higher resolution also requires more information has to be stored or transmitted. Having sampled the image, we must now describe each pixel. For a monochrome (black and white) image we can simply represent each pixel as being either black or white. This requires only one bit of information per pixel, saving storage space, but limiting quality image. For a grey scale image, we describe how light or dark each pixel is to be via a shade of grey. Most often we quantise the grey scale into 256 different shades. Grey scales convey a more realistic image than does monochrome, but at the expense of needing more information (8 bits) to represent each pixel. If we want to reproduce colour, we need to capture the colour of each pixel in the image. The RGB system uses a measure of the relative quantities of Red, Green and Blue (hence RGB) at each pixel. Since we need to quantise three colours, we end up with three times as much information as compared with grey scale images. (That is, 3 x 8 bits = 24 bits for each pixel.) As an example of the relative sizes of stored images, a typical photograph (such as the one of the microwave towers later in this text) digitised at 300 dpi can be saved as: •

2 985 984 bits (3 Mb) for 24 bit colour

•

995 328 bits (1 Mb) for 8 bit greyscale

•

124 416 bits (124 Kb) for monochrome.

Figure 5.12 shows the relative amounts of data required for each of these formats. As can be seen, it helps to think closely about whether you really need colour!

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Telecommunications engineering

Mbits of data required

3.0 2.5 2.0 1.5 1.0 0.5 0 24 Bit RGB Colour

8 Bit Grey Scale

1 Bit Monochrome

Figure 5.12 Relative amounts of data required to represent an image as colour, grey scale and black and white

Baseband transmission The term ‘baseband’ is used to describe a telecommunication system in which the message to be sent is converted directly to an electrical signal, and is then sent over a cable to its destination. The frequency range of the transmitted signal is identical to the frequency range of the original signal. (This contrasts with systems such as microwave and radio that, as we will see, use modulation of a carrier wave to transmit information at frequencies different to the original signal.) Baseband systems are very common. If you connect two PC's together with a serial cable, you are creating a baseband link. The keyboard is connected to a PC via a baseband link. A (wired) intercom system is a baseband link. The advantage of baseband systems is simplicity. All that is required for a baseband link is to convert the signal to be transmitted from its original format into an electrical signal at the sending end, and to convert it back to its original format at the receiving end. For example, speech can be converted into an electrical signal using a microphone, and converted back into sound using a loudspeaker. The wire joining the two ends is a baseband link. The fundamental disadvantage of baseband signals is that they are usually only suitable for transmissions over short distances. While a simple wire connection works well across a room, it is not suitable for connection across the world. The lengths of wire needed, the fading (attenuation) of the signal along the wire, and the susceptibility to introduced electrical noise make long distance baseband transmission impractical.

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Modulation of carriers The concept of modulation is fundamental to telecommunications systems. As a simple description, modulation involves ‘piggybacking’ information on to a carrier (or ‘transportation’) wave. You might think of the carrier wave as a truck, and the information, or message signal, as the goods to be carried on the truck. Carrier waves are better suited to transmitting signals over long distances than are baseband systems. This is because we are free to choose the frequency of carrier wave that best suits the medium over which we are transmitting. For example, if we wanted to transmit speech over a radio link using baseband frequencies, we would have to broadcast frequencies of around 300 Hz to 3500 Hz, with resulting wavelengths of hundreds of kilometres long. Hence for efficient transmission we would need antennas that were 25 kilometres high! By modulating the signal to higher frequencies for radio transmission, we can take advantage of shorter wavelengths, and hence smaller antennae. In any modulation system, we need to have three components: •

a carrier wave

•

a message (or information) signal

•

a method for ‘piggybacking’ (or modulating) the message signal onto the carrier wave.

Carrier waves The carrier wave is simply a pure sinusoidal wave at some predetermined frequency. A carrier for AM radio is the same as a carrier for FM radio, microwave or satellite links: only the frequency changes. Table 5.1 shows the range of frequencies used for carrier waves, and their common designations. Table 5.2 shows the frequency ranges of some familiar applications.

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Carrier Frequency Range

Common Description

3 – 30 kHz

Very Low Frequency (VLF)

30 – 300 kHz

Low Frequency (LF)

300 – 3000 kHz

Medium Frequency (MF)

3 – 30 MHz

High Frequency (HF)

30 – 300 MHz

Very High Frequency (VHF)

300 – 3000 MHz

Ultra High Frequency (UHF)

3 – 30 GHz

Super High Frequency (SHF)

30 – 300 GHz

Extremely High Frequency (EHF)

Table 5.1

Carrier frequencies and their common designations

Application

Carrier Frequencies

AM radio

526.5 – 1606.5 kHz

FM radio

88 – 108 MHz

HF CB and marine radio

26.965 – 27.980 MHz

Remote garage door openers

304 MHz

VHF television

45 – 230 MHz

UHF television

520 – 820 MHz

Mobile telephones

820 – 915 MHz

Microwave links

3 – 30 GHz

Satellite links

Various between 1 GHz – 100 GHz

Table 5.2

Carrier frequencies of some common applications

The carrier frequency is often used to designate different radio stations. For example, Triple J (FM) in Newcastle uses a carrier frequency of 102.1 MHz. 2NUR-FM (the radio station at the University of Newcastle) uses a carrier frequency of 103.7 MHz. ABC Local radio in Newcastle (on the AM band) uses a carrier frequency of 1233 kHz. Having established a carrier wave as a means of transporting a message, we now need to consider how to ‘piggyback’ our message onto the carrier. Figure 5.13 shows a sample information (or message) signal, together with a carrier wave.

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Message signal

Carrier wave Figure 5.13 Modulating wave and carrier wave – the inputs to a modulator

Amplitude modulation Basic technique of amplitude modulation In amplitude modulation we vary (or modulate) the amplitude of the carrier wave. Figure 5.14 shows the amplitude of a carrier wave being modulated by a triangular signal. Notice that the frequency of the carrier wave is constant, but that the amplitude of the carrier varies to reflect the triangular modulating signal. Message signal

Amplitude modulated wave Figure 5.14 Amplitude modulation of a carrier wave by a triangular signal

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Figure 5.15 shows a voice-modulated AM wave. The voice signal has a frequency of around 100 Hz. The AM wave has a carrier frequency of 1 233 kHz. The ratio of these two frequencies means that there are about 12 000 cycles of the carrier wave for each cycle of the modulating wave. Hence we cannot see the individual cycles of the carrier in this figure – we can only see the outline, or the envelope, of the modulated carrier.

Figure 5.15 Oscilloscope display of a carrier wave being amplitude modulated by a voice signal – this signal is actually from an AM radio broadcast

One way that the modulation process can be achieved is by multiplying the carrier wave with the message signal as shown in figure 5.16. Carrier wave

Amplitude modulated signal

Modulating wave Message signal) Figure 5.16 Multiplication of a carrier wave by a message signal to give an amplitude modulated (AM) signal – in this case the modulating signal is a digital waveform

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Demodulation is the process of separating the message signal and the carrier wave at the receiving end. The principle advantage of AM systems is the simplicity with which the received signal can be demodulated. This is a very significant factor … We only need one modulator to transmit an AM broadcast, but we need many demodulators to receive the signal (one inside each radio receiver picking up the broadcast). Hence the cheaper we can make demodulation, the more people will be able to afford to receive the AM broadcasts. This aspect was a prime benefit of AM transmission before low cost electronic technologies were evolved. The trade-off for relatively low cost AM transmission is the quality of sound. Because the envelope of an AM signal is always varying, the average amplitude is significantly less than the peak amplitude. This means that the signal to noise ratio (SNR) is less than could be achieved if the carrier amplitude was constant. AM demodulation can be achieved with a very simple ‘envelope detector’ circuit shown in figure 5.17. A diode is used to select only one half of the amplitude modulated signal (in this case, the positive half). A resistor and capacitor together form a smoothing filter that extracts the envelope of the signal from the modulated carrier. The envelope is the original message signal. Signal rectification

AM signal Smoothing filter Diode

Baseband signal

Envelope detector Figure 5.17 Simple envelope detector circuit for demodulation of AM signals – the incoming AM signal is rectified to extract the positive half only, and then smoothed to extract the envelope of the signal

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AM applications Amplitude modulation is used primarily for speech communication via radio. The simple modulation and demodulation processes provide low cost receivers. Radio broadcasting (on the AM stations) and CB radios, as shown in figure 5.18, both use amplitude modulation techniques.

Figure 5.18 A low cost CB radio that uses amplitude modulation to convey a message over a 27 MHz carrier

Frequency modulation Basic technique of frequency modulation As the name might suggest, frequency modulation involves varying the frequency of the carrier wave in order to convey information. Figure 5.19 shows the frequency of a carrier wave being modulated by a triangular signal. Notice that the amplitude of the carrier wave is constant, but that the frequency of the carrier changes with time. The frequency of the carrier is lower where the message signal amplitude is low, and the carrier frequency is higher where the message signal amplitude is higher. Message signal

Frequency modulated wave Figure 5.19 Frequency modulation of a carrier by a triangular signal

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AM has the inherent disadvantage that the average amplitude of the resulting modulated carrier is always less than the peak amplitude, thus reducing the signal to noise ratio. FM techniques overcome this problem since the amplitude of the FM signal is always at its maximum value. That is, the average signal to noise ratio (SNR) of FM transmission is superior to that of AM. Unlike AM which uses a single carrier frequency, FM modulation uses frequencies at and around the nominal carrier frequency. This means that an FM signal occupies a greater bandwidth than does an AM signal. This wider bandwidth means that FM offers greater channel capacity than is available with AM techniques. The circuitry required for modulation and demodulation of FM signals is generally more complicated, and hence more expensive, than those circuits used for amplitude modulation. This factor limited the widespread adoption of FM broadcasting for a number of years in Australia until more cost effective electronic circuits could be devised and manufactured.

