02. Fundamentals Of Dwdm Technology

ZTE

UNIVERSITY

English Training Manual Template (V0610)

Fundamentals of DWDM Technology Course Objectives: ·To master the purpose of DWDM networks and applicable environments ·To master the fundamentals and key technologies of DWDM

Reference: ·Optical Wavelength Division Multiplexing System ·Modern Telecommunication Base and Technology ·Principle and Test of DWDM Transmission System

Contents 1 Introduction to DWDM ............................................................................................................................. 1 1.1 Emergence and Background of DWDM ........................................................................................... 1 1.1.1 Evolution of Multiplexing Technology in Optical Network .................................................. 1 1.1.2 Evolution of Transmission Technology in Optical Network (PDH, SDH and DWDM)........ 2 1.2 Overview of DWDM Technology..................................................................................................... 5 1.2.1 Comparison of Different Multiplexing Technologies............................................................. 5 1.2.2 Relationship Between DWDM and SDH............................................................................... 8 1.2.3 Introduction to Operating Wavelength ................................................................................. 10 1.3 Features and Advantages................................................................................................................. 16 1.4 Future Trends of DWDM Technology ............................................................................................ 17 2 Overview of Optical Fiber ....................................................................................................................... 21 2.1 Basic Knowledge of Optical Fiber .................................................................................................. 21 2.1.1 Structure of Optical Fiber..................................................................................................... 21 2.1.2 Classification of Optical Fiber ............................................................................................. 22 2.1.3 Working Frequency of Optical Fiber.................................................................................... 24 2.1.4 Types and Features of Common SMFs ................................................................................ 25 2.1.5 New Optical Fiber Types...................................................................................................... 26 2.2 Transmission Characteristics of Optical Fiber ................................................................................ 27 2.2.1 Loss ...................................................................................................................................... 27 2.2.2 Dispersion ............................................................................................................................ 29 2.2.3 Non-Linear Effect ................................................................................................................ 32 3 Key Technologies of DWDM System ...................................................................................................... 36 3.1 Basic Structure of DWDM System ................................................................................................. 36 i

3.2 Light Source Technology.................................................................................................................40 3.3 Wavelength Division Multiplexing/Demultiplexing Technology ....................................................42 3.3.1 Overview...............................................................................................................................42 3.3.2 Introduction to Optical Multiplexer ......................................................................................42 3.3.3 Main Performance Indices ....................................................................................................45 3.4 OTU Technology .............................................................................................................................47 3.4.1 Overview...............................................................................................................................47 3.4.2 Working Principle and Performance Indices.........................................................................48 3.4.3 Classification and Application of OTU.................................................................................51 3.5 Optical Amplifying Technology ......................................................................................................52 3.5.1 EDFA Technology.................................................................................................................52 3.5.2 Raman Amplification Technology ........................................................................................58 3.6 Supervision Technology ..................................................................................................................60 3.6.1 Functions of Optical Supervisory Channel (OSC)................................................................61 3.6.2 Requirements for OSC..........................................................................................................61 3.6.3 Implementation of OSC ........................................................................................................62 4 Protection Principles of DWDM System.................................................................................................65 4.1 Introduction to DWDM System Hierarchy......................................................................................65 4.2 1+1 Protection..................................................................................................................................66 4.2.1 1+1 Protection in Chain Network .........................................................................................67 4.2.2 1+1 Protection in Ring Network...........................................................................................68 4.2.3 Features of 1+1 Protection....................................................................................................69 4.3 1:N Protection..................................................................................................................................69 4.3.1 Working Principle .................................................................................................................69 4.3.2 Implementation of 1:N Protection ........................................................................................70 4.3.3 Features of Channel 1:N Protection......................................................................................71

ii

4.4 Two-Fiber Bidirectional Channel Shared Protection ...................................................................... 71 4.5 Two-Fiber Bidirectional MS Shared Protection .............................................................................. 73 Appendix A Abbreviations.......................................................................................................................... 77

-iii-

1 Introduction to DWDM Key points z

Basic concepts and background of the DWDM technology

z

Future trends of the DWDM technology

1.1 Emergence and Background of DWDM The basic concepts of optical network are introduced before we start the study of the Dense Wavelength Division Multiplexing (DWDM) technology. This section describes the birth and background of DWDM based on two technologies: the multiplexing technology and transmission technology.

1.1.1 Evolution of Multiplexing Technology in Optical Network Various transmission mediums are applied in telecommunication networks, such as twisted pair cables, coaxial cables, optical fiber and electromagnetic waves. The optical fiber has the characteristics of great transmission capacity, high transmission quality, low loss, confidentiality and long regeneration distance, etc. With the constant development of wide-band and high-rate services in the information era, not only larger capacity and longer distance, but also convenient and rapid exchanges are needed for optical transmission systems. Then the multiplexing technology is introduced into optical transmission systems. This technology enables the transmission of multiple-channel signals through a single fiber or fiber cable with the broad frequency band and large-capacity features of fibers. In transmission systems for multiple signals, the multiplexing mode affects the performance and cost of the system greatly. The multiplexing technology of fiber optic transmission network has gone through three development stages: Space Division Multiplexing (SDM), Time Division Multiplexing (TDM), and Wavelength Division Multiplexing (WDM). The SDM technology features simple design and practicability. But it requires that the quantity of fiber transmission cores must be configured in accordance with the quantity 1

Fundamentals of DWDM Technology

of multiplexing channels, which results in poor investment profit. The TDM technology is widely applied, which is the basis of Plesiochronous Digital Hierarchy (PDH), Synchronous Digital Hierarchy (SDH), Asynchronous Transfer Mode (ATM), and IP technologies. Its disadvantage is the low utilization ratio of lines. The WDM technology supports multiple wavelengths (channels) to be borne on one fiber. So, it is the major means to expand the current fiber communication network capacity and is mostly used in trunk network.

1.1.2 Evolution of Transmission Technology in Optical Network (PDH, SDH and DWDM) The traditional fiber optic transmission technologies, such as PDH and SDH, employ the one-wavelength-in-one-fiber transmission mode. Due to the restriction caused by the characteristics of their own components, neither the transmission capacity nor the capacity expansion mode can meet the requirements of the rapid development of communication networks, leaving the massive bandwidth resources of fibers far from being fully exploited. The DWDM technology allows the transmission of multiple wavelengths over a single fiber, which has become the most economical and practical means for the fiber capacity expansion. With its unique technical advantages, the DWDM technology becomes a simple and economical means to expand the fiber transmission capacity in a rapid and effective manner. It can fully meet the current needs of the network broadband service development and lays a solid foundation for the development of the future fully-optical transmission network. The development process of PDH, SDH and DWDM, and the interface specifications of each technology are briefly described as follows. 1.

PDH The early optical transmission system uses PDH, which introduced Pulse Code Modulation (PCM) digital transmission technology based on the former analog telephone network. It multiplexes signals at low rate level into high-speed signals by means of bit stuffing and byte interleaving. The signals of the primary group of the PDH system adopts the synchronous TDM mode, and the multiplexing of other high-order groups adopts plesiochronous (or called asynchronous) TDM mode.