FM applications Frequency modulation is used for speech, music and data transmissions, primarily using radio links. Radio broadcasting using FM techniques provides a better sound quality than is obtainable from AM broadcasting. Television stations also use FM techniques to broadcast the soundtrack to the programs. The carrier frequencies used for television sound are very close to the carrier frequencies used for the vision signal.

Phase modulation Phase modulation, or PM, varies the instantaneous phase (or angle) of the carrier wave to represent the message. PM can be and is used, though primarily only for specialised applications. A PM signal has a constant amplitude waveform like FM, but requires less bandwidth.

Turn to the exercise section and complete exercise 5.7.

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Digital modulation techniques The modulation schemes described above are analogue or continuous wave modulation techniques. That is, both the carrier wave and the message signal are analogue signals. In many applications we want to be able to transmit digital data over a modulated link. Digital modulation techniques modulate an analogue carrier with a digital signal in order to transmit digital data. The concept is surprisingly easy to understand having already seen analogue modulation techniques. The terms ‘Amplitude Shift Keying’ (ASK), ‘Frequency Shift Keying’ (FSK) and ‘Phase Shift Keying’ (PSK) are used to describe the digital variants of AM, FM and PM respectively.

Amplitude Shift Keying Amplitude Shift Keying involves switching the carrier wave between two different amplitudes. In its most simple form, we simply have full amplitude and zero amplitude as the two levels. Figure 5.20a illustrates a binary ASK waveform. In more sophisticated systems, multilevel ASK is used. This allows us to represent more than one bit of information for each signaling level. For example, if we use four different amplitude levels, we can encode two bits of information in each signaling interval. Eight amplitude levels allows three bits of information per signaling interval.

Frequency Shift Keying In Frequency Shift Keying we use two or more different carrier frequencies to convey digital data. When we have two frequencies we can arbitrarily assign one frequency to a logic ‘High’ and the other frequency to a logic ‘Low’. If we use four different frequencies we can represent two bits of information per signaling interval. Figure 5.20b illustrates a binary FSK waveform.

Phase Shift Keying In Phase Shift Keying we use different phases of the carrier wave to convey digital data. For binary digital data, we simply invert the phase of the waveform at each transition between logic levels. Figure 5.20c illustrates a PSK waveform.

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Message signal

Amplitude Shift Keying

Frequency Shift Keying

Phase Shift Keying Figure 5.20 Digital modulation techniques: a Amplitude Shift Keying (ASK); b Frequency Shift Keying (FSK); c Phase Shift Keying (PSK)

More sophisticated keying schemes In practice, we often use several keying schemes concurrently. For example, the combination of ASK and PSK is commonly used in modem and fax machines. The various modem standards such as V.33, V.34 and V.90 specify how many combined amplitude and phase keying levels are to be used to represent digital data. Table 5.3 shows the maximum data rate, the number of different amplitude/phase levels, and the number of bits of information represented in each signaling interval, for a number of modem standards.

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Telecommunications engineering

Standard

Data Rate

Number of different ASK/PSK levels

Number of bits per signaling interval

V.32

9,600 bits per sec

32

5

V.33

14,400 bits per sec

128

7

V.34

28,800 bits per sec

V.90

56,000 bits per sec

Table 5.3

Common standard modem specifications

Turn to the exercise section and complete exercise 5.7.

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Transmission media In this section, we look at some of the common types of transmission media used in telecommunications, and identify their advantages and disadvantages. However, before looking at particular examples of transmission media it is helpful to consider the electromagnetic spectrum from which all signals are derived.

The electromagnetic spectrum The electromagnetic spectrum describes all of the frequencies used for telecommunications. We saw previously, in table 5.3, that various frequency ranges are given particular names. For example, HF describes the High Frequency band from 3 to 30 MHz. UHF represents the band from 300 MHz to 3 GHz. In Australia, and indeed around the world, the electromagnetic spectrum is divided into various bands for particular applications. These applications include: •

aeronautical navigation

•

broadcasting

•

land mobile transmissions

•

maritime mobile

•

meteorological aids

•

radio navigation

•

satellite communications

•

amateur radio.

The administration and supervision of broadcasting in the electromagnetic spectrum is very closely monitored by government authorities. It is illegal to transmit in bands for which you are not licensed. With increasing demand for access to telecommunications systems, available space in the electromagnetic frequency spectrum is becoming a scarce resource. One way we can ‘create’ more frequency spectrum is by using increasingly higher frequencies. This in turn brings its own technological problems. Another way to address the problems of limited resources is to make better use of the currently used bands. Essentially this means squeezing

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Telecommunications engineering

more users into the available frequency bands. This is where techniques for reducing the required bandwidth for a signal become important. You may have heard of discussions both here in Australia and overseas about auctioning off parts of the spectrum to the highest bidder. Radio broadcasting and mobile telephones are probably the two most common areas where new operators are seeking to gain access to the allocated frequency bands. Literally billions of dollars have been paid for nothing more than the right to use certain frequencies! The electromagnetic spectrum has become a commercially valuable commodity! (Who ‘owns’ the frequency spectrum anyway?) If you have access to the Internet visit the following sites: for more details regarding the allocation of the radio frequency to view a chart of the different frequencies and their applications (accessed 04.12.01). The reference pages of electronic store catalogues (such as Dick Smith and Jaycar) also contain descriptions of the broadcast bands. Figure 5.21 shows a simple overview of the electromagnetic spectrum. Band

VLF LF

MF HF VHF UHF SHF EHF

Frequency (Hz) 104 105 106 107 108 109 1010 1011 1012 1013 1014

Television

Music

Mobile phones

Radio

Speech

AM radio

FM radio

Twisted pair cable Mains wiring

Optical fibres

Microwave Satellite

Infrared

Coaxial cable Waveguides

Wavelength

105 104 103 102 101 100 10–1 10–2 10–3 10–4 10–5 10–6

Figure 5.21 Overview of the electromagnetic spectrum

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Classification of media In practice, transmission media can be divided into two broad groups: guided and unguided (or free space) media. Guided media, as the name implies, refers to transmission media that carry signals along a conductor. Examples include telephone wires and optical fibres. Guided media share the common characteristic of requiring a physical link between the sending and receiving ends of the communications link. That is, a cable must physically connect the two ends. This is not a problem over short distances, but can result in expensive infrastructure across long distances or difficult terrain. Unguided (or free space or wireless) media transmit their signals through air and/or space. The signals are not guided by any physical conductor. Unguided media have the obvious advantage of not needing a physical connection, and hence are very suitable for communications over water or difficult terrain and long distances.

Guided media Cables are the principal media used for guided transmission. Cable types used include twisted pair cable, coaxial cable and optical fibres. Another medium used for guiding electromagnetic radiation is called a waveguide. The principal characteristics that distinguish various types of guides include:

32

•

bandwidth – we have seen that the capacity of a channel to convey information is directly proportional to the channel bandwidth.

•

signal attenuation – signals travelling along a cable are subject to attenuation, or reduction in amplitude. This attenuation results from current flowing in the cable interacting with the resistance of the cable resulting in some power loss in the signal. We have already seen that the capacity of a channel is dependent on the signal amplitude.

•

interference – noise is induced in a cable from electromagnetic interference from external sources. Increasing noise in a cable decreases the signal to noise ratio, and hence reduces the channel capacity of the cable.

•

cost – quite clearly everyone tries to reduce the cost of their projects. In telecommunications we try to use the lowest cost cable that still performs to the required specifications.

Telecommunications engineering

Twisted pair cable A twisted pair cable is simply two parallel insulated copper wires that are twisted together in a spiral pattern. The interweaving is done to help reduce the electrical noise that is induced into the cable from external sources. In many instances, multiple pairs of wires are bundled together into a single cable, and encased by a tough protective sheath. Such a bundle might contain hundreds of pairs. Twisted pairs are probably the most common medium in use today. They are used for both analogue and digital signalling, and are used extensively in the telephone network, principally for the final stage of connections to homes. Twisted pair cables can accommodate frequencies up to around 1 MHz, or even up to 10 MHz for short distances. Attenuation rates are of the order of 3 dB per kilometre, equating to a reduction of signal amplitude of 0.5 for each kilometre of cable. Repeater stations, used to boost the signal levels after attenuation, are spaced at around 2 km intervals. While the twisting does reduce susceptibility to interference, twisted pairs have only moderate immunity to interference. For some special applications, an electrical shield consisting of a thin metal foil is placed around each twisted pair to further improve noise immunity. Such cables are known as shielded twisted pairs. Twisted pair cabling is relatively easy to work with, and is quite inexpensive.