2

Chapter 1

Introduction to DWDM

The PDH system includes three kinds of regional rate level standards respectively for Europe, North America, and Japan, as listed in Table 1.1-1. Table 1.1-1 Country/ Region

Primary Group

Europe and

2.048 Mbit/s

China

30 channels

North

1.544 Mbit/s

America

24 channels

Japan

PDH Bit Rate

Secondary Group

1.544 Mbit/s 24 channels

Tertiary Group

Quartus Group

8.448 Mbit/s

34.368 Mbit/s

139.264 Mbit/s

120 channels

480 channels

1920 channels

(30×4)

(120×4)

(480×4)

6.312 Mbit/s

44.736 Mbit/s

274.176 Mbit/s

96 channels

672 channels

4032 channels

(24×4)

(96×7)

(672×6)

6.312 Mbit/s

32.064 Mbit/s

97.728 Mbit/s

96 channels

480 channels

1440 channels

(24×4)

(96×5)

(480×3)

From early 1970's to 1980's, the PDH system and devices were popularly used in the digital network. However, along with the developing fiber communication technology and user's increasing demands for communication services, the disadvantages of PDH can not be ignored any longer. 1)

The compatibility between the three rate standards is not available, which obstructs the development of international interconnection.

2)

There is no worldwide standard optical interface specification. Private optical interfaces developed by different manufacturers are not compatible with each other, which limits the networking flexibility and increases the network complexity and operation costs.

3)

PDH is a multiplexing structure based on the point-to-point transmission. It only supports point-to-point transmission, but cannot accommodate complicated networking.

4)

The operation, management, and maintenance must depend upon manual digital signal cross-connection and service-suspension test, which cannot meet the monitoring

and

network

management

requirements

of

the

modern

communication network. 5)

Along with the rate increase, it is more and more difficult to implement 3

Fundamentals of DWDM Technology

multiplexing of high-order groups through the PDH technology, and requirements of fiber digital communication for large-capacity and super-high speed transmission cannot be satisfied. 2.

SDH In mid-1980's, the Bell Communication Research Institute in USA put forward the concept of Synchronous Optical Network (SONET). In 1988, the CCITT (former ITU-T) accepted the SONET concept, and formed the worldwide unified technology standard for transmission network, and renamed it as SDH. The SDH signals use the synchronous multiplexing mode and a flexible multiplexing and mapping structure. Code streams at different levels are arranged regularly in the payload of the frame structure. The payload is synchronous with the network, so the corresponding software can be used to directly demultiplex a high-speed signal into the low-speed tributary signal at a time, called “one-step” demultiplexing”. The rate specifications of the SDH system are shown in Table 1.1-2. Table 1.1-2 SDH Signal Levels

SDH Level (ITU-T)

OC Level (SONET)

Line Rate (Mbit/s)

STM-1

OC-3

155.520

STM-4

OC-12

622.080

STM-16

OC-48

2488.320

STM-64

OC-192

9953.280

SDH standardizes the features of the digital signals, such as frame structure, multiplexing mode, transmission rate level, and interface code pattern. It provides a frame that is supported globally, on which a world-class telecom transmission network has been developed, featuring flexibility, reliability and easy management. This kind of transmission network is easy to expand and applicable to the development of the new telecom services. In addition, it makes possible the interworking between the devices of different manufacturers. When the transmission rate exceeds 10 Gbit/s, however, the system dispersion and other negative influences increase difficulty of long-distance transmission. Furthermore, the SDH system is the TDM system based on single wavelength. 4

Chapter 1

Introduction to DWDM

The single-wavelength transmission cannot fully utilize the huge bandwidth of fibers. Therefore, the WDM (Wavelength Division Multiplexing) technology is introduced in the backbone network, to greatly enlarge the transmission capacity of fibers. 3.

DWDM DWDM (Dense Wavelength Division Multiplexing) is one kind of WDM technologies. By “dense”, this is because the interval of the adjacent wavelengths is relatively small (1 nm to 10 nm). At present, the practical DWDM system works in the 1550 nm window, for the convenience of using the gain spectrum feature of the EDFA amplifier to directly amplify the composite optical wavelength signals. To meet the transverse compatibility between systems, the central wavelength of the optical channel must comply with the ITU-T G.692 Recommendation. In the DWDM system, each optical channel can bear different customer signals, such as SDH signal, PDH optical signal and ATM signal. Since the optical fiber communication and its networking technologies have unique advantages in accommodating multi-service and broadband requirements, the high-speed SDH system, N×2.5 Gbit/s and N×10 Gbit/s DWDM systems have become the major part and backbone of the core network.