Coaxial cable A coaxial cable is comprised of two concentric conductors separated internally by an insulating dielectric material. The concentric construction means that coaxial cables are much less susceptible to external noise sources than are twisted pairs. Coaxial cables can accommodate frequencies up to around 100 MHz, and up to 1 GHz for shorter distances. As such, they are much better suited to carrying modulated signals (such as television) and high speed digital signals than are twisted pairs. Attenuation rates for coaxial cables tend to be higher than for twisted pairs. Typical values are of the order of 7 dB per kilometre. Repeater

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stations spacings depend on the frequencies used, and can vary from between 1 km to 10 km. Coaxial cabling is moderately easy to work with, and is moderately inexpensive. Special (though inexpensive) tools are required to properly join and connect cables. Figure 5.22 shows examples of various types of cables.

Figure 5.22 Examples of cable types, including from top to bottom: optical fibre with terminating connector; coaxial cable; shielded twisted pair, mains cable

Optical fibres Optical fibres are simply strands of glass, surrounded by cladding and sheaths to contain the light in the core, and to protect the delicate fibre from breakage. They are lighter and thinner than twisted pair and coaxial cables. Often, many fibres are bundled together in much the same way as twisted pairs. The additional infrastructure cost of running multifibre cable is more than offset by the additional capacity provided by the additional fibres. Optical fibres have excellent immunity to induced noise. Because optical fibres only conduct in and near the visible light spectrum, interference from other parts of the spectrum (such as radio waves and 50 Hz power supply noise) is not a problem. The only source of interference is from other light entering the cable – and this is unlikely since the fibre is encased in an opaque sheath.

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Optical fibres have a frequency range of between 180 and 370 THz (TerraHertz, that is, 10 12 Hz). This results in a very large useable bandwidth, and hence immense channel capacity. The attenuation of signals in optical fibres (around 0.5 dB per km) is significantly less than that found in coaxial and twisted pair cables. This results in less need for repeater stations. The only significant disadvantage of optical fibres is the difficulty of terminating and repairing damaged cables. Such tasks require the use of expensive equipment. For this reason, installation of optical fibres into individual homes and workplaces is not (yet) an economically viable proposition. Optical fibres are used almost exclusively for digital applications. Signalling is almost exclusively based on a binary (on-off) system.

Mains wiring While mains wiring may not obviously be part of a telecommunications system, it is used for some specific applications. Perhaps the most common of these is to control the switching of off-peak hot water systems. You might know that off-peak hot water systems are used to distribute the load on the power grid so that the generating stations can run closer to constant load conditions at all times. In order to be able to control the load on the power stations, we need a way of switching hot water systems on and off. The most common method for controlling the switching is called ‘audio frequency injection’ or ‘ripple control’. This technique involves sending coded pulses of audio frequency signals along the power lines. The signaling frequencies used – around 200 to 300 Hz – are in the audio frequency range, hence the name ‘audio frequency injection’. Different frequencies are used by different distribution authorities so as not to interfere with one another. This scheme is actually Amplitude Shift Keying. The carrier wave is a 200 – 300 Hz tone, while the message signal is the coded signal to tell particular hot water systems to turn on or off. A frequency sensitive detector in the electrical switchboard is tuned to the particular frequency used, and operates a switch supplying power to the hot water system. You may have heard some strange noises coming from electrical appliances in your home late at night and very early in the morning. These times correspond to the times that off-peak hot water are turned on and off respectively. The noises you can hear are the coded pulses

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flowing in the various appliances in your home. Some appliances are more susceptible to emit the audio signals than others. Figure 5.23 shows a ripple control unit mounted in a domestic switchboard.

Figure 5.23 Ripple control unit on the right hand side is designed to detect and interpret audio frequency signals injected onto mains wiring to control an off-peak hot water system

Waveguides Waveguides are a special type of 'cable' used for high frequency signals. You might recall from our discussion of power line conductors in Personal and public transport that electrical current has a tendency to concentrate close to the surface of conductors, and not to be uniformly spread across the cross-section of the material. This effect was termed the ‘skin effect’. The skin effect is dependent on frequency: the higher the frequency, the more tendency there is for the energy to be concentrated on the surface. At very high frequencies, the energy all but escapes from the conductor, and instead propagates in free space around the conductor. A waveguide, as the name might suggest, is simply a means of guiding electromagnetic energy in a particular direction. Typically a waveguide is a rectangular metal tube (hollow in the middle). The dimensions vary according to frequencies being used, but dimensions of tens of centimetres by centimetres are typical.

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Applications of waveguides are quite specialised. Places where they are often found include military and air traffic control radars (have a look next time you are taxiing around Sydney Airport), and high powered high frequency transmitters (such as television towers).

Figure 5.24 Some laboratory waveguide equipment – the device on the left is used for measuring microwave frequency; the horns on the right hand side are antennae used to transmit and receive microwave signals

Table 5.4 summarises the key parameters for the various types of cable. Cable type

Useful distance

Frequency range

Typical applications

Twisted pair

2 km

0 – 10 MHz

baseband signals, telephones, office LANs

Coaxial

1 – 10 km

0 – 1 GHz

modulated signals, high speed digital

Optical fibre

40 km

180 – 370 THz

very high speed digital

Waveguides

10 m

3 – 30 GHz

radar, high power transmitters

Mains wiring

50 km

0 – 400 Hz

ripple control

Table 5.4

Summary of principal cable types used in telecommunications

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Unguided media Unguided media is the general term used to describe communication links that are not physically connected. The term ‘wireless’ is often used in such cases. Wireless links are becoming increasingly popular in a range of applications as a result of the flexibility they offer. For example, infrared links between computers and peripherals eliminate the need for unwieldy cables. Mobile telephones have revolutionised the way we think about personal communications. The principal characteristics used to describe unguided media include: •

frequency – the frequencies used for wireless links range from around 10kHz (for submarine radio communications) to 200 GHz for some satellite communications.

•

range – the effective range of ‘wireless’ links varies from around 10 metres (for infrared and ‘Bluetooth’) technologies, to tens of thousands of kilometres in satellite applications.

•

directionality – some links are designed to be point-to-point (such as in microwave links) and others are used in broadcasting to many receivers.

•

half- or full-duplex communications – ‘half-duplex’ communications refers to a link in which the information flow is in one direction only. A full-duplex link allows information to flow in both directions.

Radio The term ‘radio’ is most often used to describe non-directional transmissions in the spectrum range from 3 kHz to 1 GHz. Both half- and full-duplex systems are used. Applications of radio to telecommunications include: •

AM Broadcasting – The AM broadcast band in Australia extends from 526.5 kHz to 1606.5 kHz. This band is divided into separate sub-bands (or frequency ‘slots’) for each radio station. Each frequency slot has a bandwidth (or range of frequencies) of 9 kHz. This means that the existing AM band can accommodate up to 120 different broadcast stations without replicating frequencies. The 9 kHz bandwidth of each station limits the usable audio bandwidth to the range 100 Hz to 4.5 kHz. (It may not be apparent why this maximum frequency isn't twice as high, that is 9 kHz. The reason is buried in mathematics that will become more apparent when you study telecommunications at university!)

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As we saw previously, this limited bandwidth results in low-fidelity sound that does not reproduce music particularly well, but is still quite satisfactory for speech. Reception range limits vary from 100 km to 1000's of km (the latter in ideal conditions such as on a clear night). Transmitting antennae are generally omnidirectional – that is, they transmit signals in all directions equally. Some AM broadcast transmitters do have some directionality – this is done to broadcast most of the signal power to areas of highest populations, and to avoid wasteful broadcasts over unpopulated areas. Directional broadcasting is achieved by having multiple antennae – for AM broadcasting this is usually two antennae, located perhaps 50 metres to 100 metres apart. Figure 5.25 shows a directional antenna used in the Newcastle area. Note the two towers used. (The two antennae are actually vertical – the apparent lean on the towers is caused by the camera lens!) Receiving antennae (most often inside the radio receiver) are usually quite insensitive to direction.

Figure 5.25

•

AM antenna comprised of two towers to offer some degree of control of signal transmission direction

FM Broadcasting – The FM broadcast band in Australia extends from 88–108 MHz. The higher frequencies used by FM broadcasting in comparison to AM broadcasting result in shorter wavelengths, and hence an

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increased susceptibility to interference from solid objects. You may have noticed a tendency for FM stations to drop out (or become unclear) when driving around in a car. This occurs because objects (buildings, hills, trees) block the direct line between transmitter and receiver from time to time. The FM band is divided into separate sub-bands (or frequency ‘slots’) for each radio station. Each frequency slot has a bandwidth of 200 kHz, and can accommodate an audio soundtrack of 50 Hz to 15 kHz. This wider range of audio frequencies provides a higher fidelity sound than is obtained from AM broadcasts. The channel capacity of an FM broadcast link is greater than for an AM link due to two factors: 1

The SNR of FM is greater than for AM (assuming equal amounts of noise power) because the FM waveform is always at the maximum amplitude, whereas the amplitude of an AM signal is variable.

2

The bandwidth of the FM broadcast link is 200 kHz compared with the 9 kHz bandwidth of the AM link.

Transmitting antennae are usually omnidirectional. Receiving antennae are moderately directional (more so than their AM counterparts) – reception can often be improved by adjusting the aerial. •

Television broadcasting – These systems operate in the 45–230 MHz (for VHF) and 520–820 MHz (for UHF) bands. The range of reception varies from several kilometres (in hilly terrain) to hundreds of kilometres (in flat terrain). Television broadcasting differs from radio broadcasting most noticeably in the type of receiving antennae required. Television receiving antenna are highly directional – it is often difficult to get any picture at all unless your aerial has been tuned to the correct direction.

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Telecommunications engineering

Figure 5.26 Television receiving antenna – this antenna is highly directional, and must be pointed towards the signal source for best results

•

Mobile communications – These include air-to-air and air-to-ground, marine and terrestrial mobile radio. All are full-duplex allowing communication in both directions. Nondirectional antennae are most often used to maintain useful performance for the system while the vehicle orientation changes. A broad range of frequencies are used including MF, VHF and UHF ranges. Mobile telephones are quickly becoming the dominant example of this type of link.

Figure 5.27 Two way radio used for mobile communications

•

Navigation systems – Airborne and coastal navigation systems use a variety of radio beacons to determine position. A broad range of frequencies are used including VLF, LF, VHF and UHF ranges. These systems are rapidly being superceded by global positioning systems (GPS) technologies.

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Microwave The term ‘microwave’ is generally used to refer to highly directional signals in the SHF and EHF ranges (2–40 GHz). At these frequencies, the electromagnetic waves have wavelengths of metres down to centimetres, and hence can be focussed into narrow beams using parabolic dishes. (Lower frequencies are much harder to focus because their longer wavelengths require impractically large dishes.) The short wavelengths also result in susceptibility to adverse weather conditions: at frequencies above 10 GHz the relative size of rain droplets is sufficient to cause signal degradation. Microwave links are used both for full-duplex point-to-point communications and for half-duplex one-to-many transmissions. Both applications require line-of-sight between transmitter and receiver. The full-duplex point-to-point links are usually between towers located on prominent landmarks. The dishes at either end require precise alignment to ensure maximum signal power (and hence channel capacity). Applications of such links include high volume digital telephony and data transmissions. The half-duplex links are most often used for delivery of (pay-) television. In these applications a transmitting tower broadcasts a semifocussed beam across a region. Receiving dishes are aligned towards the transmitting tower. Such systems offer an attractive (economical) alternative to ‘cable television’ which requires expensive ‘cable roll-out’ to connect all subscribers.

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Figure 5.28 Microwave dishes used for point-to-point transmissions mounted high on a telecommunications tower – the covers over the dishes are in place to prevent birds and insects from nesting in the focal point of the parabolic dish

Satellite communications While satellite communications are often thought to be a specialised form of telecommunications, they are in fact simply a microwave link that uses an orbiting ‘tower’ to extend the line-of-sight. The satellites may be geostationary, meaning that they appear stationary above the earth (they in fact orbit the earth at the same rate that the earth spins on its axis), or they may be in low earth orbits meaning that they move across the sky relative to the earth. Geostationary satellites are further away from the earth than orbiting satellites meaning that their transmission delays are longer. On the other hand, it is possible to point a fixed satellite dish at a geostationary satellite and maintain good reception, whereas a low earth orbiting satellite needs to be tracked across the sky. The satellite receives the land-to-space (up-link) signal, reconstructs the signal to eliminate accumulated noise, and then retransmits the signal to earth (downlink) using a different frequency.

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If the same frequency was used in both uplink and downlink, the two signals would interfere with each other. For this reason, satellite links are characterised by having separate uplink and downlink frequencies. Frequencies used for satellite communications are a subset of those used for terrestrial microwave. Useful frequencies are bounded at the lower end by land based sources of interference that become too prominent. Above 10 GHz the earth's atmosphere tends to significantly attenuate the microwave signals. Satellite systems can be used to provide one-to-one or one-to-many communications. Figure 5.29 illustrates both situations.

Point to point satellite link Up link

Down link

Satellite broadcasting

Figure 5.29 Satellites used to provide point to point and broadcast facilities

Infrared Infrared signals are normally defined to be those just below the visible spectrum. Typical wavelengths used are 880 to 950 nm (nanometres). Transmission is by line-of-sight, and useful range is up to 10 metres. Most applications use a simple half-duplex system. Applications requiring bidirectional data transmission usually achieve this by using two half-duplex systems.

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The relatively low cost of infrared transmitters and receivers has popularised their use in many and varied situations. Remote controllers for television, video and audio systems, cordless connections between computers and peripherals and security applications are now relatively common.

Figure 5.30 A common application of infrared transmission – in this case a remote controller from an audio system

Turn to the exercise section and complete exercise 5.8.

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Evaluating telecommunications systems In the preceding work we have learned about a number of telecommunications principles, including: •

analogue and digital signals

•

the effects of bandwidth and signal to noise ratio on channel capacity

•

images for transmitting information

•

modulation techniques (to enhance signal propagation)

•

transmission media, including guided and unguided forms.

We can now look at any telecommunications system and evaluate them in terms of the above parameters. In this way we appreciate why some systems work better than others, why some systems cost more than others, and how telecommunications systems might evolve in the future.

Analogue and digital data and systems We have seen analogue and digital signals, and have also seen analogue and digital transmission systems. There are four combinations: •

analogue data over analogue links

•

analogue data over digital links

•

digital data over analogue links

•

digital data over digital links.

Table 5.5 summaries the various combinations, and gives examples of each. Analogue transmission

Digital transmission

Analogue Data

Signals are sent at baseband or modulated to higher frequency using AM or FM. Example: radio and television broadcasting.

Sample and quantise analogue signal to form digital data. Example: digital mobile telephone.

Digital Data

Use Shift Keying methods (ASK, FSK, PSK) to modulate an analogue carrier. Example: computer modem connected to telephone system.

Data formed into packets and sent using digital signalling. Example: local area computer networks (LANs).

Table 5.5

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Combinations of analogue and digital data and transmission

Telecommunications engineering

Describing a telecommunications system Modern telecommunications systems are very complex. Invariably, a particular application (such as a mobile telephone, a fax machine or television transmission) will consist of many subsystems. As such, it is difficult to answer simplistic questions such as ‘Describe how a mobile telephone works.’ To answer this completely and correctly, we would need to identify every link in the system, and describe each according to its key parameters. Another approach to describing telecommunications applications is to identify what characteristic or characteristics are unique to the particular application and differentiate it from other similar or related systems. For example, a mobile telephone shares many characteristics with a fixed telephone: they are both used to convey analogue signals in the range from 300 Hz to 3 500 Hz from one handset to another. Mobile telephony differs from fixed telephony in that a mobile phone uses a modulated carrier at microwave frequencies across an unguided medium to link it to the fixed telephony network, whereas a fixed telephone uses a twisted pair cable at baseband frequencies to link it with the fixed telephone network. The fact that most mobile telephones convert the analogue voice signal to a digital signal before transmission does not necessarily distinguish mobile telephones from fixed telephones: a fixed telephone could also convert its analogue input into a digital signal for communication with the network. To assist with understanding and categorising telecommunications systems, it is sometimes insightful to ask the following questions: 1

Is the information speech/music, text/numerics, images and/or video?

2

Is the information analogue or digital?

3

Is the transmission from point-to-point, or is it broadcast to many?

4

Is the device in question part of a much larger and commonly used system? In some cases we can investigate the technicalities more closely.

5

How many different links are used between sending and receiving ends? For each link: –

Is the link between sending and receiving ends analogue or digital?

–

Is the link a guided or unguided medium?

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–

What medium is used?

–

Is modulation used to propagate the signal?

The specific answers to these latter questions are often not obvious to the casual observer. Only an engineer or technician involved in that particular industry would be able to respond with certainty. Let us try an example.

Facsimile machine

Figure 5.31 Facsimile machine that transmits digitised images over a telephone line

1

Is the information speech/music, text/numerics, images and/or video? A facsimile machine is used to transmit text and images from a printed page. The text, however, is treated as though it were an image, and is not transmitted as a series of characters or symbols. So to be correct, we should say that only images are sent.

2

Is the information analogue or digital? The images are treated as continuous or analogue signals. The fax machine converts the analogue image into digital data using sampling and quantisation. The sampling is at around 150 dpi, and the quantisation is monochrome (black and white).

3

Is the transmission from point-to-point, or is it broadcast to many? The transmission is point-to-point. (A mass faxing to many recipients is actually handled as many separate fax transmissions.)

4

Is the device in question part of a much larger and commonly used system? A facsimile machine is part of a much larger telephone system. The image is digitised, and audio tones are used to represent the digital

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Telecommunications engineering

information. These tones are then handled by the telephone system as if it were a telephone call. 5

How many different links are used between sending and receiving ends? At least several, often many. There is a link from the facsimile machine to the exchange, from exchange to exchange, and then from exchange to receiver. Between exchanges there could be additional links. –

Is the link between sending and receiving ends analogue or digital? The link to the exchange is analogue. Links between exchanges are most often digital.

–

Is the link a guided or unguided medium? What medium is used? The link from machine to exchange is guided via twisted pair cable. Between exchanges the link is likely to be by optical fibres or microwave links.

–

Is modulation used to propagate the signal? The link to the exchange uses audio tones to represent digital data. A combination of Amplitude and Phase Shift Keying is used. The optical fibre also uses Amplitude and Phase Shift Keying to modulate a light source. The microwave link uses a combination of Amplitude and Phase Shift Keying to modulate a microwave carrier.

Colour television

Figure 5.32 Colour television used to receive broadcast video and sound signals

1

Is the information speech/music, text/numerics, images and/or video? The information received is video and sound. The video is in RGB colour.

2

Is the information analogue or digital?