1.2 Overview of DWDM Technology With the DWDM technology, multiple optical carriers with information (analog or digital) can be transmitted on one fiber, and the capacity of transmission system can be expanded easily by increasing wavelengths (channels). It combines (multiplexes) optical signals with different wavelengths for transmission. At the receiving end, it separates (de-multiplexes) the combined optical signals and then sends them to corresponding communication terminals respectively. In other words, the DWDM technology provides multiple virtual fiber channels on one physical fiber.

1.2.1 Comparison of Different Multiplexing Technologies The following compares different multiplexing technologies commonly used in optical transmission networks.

5

Fundamentals of DWDM Technology

1.

Time Division Multiplexing (TDM) In the TDM mode, multiple-channel signals are transmitted in different time spacing (time slots) through the same fiber. The TDM technology is widely used in various systems, such as PDH, SDH, ATM and IP. The advantage of TDM is that the fixed arrangement of time slots makes it flexible to adjust and control these time slots. Thus the TDM mode is applicable to the transmission of data information. The drawback of the TDM mode is low utilization ratio of lines. When a signal source has no data for transmission, its corresponding channel will be idle. At the same time, other busy channels can not make use of this idle channel. And due to the limitation of high-speed electron devices and modulation capability of lasers, transmission systems with capacity over 40 Gbit/s can not be achieved. Therefore, it is difficult to upgrade lines and expand the network capacity with the TDM technology.

2.

Space Division Multiplexing (SDM) The SDM technology divides the space into different channels to implement wavelength multiplexing. For example, more cores or fibers are involved in the optical cable to form different channels. The SDH performs optical intensity modulation to each channel of baseband signals respectively. Each channel of signal is transmitted by one fiber, and different channels will not affect each other, leading to best transmission performance. The SDM technology is easy for design and practice, but it requires that fiber cores of certain number must be configured according to the channel quantity of signals to be multiplexed. And this results in poor investment profit.

3.

Sub-Carrier Multiplexing (SCM) The SCM technology modulates multiple baseband signals into different microwave carrier frequencies to implement the electrical Frequency Division Multiplexing (FDM), and then uses this bit stream to modulate a single optical carrier into the fiber. At the receiving end, the photoelectrical detector picks the electrical FDM aggregate signals. And then different microwave carriers are divided into original baseband signals with the microwave technology. 6

Chapter 1

Introduction to DWDM

The SCM technology is mainly used in the Cable Television (CATV) multi-band transmission system of access networks. 4.

Wavelength Division Multiplexing (WDM) The WDM technology enables a single fiber to carry multiple wavelength (channel) systems, converting one fiber into multiple “virtual” fibers, each of which works on different wavelengths independently. Due to its economical efficiency and practicability, the WDM becomes the major wavelength multiplexing technology widely used in current fiber communication networks. The WDM is divided into three multiplexing modes: 1310 nm/1550 nm wavelength multiplexing, Coarse Wavelength Division Multiplexing (CWDM) and DWDM.

1)

1310 nm/1550 nm wavelength multiplexing In early 1970's, this multiplexing technology only used two wavelengths: one in 1310 nm window and the other in 1550 nm window. It implemented single-fiber dual-window transmission through the WDM technology, which was the initial wavelength division multiplexing case.

2)

CWDM The CWDM technology refers to the WDM technology with large spacing (usually no less than 20 nm) between adjacent wavelengths. Generally, the wavelength quantity is 4 or 8 (16 at most). The CWDM uses 1200 nm - 1700 nm windows. The cost of CWDM system is lower than DWDM because it adopts non-cooling lasers and does not need optical amplifying components. The disadvantages of CWDM are low capacity and short transmission distance. Therefore, the CWDM technology is applicable to the communication situations with short distance, broad bandwidth and dense access points, for example, the network communication inside a building or between buildings.