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Both the vision and sound are analogue signals (though soon to be replaced by digital television through the next decade!). 3

Is the transmission from point-to-point, or is it broadcast to many? The television signal (video and sound) is broadcast to many receivers.

4

Is the device in question part of a much larger and commonly used system? The television broadcast network is virtually stand-alone. There is a small market for teletext information systems that are ‘piggy backed’ onto the television broadcasts. Television receivers are usually used for the sole purpose of receiving television broadcasts.

5

How many different links are used between sending and receiving ends? There are generally two or three links in the transmission system: –

from studio to broadcast transmitter, or

–

from studio to relay tower (to relay tower …) to transmitter, and

–

from transmitter to receiving antenna.

We will consider only the final transmitter to receiver link. –

Is the link between sending and receiving ends analogue or digital? The transmitter broadcasts two analogue signals to receivers: one for vision, and a separate signal for sound.

–

Is the link a guided or unguided medium? The transmitter to receiver is via unguided VHF or UHF transmissions.

–

What medium is used? The signals are broadcast through the air.

–

Is modulation used to propagate the signal? The broadcast signal uses Amplitude Modulation for the vision, and Frequency Modulation for sound.

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Telecommunications engineering

Internet browsing via a modem

Figure 5.33 Modem used to connect a digital computer to analogue telephone lines

1

Is the information speech/music, text/numerics, images and/or video? Internet browsers access all types of information.

2

Is the information analogue or digital? All of the sources of information are digital, or are digitised to be distributed and interpreted by computers. The original sources of information are a mixture of analogue (music, voice, images) and digital (text, numerics).

3

Is the transmission from point-to-point, or is it broadcast to many? The communication links are point-to-point between the web server and the browser.

4

Is the device in question part of a much larger and commonly used system? The modem invariably uses the telephone system to connect the browser to the service provider. The service provider is connected to the ‘computer network’ by high speed shared or dedicated telephone lines. The ‘computer network’ is a very large interconnection of web servers spread across the world.

5

How many different links are used between sending and receiving ends? Many! Indeed it is this aspect that characterises the internet. Its flexibility, size and scope is a function of the large number of interconnected machines. It is likely that there is no other application of telecommunications that uses more and varied links to achieve its purpose! For the purposes of illustration, let us consider only the modem connection to the telephone network. –

Is the link between sending and receiving ends analogue or digital? The link between the modem and the telephone system is an analogue connection, designed originally for voice communication (300 Hz to 3 500 Hz).

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–

Is the link a guided or unguided medium? What medium is used? The link is a (guided) twisted pair.

–

Is modulation used to propagate the signal? The data in and out of the computer is digital (binary). The modem is used to interface the digital computer to the analogue line. The term ‘modem’ is actually an abbreviation of ‘modulator – demodulator’. Its function is to encode the digital data from the computer into audio tones for transmission across the telephone network. A combination of amplitude and phase shift keying is used. By using combinations of amplitude and phase modulation, we effectively create a multilevel signal (like that shown in figure 5.2) that can transmit two or more bits of information at a time. The precise format of the modulation used is dependent on the modem standard being used. For example, V.34 and V.90 use different levels and combinations of amplitude and phase shift keying schemes to represent data.

While we are talking about modems, we might ask "What causes the 'beeping' that is heard when a modem first connects to the service provider"? The modem connects to the service provider by dialing the provider's telephone number. In this situation, the modem is behaving as though it were a telephone. The various tones that are heard are used to signal the keys (numbers) on the dialing keypad. Each key is represented by the combination of two audible tones. Table 5.6 sets out the tones used for each symbol. Tone

1209 Hz

1336 Hz

1477 Hz

697 Hz

1

2

3

770 Hz

4

5

6

852 Hz

7

8

9

941 Hz

*

0

#

Table 5.6

Tones used in tone dialed telephone systems

When a particular key is pressed, the corresponding two tones are sent down the line. Since the two tones are at different frequencies, we can

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Telecommunications engineering

distinguish between them at the receiving end, even though they are transmitted concurrently. You might also hear the same tones when you use an ordinary fixed telephone handpiece. The system is called DTMF, or Dual Tone Multiple Frequency dialing.

Figure 5.34 Telephone handset that uses Dual Tone Multiple Frequency (DTMF) dialing, giving a series of 'beeps' at various frequencies as it dials the numbers

Turn to the exercise section and complete exercise 5.9.

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Telecommunications engineering

Exercises

Exercise 5.1 Describe the two main differences between analogue and digital signals. 1

_______________________________________________________ _______________________________________________________

2

_______________________________________________________ _______________________________________________________

Exercise 5.2 Explain why a radio transmitter for an AM broadcast station is much larger than the antenna on a mobile telephone. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Exercise 5.3 Why does digital signaling usually perform better than analogue signaling in a noisy communications channel? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________

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Exercise 5.4 Give four reasons why telecommunications systems are increasingly becoming digital rather than analogue. __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ Exercise 5.5 Explain why you would either (a) double the bandwidth, or (b) double the signal amplitude if you had money to spend to improve the channel capacity of a telecommunications link. __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ Exercise 5.6 a

Give a reason why Amplitude Modulation is preferable to Frequency Modulation for radio broadcasting? _______________________________________________________ _______________________________________________________

b

Give a reason why Frequency Modulation is preferable to Amplitude Modulation for radio broadcasting? _______________________________________________________ _______________________________________________________

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Telecommunications engineering

Exercise 5.7 Describe three ways in which digital information can be represented as an analogue signal. 1

_______________________________________________________

2

_______________________________________________________

3

_______________________________________________________

Exercise 5.8 State one advantage in favour of using each of the following media: a

twisted pair cable

___________________________________________________________ b

coaxial cable

___________________________________________________________ c

optical fibre

___________________________________________________________ d

AM radio

___________________________________________________________ e

microwave links

___________________________________________________________ f

satellite links.

___________________________________________________________

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Exercise 5.9 Select the alternative a, b, c, or d that best completes the statement. Circle the letter. 1

2

3

4

5

6

58

An analogue signal: a

is one that has an infinite number of possible values of amplitude

b

can be sampled at any instant in time

c

can be represented by a continuous graph of amplitude plotted against time

d

all of the above.

A digital signal: a

is what we hear from a digital mobile telephone

b

has only two possible amplitudes

c

should only be evaluated at the correct instants in time

d

is a gesture made by raising any number of fingers.

A multilevel signal: a

has only two possible values, but at any voltages

b

can represent two or more bits of information in one signaling interval

c

is obtained from a binary signal by transmitting bits more quickly

d

is the result of faulty equipment.

A binary digital signal generally occupies: a

less bandwidth than a single sinusoidal signal

b

more bandwidth than a single sinusoidal signal

c

the same bandwidth as a single sinusoidal signal

d

the house next door to the mad scientist with the yellow parrot.

Electrical noise is present in all electrical circuits, but: a

we can ignore it

b

we can turn it down

c

we can overcome its effects in any circuit

d

we can overcome its effects in some instances.

The bandwidth of a telecommunications link: a

tells us how large the microwave dishes have to be

b

describes the range of frequencies that it can propagate

c

increases with antenna height

d

is generally insignificant in telecommunications engineering.

Telecommunications engineering

7

8

9

The capacity of a telecommunications link is dependent on: a

the number of computers connected to it

b

the height of the towers supporting the microwave dishes

c

whether it transmits analogue or digital data

d

the bandwidth and signal to noise ratio.

The dynamic range of a signal is: a

the difference between the minimum and maximum amplitudes

b

the distance it can travel in free space

c

the speed it can travel in an optical fibre

d

all of the above.

The size of a stored digital image is: a

determined by the resolution (in dpi) and the number of pixels used

b

determined by the number of pixels and the number of colours used

c

determined by the resolution (in dpi) and the number of colours used

d

the size of the computer's hard disc.

10 Modulation involves: a

sampling and quantisation

b

using different frequencies for radio stations

c

eliminating noise

d

varying a parameter of a carrier wave according to a message signal.

11 Modulation is used to: a

increase the bandwidth of a signal

b

increase the amplitude of a signal

c

increase the signal to noise ratio of a signal

d

allow the modulated signal to propagate more efficiently through a given medium.

12 Amplitude modulation: a has a lesser signal to noise ratio than Frequency Modulation b is relatively cheap to generate and decode c is quite suitable for voice communications d

all of the above.

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13 Frequency modulation: a varies the frequency of the carrier in proportion to the message signal's amplitude b varies the frequency of the carrier in proportion to the message signal's frequency c varies the amplitude of the carrier in proportion to the message signal's frequency d

varies the amplitude of the carrier in proportion to the message signal's amplitude.

14 The electromagnetic spectrum: a

describes the range of frequencies used by all types of electromagnetic communications equipment

b

is a common resource that anyone can use as they please

c

is a telecommunications system that was built by foreign interests

d

is limited by the range of channels on a television receiver.

15 Twisted pair cables: a

are used to scramble digital signals so that they can't be intercepted by a third party

b

result from frequent wiring and rewiring of telephone exchanges

c

are a low cost guided medium used for telephone and low speed digital communications

d

are a suitable replacement for optical fibres in high speed applications.

16 Coaxial cables: a

have a wider bandwidth than optical fibres

b

are constructed from a central glass fibre surrounded by a metal foil shield

c

have a wider bandwidth than twisted pair cables

d

can carry larger signal amplitudes than twisted pair cables.

17 The extremely wide bandwidth of optical fibres is a result of:

60

a

using light instead of electrical current for signaling

b

the very short wavelength of visible light

c

having very good electrical noise rejection

d

a fundamental physical property of the medium.

Telecommunications engineering

18 The guided medium with the highest channel capacity is: a

mains wiring

b

twisted pair cabling

c

coaxial cabling

d

optical fibres.

19 Unguided media: a

are journalists who do as they like

b

is the term used to describe telecommunication links that are not physically connected

c

is the term used to describe telecommunication links that use microwave links

d

is another term for radio communications.

20 The usable range of an unguided medium transmission depends on: a

the number and size of obstacles in the path

b

the power of the transmission signal

c

the amount of noise in the channel

d

all of the above.

21 FM radio broadcasts have a superior sound quality because: a

they are a newer technology

b

they don't broadcast advertisements

c

they have a wider bandwidth and better signal to noise ratios

d

they use higher frequencies and shorter wavelengths.

22 Television receiving antenna are an unusual shape because: a

they are essentially a structural component for a house roof

b

they are designed to receive frequency modulated signals

c

they are fashionable

d

they are highly directional to increase signal to noise ratios.

23 A microwave signal is one that: a

only works in conjunction with amplitude and Phase Shift Keying

b

has a wavelength that allows beams to be shaped by parabolic dishes

c

can only be used between transmitters located on tall towers

d

only works in fine weather.

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24 Satellite communications are:

62

a

a unique type of telecommunications link that uses special signaling methods

b

expensive because of the distance the signals have to travel

c

achieved by reflecting a signal off an orbiting dish

d

is really the same as microwave signaling, but with receivers and transmitters located in space.

Telecommunications engineering

Progress check

In this part you examined the principles that underpin many of the telecommunications technologies currently in use.

✓ ❏

Disagree – revise your work

✓ ❏

Uncertain – contact your teacher

Uncertain

Agree – well done Disagree

✓ ❏

Agree

Take a few moments to reflect on your learning then tick the box which best represents your level of achievement.

I have learnt about: •

telecommunications – analogue and digital systems, modulation, demodulation, radio transmission (AM, FM), television transmission (B/W, colour), telephony (fixed and mobile), transmission media (cable, microwave, fibre-optics) •

satellite communication systems, geostations.

I have learnt to: •

• • •

describe the basic concepts and applications of modulation and transmission systems in telecommunications distinguish the communication bands in the electromagnetic spectrum contrast the differences in transmission media describe the basic principles of satellite communication systems.

Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to for original and current documents.

In the next part you will continue to develop skills in representing objects using freehand and technical drawing, and be given opportunity to develop CAD skills, applying AS1100 drawing standards where appropriate.

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Telecommunications engineering

Exercise cover sheet

Exercises 5.1 to 5.10

Name: _______________________________

Have you have completed the following exercises? ❐ Exercise 5.1 ❐ Exercise 5.2 ❐ Exercise 5.3 ❐ Exercise 5.4 ❐ Exercise 5.5 ❐ Exercise 5.6 ❐ Exercise 5.7 ❐ Exercise 5.8 ❐ Exercise 5.9 Locate and complete any outstanding exercises then attach your responses to this sheet. If you study Stage 6 Engineering Studies through a Distance Education Centre/School (DEC) you will need to return the exercise sheet and your responses as you complete each part of the module. If you study Stage 6 Engineering Studies through the OTEN Open Learning Program (OLP) refer to the Learner’s Guide to determine which exercises you need to return to your teacher along with the Mark Record Slip.

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Telecommunications engineering

Part 6: Telecommunications engineering – communications

Part 6 contents

Introduction ..........................................................................................2 What will you learn?...................................................................2

Communication tools of the engineer..............................................3 Teamwork .................................................................................4 Orthogonal drawing....................................................................5 CAD advantages .......................................................................6 Electronic/electrical component representation ............................6 Technical representation in detail................................................9

Exercises............................................................................................15 Progress check .................................................................................35 Exercise cover sheet........................................................................37 Bibliography.......................................................................................39 Module evaluation.............................................................................43

Part 6: Telecommunications engineering – communications

1

Introduction This communication part of the module will further consolidate and review the communications content presented to you in earlier modules. You will continue to develop your skills in representing objects using technical drawing and freehand communication. You will be given the opportunity to develop your CAD skills, and be asked to consider the relevance of communicating technical information as it might relate to an engineer working in the telecommunication area. You will be expected to apply AS1100 drawing standards where appropriate. You will be expected to complete several exercises in this module part in order to develop and demonstrate your communication skills.

What will you learn? You will learn about: •

freehand and technical drawing, pictorial and dimensioned orthogonal drawing

•

Australian standards AS1100

•

computer graphics, computer assisted drawing (CAD)

•

collaborative work practices.

You will learn to: •

produce orthogonal drawings applying appropriate Australian Standards (AS1100)

•

apply dimensions to drawings to AS1100 standard

•

justify graphics as a communication tool for telecommunications engineering

•

appreciate the value of collaborative working.

Extract from Stage 6 Engineering Studies Syllabus © Board of Studies, NSW, 1999. Refer to for original and current documents.

2

Telecommunications engineering

Communication tools of the engineer

You might recall the notes on communication – technical drawing. An engineer working in telecommunications, like all engineers, will be expected to communicate technical information accurately. The importance of accuracy in communication is critical. So what tools does the engineer have?

You might consider the traditional ‘tools’ such as drawing equipment, but there are a new and emerging array of alternatives. In addition to the ‘hardware and software’ tools you may have thought about, other forms of tools might be considered. The Australian Standards provide a framework to be used as a tool for accurate communication. Without that tool, mis-communication would be a regular occurrence and could easily lead to disaster. The written language and verbal communication are also essential. Engineers are required to write engineering reports and make presentations. List the ‘tools’ available for the engineer to use in order to communicate. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Did you answer? Some of the ‘tools’ incude: • computer links – email, video • telephone, fax, video • conferences, meetings • standards • CAD software.

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Team work All engineers will be required to work collaboratively. Perhaps during the early 1900s some gifted engineers might have completed projects as individuals. In the engineering world we live in, this would now be a rare event. There is much more likely to be a structure in place for a variety of technical groups to work collaboratively on any project. A team approach is a proven technique for attaining the optimum results. In the telecommunication field, it is difficult to imagine one person developing a product or system in isolation. While individuals may well have the inspirational idea, or provide a key breakthrough, it will require a team to turn that idea into a commercial application. Consider the development of one telecommunication product. Make a list of the technical skills that the collaborative team might have needed during the development of the product. The list might start with ‘a great idea’. A great idea – an inspiration – creative thinking Then: __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ Did you answer? Some of the technical skills required may include: • electrical engineering • computer engineering • mechanical engineering • production engineering • technical drafting • marketing and sales.

4

Telecommunications engineering

Orthogonal drawing At this stage of the course you will have read about many AS1100 standards. You would have had the opportunity to test your own skills at applying these standards. You would be aware of the importance of these standards in all engineering fields. The standards are applied in technical drawings completed using drawing instruments or when using a CAD program. Software drawing programs do not necessarily apply drawing standards just as drawing instruments do not apply them. The CAD program is a drawing tool. The user must have acquired technical drawing knowledge if a standards drawing is to be produced. It is important to note that drawing standards do not stop when freehand technique is used. While not all standards can be applied accurately with freehand drawing, the concepts should always be applied if accurate communication is to occur.

Figure 6.1 Instrument and freehand orthogonal drawing

Part 6: Telecommunications engineering – communications

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CAD advantages The engineer working on a design concept is very likely to begin the process with freehand sketches. These freehand sketches are likely to be refined several times before an instrument or CAD drawing is produced. The main advantage of all CAD drawings is that they can be continually modified without the need to start the drawing over again. CAD drawing is not necessarily faster to do, the advantage is much more likely to be the ease of modification and alteration. The CAD drawing is also flexible in that versions can be produced specifically targeted for various needs. For instance, the electrical/electronic content can be removed (not displayed) when presenting specification drawings to the polymer manufacturer involved in moulding the case of the product. Another consideration is the ease of information transfer using CAD. Because the information is stored digitally, it can be transferred electronically. This not only allows nearly instant communication amongst the team in the office, it allows information to be transferred immediately throughout the computer network around the world. With CAD software technology developing rapidly, it has become common place for objects drawn in 2 dimensional orthogonal projection, to be transformed into 3 dimensional images. The 3 dimensional image is able to have its surfaces automatically rendered to selected surface textures. The object can be viewed from any direction and rotated as a moving picture. ‘Walk through’ features in CAD programs allows the spectator to have a ‘real’ three dimensional view of the object even from inside – commonly used on house plan designs or for technical presentations. Now turn to the exercise sheet and complete exercise 6.1.

Electronic/electrical component representation In order to communicate electrical circuitry, electrical and electronic engineers will need to be able to read electrical circuit diagrams and design electrical circuits. You might be aware of basic circuitry diagrams from studying house plans or from looking at electronic magazines. What communication skills are required for electrical drawings?

6

Telecommunications engineering

Like all technical drawing, there are specific standards that apply when drawing circuits. The list of symbols used to represent components in the circuit appear to be endless. However, there is no need for you to be familiar with the vast array of symbols. Concentrate on being able to recognise the more common circuit symbols. Because the field of electrical control is so vast, engineers specialise. Representing the electrical circuit in a building will be significantly different from representing the circuit components used in an electronic control device. There are symbols used when designing the larger system of control devices (switches, cabling, lighting, power outlets, etc.), and there are symbols used specifically when designing electronic components that are attached to a printed circuit board (transistors, resistors, capacitors, etc.). You should develop an elementary knowledge of identifying both electronic circuit board components and electrical system components. A selection of electronic symbols are shown in the following table. Component

Symbol

Resistor Capacitor (non polarised) Capacitor (polarised) Battery

....

Diode

+

Integrated circuit

1 4 2 3

Transistor

+

C B E

Light Emitting Diode

Part 6: Telecommunications engineering – communications

7

A selection of electrical symbols is shown in the following table. Component

Symbol

Switch Light Power outlet

Transformer

Earth or

Fuse Motor

M

or

M ~

o

Now turn to the exercise sheet and complete exercise 6.2. You are required to freehand draw the appropriate symbol for each of the components indicated.

8

Telecommunications engineering

Technical representation of the detail You may recall the following topics from past modules: Civil structures •

Developments In engineering, the design of sheetmetal objects is done using flat surfaces. The flat shapes obtained are folded to form the required object. The method used to create the correct flat shape include parallel development (for simple shapes), radial developments (for cones or pyramids) and dividing the shape into triangular segments (triangulation) when developing transition pieces.

•

True length of lines To develop objects to specified sizes, developments must use true sizes (scaled) rather than apparent sizes which are often created when objects are drawn using orthogonal projection. True lengths are determined using several methods, including the: –

rotation method

–

auxiliary plane method

–

offset method.

Each of these methods relies on the fact that a line will be seen as a true length if it is projected from a line that is parallel to the projection plane. •

Transition pieces The common application of transition pieces is in ventilation ducts. Connection segments are often required to join different shaped ducts. The connection segments are called transition pieces.

•

Representing threads, nuts and bolts Drawing, or representing, threaded components requires many AS 1100 standards to be applied. Representing threaded devices is critical because drawing the actual shape of the thread would be time consuming. Applying AS 1100 standards accurately is essential.

Part 6: Telecommunications engineering – communications

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•

Sectioning Viewing the internal shape of objects is regularly required in order to gain a full understanding of how the components are assembled or for manufacturing detail. Rather than show hidden detail construction, the object is viewed as if cut. Many AS 1100 rules need to be applied when showing objects in section.

Personal and public transport •

Drawing sheet borders, title blocks and 3rd angle logo Students often underestimate the significance of these sheet details. The technical drawer needs to communicate accurately. This is only possible if sheet details are displayed. There are AS 1100 standards to be applied for consistency.

•

AS1100 symbols The use of clear and unambiguous symbols has an advantage over written comments. Symbols reduce drawing time and allow the drawing to cross international borders. A few of the symbols include:

•

–

3rd angle logo

–

square holes

–

depth of holes

–

diameter of holes

–

countersunk holes

–

counterbore holes

–

spotfacing

–

spherical radius

–

chamfered corners

–

springs

–

knurls

–

flat surfaces

–

breaks.

Auxiliary views Often when a single orthogonal view is drawn, the location and shape of some details need to be determined by projection from another view. The view drawn in order to determine these details is called an auxiliary view. The auxiliary view does not need to be a complete orthogonal view, but

10

Telecommunications engineering

rather, construction can be limited in order to supply only the required information. •

Freehand drawing Engineers are required to convey information quickly but with accuracy. An important skill is the ability to produce freehand drawings. Freehand drawings need to use all the AS 1100 concepts. Without these standards the drawing will be less able to communicate accurate details. While some AS 1100 standards are difficult to apply using freehand, the fundamental concepts should always be applied.

Lifting Devices •

Representing repeat features When a component contains regular repeating features such as holes or slots, the AS 1100 standards allow these repeat features to be shown as full outline or alternatively, by a conventional representation. Examples include drilled holes at a set distance from a central hole. Rather than draw a series of holes, one hole is drawn, then the details of the remaining holes are indicated using symbols. Pitch circle diameter (PCD) refers to the diameter that the hole centres are located from the centre point.

•

Material lists Material lists or parts lists should be used when several components are detailed in one drawing, or a number of components are shown in one assembly drawing. The material list should be positioned near the sheet title block.

•

Itemising Often a component on an assembly drawing is assigned an item number. The number is used to identify the component and is referenced to the material list. Leaders (lines) are continuous thin dark lines drawn from the item number to the component.

•

Square threads Where screw threads are used to transmit large forces, such as in lifting devices, square thread is used rather then the more common v-thread. To differentiate a square thread from a v-thread a section of the detail view is drawn to illustrate the thread shape.

•

Tangency Tangency refers to the joining of lines. These lines can be the edge of a circle or an arc meeting a straight line, or can be arc to arc, or circle to circle. Construction techniques are required to ensure accurate tangency.

Part 6: Telecommunications engineering – communications

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Aeronautical engineering •

Drawing scales Drawing scales are required in order to accurately draw objects that are too large to represent full size, and for objects that are too small to draw accurately at actual size. Rather than have endless variation between drawings, a series of preferred scales are used. Enlarging scales are written 10:1 etc. Reduction scales are written 1:10 etc. A simple way of determining if a scale is an enlarging or reducing scale is to consider the : sign as a / sign. Thus, a 1:10 scale can be thought of as a 1/10 th size drawing.

•

Selection of views When selecting the views required, the drawer should aim to:

•

–

reduce the number of views required to give a full shape description

–

avoid repetition of detail or views

–

avoid hidden detail.

Partial and auxiliary views Partial views are used where full shape views do not provide good shape description. They apply where a component has an inclined face and are often used in conjunction with an auxiliary view.

•

Symmetrical parts When components or parts are symmetrical, time can be saved, without loss of accuracy, by drawing half shapes or patterns. AS 1100 standards are applied to indicate lines of symmetry.

It would be advisable to review each of the parts in each module, and refresh your memory of the details. This knowledge will need to be recalled in order to complete the exercises that follow. Collecting the data for exercise 6.3 Prepare yourself with a tape measure, paper and a few sharp pencils. Find a public telephone. Make as many ‘in the field’ sketches of the telephone, and the telephone booth, as you can. Take as many measurements as you can. You will need the dimensions of the telephone booth, concrete footing, height of telephone above the floor level. Before you begin to collect this information, turn to exercise 6.3 to read the instructions.

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Telecommunications engineering

Figure 6.2 Telephone booth

Now turn to the exercise sheet and complete exercises 6.3 to 6.6.

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Telecommunications engineering

Exercises

Exercise 6.1 This exercise requires a CAD drawing. This CAD drawing will be a very simple icon representation of a telephone booth. You should allocate approximately 20 minutes for this exercise. This icon should be a suitable simple and clear design for inclusion on a site map. There is no requirement for a detailed technical drawing, you need to represent the telephone booth without detail. The icon should be easily ‘read’ when reproduced within a 20mm by 20mm square. An enlarged image 100 x 100 mm, and a 20 x 20 mm image, should be printed off and attached to the spaces provided on the next page.

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Telephone icon – enlarged view (100 mm x 100 mm)

Telephone icon – standard view (20 mm x 20 mm)

16

Telecommunications engineering

Exercise 6.2 This exercise requires the drawing of several electronic component symbols. These symbols should be drawn freehand in the appropriate positions indicated on the following circuit diagram. resistor

diode

resistor

transistor resistor

transistor

capacitor

capacitor

transistor

+ LED battery

Figure 6.3 Electronic circuit requiring component symbols

Part 6: Telecommunications engineering – communications

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Exercise 6.3 This exercise requires a freehand orthogonal front view of the telephone booth to be drawn. Add general detail and show overall sizes. Although freehand technique is to be used, apply AS1100 concepts and techniques to the drawing. This technical drawing should be used to give the viewer overall reference of the telephone booth design, and be able to be used to identify the location of the smaller components of the telephone box. Construct a full title block and item list on this drawing sheet. Show projection angle. Use a scale of 1:10.

Figure 6.4 Telephone booth image 1

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Telecommunications engineering

Figure 6.5 Telephone booth image 2

Part 6: Telecommunications engineering – communications

Figure 6.6

Telephone booth image 3

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Telecommunications engineering

Part 6: Telecommunications – communication

SCALE 1:10 Page 21

Ex 6.3

Exercise 6.4 From the photographs on this page and the next, draw a three view (left end, front and right end) freehand orthogonal view that represents the threaded fixing device shown, using a scale of 2:1. These views will best fit on one centre, with the drawing sheet orientated in a landscape position. The bolt is 45 mm long with a thread of M10. All additional sizes to those provided must be estimated from the photographs. Fully dimension your drawing, with enough detail to allow for manufacture of the bolt.

Figure 6.7 Threaded fixing device image 1

Figure 6.8 Threaded fixing device image 2

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Figure 6.9 Threaded fixing device image 3

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Telecommunications engineering

Part 6: Telecommunications – communication

SCALE 1:1 Page 25

Ex 6.4

Exercise 6.5 From the details provided in the illustrations, draw a freehand pictorial view of the satellite dish and the supporting structure. Drawing space is provided after the illustrations. All sizes should be estimated from the illustrations. Alternatively, if you have access to a satellite dish, you may draw the details of that dish and its supporting bracket.

Figure 6.10 Satellite dish image 1

Figure 6.11 Satellite dish image 2

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27

Figure 6.12 Satellite dish image 3

Figure 6.13 Satellite dish image 4

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Telecommunications engineering

Part 6: Telecommunications – communication

SCALE 1:1 Page 29

Ex 6.5

Exercise 6.6 Select the most appropriate alternative a, b, c or d that best answers the question or best completes the statement. 1

2

3

4

5

When indicating a line of symmetry: a

a thick chain line is used; two short lines are drawn at each end at right angles to the line of symmetry

b

a thin chain line is used; two short lines are drawn at each end at right angles to the line of symmetry

c

a thin chain line is used; the ends of the chain line are thickened, and two short parallel lines are drawn across the line of symmetry

d

a thick chain line is used; the ends of the chain line are thickened, and two short parallel lines are drawn across the line of symmetry.

When selecting views: a

avoid hidden outline

b

avoid hidden outline and repetition of views

c

avoid hidden outline, repetition of views and have the minimum number of views that provide full shape description

d

avoid hidden outline, repetition of views and have the minimum number of views that provide full shape description but include a sectional view.

Scaled views are regularly required because: a

objects are never the appropriate size for the paper size

b

true sizes do not always allow sufficient detail to be indicated

c

to provide full shape description, objects must be drawn larger than full size

d

objects must be drawn at a size that is convenient and appropriate.

PCD refers to: a

pitch circle diameter

b

partial CAD drawing

c

electrical symbols used on PCB’s

d

the diameter of a thread on a bolt or nut.

When dimensioning a technical drawing: a

actual sizes are indicated

b

drawn sizes are indicated

c

scaled sizes are indicated

d

actual sizes are scaled.

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6

7

8

9

Transition pieces are: a

missing parts between components

b

disassembled parts of a development

c

developed drawings

d

connection segments joining different shaped ducts.

A 20 x 20 mm sheetmetal cube, placed in a standard position in an orthogonal drawing: a

will have edges of apparent length 20 mm in a top view

b

will have all parallel edges shown as 20 mm

c

will have all edges shown as true lengths

d

will have all edges that are parallel to the viewing plane shown as true lengths.

Triangulation is: a

a method of dividing a surface into segments

b

finding the location of points

c

dividing square surfaces into triangles

d

none of the above.

Hexagonal nuts and bolt heads should be viewed: a

with three sides shown in the front view

b

shown with a spiral cut thread in orthogonal views

c

shown with the thread as hidden detail lines

d

drawn twice the diameter of the shaft of the bolt.

10 AS 1100 standards use the following letters as symbols: a

SR, S, R, and M

b

A/F, S, M and QD

c

SR, S, QD and M

d

R, SR, A/F and O.

11 AS 1100 rules regarding centre lines state:

32

a

centre lines should extend only a short distance past the feature unless required for dimensioning

b

centre lines should cross one another on a dash part of the line when they define a centre point

c

centre lines are drawn as chain lines

d

all of the above.

Telecommunications engineering

12 A detail drawing: a

gives a full size and shape description of the object

b

states the material that the object is made

c

provides sufficient information for the manufacture of the object

d

all of the above.

13 Which of the following lists only contain terms for physical features of engineering objects? a

radius, collar, web, blind hole, and shoulder

b

shaft, taper, boss ,fillet and countersunk

c

counterbore, step, flange, thread and spigot

d

all three of the lists above.

14 Engineering is about: a

evaluating alternatives and designing the best criteria

b

setting criteria and calculating the costs of the best design

c

working collaboratively to ensure a consensus is always reached

d

determining the best solution based on the criteria.

15 At what stage of the course module work are you at now? a

very close to finished

b

ready to review all past work

c

ready for the HSC examination

d

none of the above

e

some of the above.

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Telecommunications engineering

Progress check

During this part you have learned more about the use of drawings in communicating information and have reviewed the work you have done in previous modules on drawing to AS1100 standards.

✓ ❏

Disagree – revise your work

✓ ❏

Uncertain – contact your teacher

Uncertain

Agree – well done Disagree

✓ ❏

Agree

Take a few moments to reflect on your learning then tick the box that best represents your level of achievement.

I have learnt about: •

freehand and technical drawing, pictorial and dimensioned orthogonal drawing

•

Australian standards AS1100

•

computer graphics, computer assisted drawing (CAD)

•

collaborative work practices.

I have learnt to: •

produce orthogonal drawings applying appropriate Australian Standards (AS1100)

•

apply dimensions to drawings to AS1100 standard

•

justify graphics as a communication tool for telecommunications engineering

•

appreciate the value of collaborative working.

Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to for original and current documents.

Congratulations! You have completed the module on Telecommunications engineering.

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Telecommunications engineering

Exercise cover sheet

Exercises 6.1 to 6.6

Name: _______________________________

Check! Have you have completed the following exercises? ❐ Exercise 6.1 ❐ Exercise 6.2 ❐ Exercise 6.3 ❐ Exercise 6.4 ❐ Exercise 6.5 ❐ Exercise 6.6 Locate and complete any outstanding exercises then attach your responses to this sheet. If you study Stage 6 Engineering Studies through a Distance Education Centre/School (DEC) you will need to return the exercise sheet and your responses as you complete each part of the module. If you study Stage 6 Engineering Studies through the OTEN Open Learning Program (OLP) refer to the Learner’s Guide to determine which exercises you need to return to your teacher along with the Mark Record Slip.

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Bibliography

AS 1100.101 – 1992 Technical Drawing Part 101 general principles AS 1100.201 – 1992 Technical Drawing Part 201 Mechanical engineering drawing AS 1100.301 – 1992 Technical Drawing Part 301 Architectural drawing AS 1100.501 – 1992 Technical Drawing Part 501 Structural engineering drawing Avner, S.A. 1974, Introduction to Physical Metallurgy; McGraw-Hill, Singapore. Benade, A. H. 1976, Fundamentals of Musical Acoustics, Oxford University Press, London. Board of Studies, 1999, Engineering Studies Stage 6 Examination, Assessment and Reporting, Board of Studies NSW, Sydney. Board of Studies, 1999, Engineering Studies Stage 6 Specimen Paper, Board of Studies NSW, Sydney. Board of Studies, 1999, Engineering Studies Stage 6 Syllabus, Board of Studies NSW, Sydney. Brown, B. and Carr P. 1979, Electronics; a practical introduction, Heinemann Educational Australia, Victoria. Davis, Troxell & Wiskocil. 1964, The Testing and Inspection of Engineering Materials , McGraw-Hill, Tokyo. Duncan, T. 1989, GCSE Electronics, John Murray, London. Duncan, T. 1991, Electronics for Today and Tomorrow, John Murray, London. Duncan, T. 1993, Electronics for Today and Tomorrow, John Murray, London. Encyclopaedia Britannica CD 99. 1999. [CD-ROM]. Halliday, D. and Resnick, R. 1966,Physics, John Wiley and Sons, Sydney.

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Haykin, S. 1988, Digital Communications, John Wiley and Sons, New York. Hibbler, R.C. 1989, Engineering Mechanics – Statics, Macmillan, New York. Higgins, R.A. 1987, Materials for the Engineering Technician, Edward Arnold, London. Hioki, W. 2001,Telecommunications, Prentice-Hall Inc, New Jersey. Holden, R. 1991, A Guide to Engineering Mechanics, Science Press, Marrickville, NSW. Illingworth, V. 1999, The Penguin Dictionary of Electronics, Penguin Books, United Kingdom. Jacobowitz, H. 1972, Electronics Made Simple, Doubleday and Co, London. Jensen, P.R. 2000, From the Wireless to the Web, UNSW Press, Sydney. John, V.B. 1985, Introduction to Engineering Materials, Macmillan Publishers Ltd, London. Kaplan, W. and Lewis, D.J. 1971, Calculus and Linear Algebra , John Wiley and Sons, New York. Mackie, D. and Hayes P. 1987, Communications, CHP Books, Burlington. Mullins R, K. 1983, Engineering Mechanics, Longman, United Kingdom. Rochford, J. 2000, Engineering Studies – Student’s Handbook, KJS Publications, Gosford. Schlenker, B. and McKern, D. 1983, Introduction to Engineering Mechanics, Jacaranda Press, Sydney. Schlenker, B.R. 1974, Introduction to Materials Science, Wiley, Sydney. Serritella, M. 1977, Operation Electronics Manual, Macmillan, South Melbourne. Stallings, W. 2000, Data and Computer Communications, Prentice-Hall International, New Jersey. Stallings, W. 2001,Business Data Communications, Prentice-Hall International, New Jersey. Soden, F.A. et al, 1996, 100 Years of the Telephone 1896–1976, Wellman Printing Co Pty Ltd, Victoria. Taylor, A. Barry, O. 1975, Fundamentals of Engineering Mechanics, Cheshire, Melbourne.

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Telecom Australia, 1981, Engineer Development Programme, South Melbourne. University of Newcastle, 2001, Digital Communications Lecture Notes, Department of Electrical and Computer Engineering, University of Newcastle NSW. World Book Encyclopaedia, 1985, World Book, United States of America. Wolf, L. 1990, Statistics and Strength of Materials: a parallel approach to understanding structures, Merrill, New York. World Book Multimedia Encyclopedia, 1998. [CD-ROM] World Book, USA.

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