3)

DWDM The DWDM technology refers to the WDM technology with small spacing between adjacent wavelengths, with the operating wavelength in the 1550 nm window. It can carry 8 - 160 wavelengths on one fiber, and is mainly used in long-distance transmission systems. 7

Fundamentals of DWDM Technology

1.2.2 Relationship Between DWDM and SDH 1.

Relationship between DWDM and SDH on the transmission layer of optical networks Both the DWDM system and the SDH system belong to the transport network layer. They are the transmission means established on the fiber transport medium. Their relationship of them in the transport network is shown in Fig. 1.2-1.

Fig. 1.2-1

Relationship between DWDM and SDH in Transport Network

The SDH system implements multiplexing, cross-connection and networking on the electrical channel layer. The WDM system implements multiplexing, cross-connection and networking on the optical domain. 2.

Multiplexing modes of DWDM and SDH for carrier signals The SDH is a kind of TDM system based on single wavelength (one fiber transmitting one wavelength channel). When the transmission rate exceeds 10 Gbit/s, the system dispersion and other negative influences will make the long-distance transmission more difficult. The DWDM technology simultaneously transmits multiple optical carrier signals of different wavelengths in the same fiber, fully utilizing the bandwidth resources of the fiber and increasing system transmission capacity. 8

Chapter 1

3.

Introduction to DWDM

Capability of DWDM to transmit signals of different types at the same time The wavelengths used in the DWDM system are mutually separated and unrelated with the formats of service signals. Therefore, each wavelength can carry the optical signal with totally different features from the other one. In this way, the DWDM can implement the hybrid transmission of various signals. The relationship between DWDM system and some common services is shown in Fig. 1.2-2. IP

ATM

SDH

SDH

ATM

Ethernet

Other

Open Optical Interfaces

DWDM Fiber Physical Layer

Fig. 1.2-2

4.

Relationship between DWDM and Common Services

Optical interface standards of DWDM and SDH signals The optical interfaces of SDH devices should accord with the ITU-T G.957 recommendation, which does not specify the central operating wavelength. The optical interfaces in DWDM systems must accord with the ITU-T G.692 recommendation, which specifies the reference frequency, channel spacing, nominal central frequency (central wavelength), central frequency offset and other parameters of each optical channel. Therefore, the DWDM system can be either an open system or an integrated one.

·

Open DWDM system: The transmitting side of the system provides the Optical Transponder Unit (OTU) to converts the customer signals with non-standard wavelength into the standard wavelength compliant with ITU-T G.692. The "Open" means that the DWDM system has no special requirements for the operating wavelength of input signals. For example, the signals are accessed through “Open Optical Interfaces” as shown in Fig. 1.2-2. 9

Fundamentals of DWDM Technology

·

Integrated DWDM system: All the customer signals accessed to the DWDM system must comply with ITU-T G.692. For example, some signals are accessed to the DWDM system not through “Open Optical Interfaces” as shown in Fig. 1.2-2.

5.

Integrated application of DWDM and SDH The transmission capacity of fiber networks can be effectively improved through the integrated application of DWDM and SDH.

1.2.3 Introduction to Operating Wavelength 1.2.3.1 Operating Wavelength Range The quartz fiber has three low-loss windows: 860 nm, 1310 nm and 1550 nm, as shown in Fig. 1.2-3.

O: Original Band

E: Extend Band

S: Short Band C: Conventional Band

L: Long Band

Fig. 1.2-3 Low-Loss Windows in Fiber Communication

1.

860 nm window The wavelength range is 600 nm - 900 nm. It is always used in multi-mode fiber, and the transmission loss is large (2 dB/km averagely). The 860 nm window is applicable to short-distance access networks, such as for Fiber Channel (FC) services.

10

Chapter 1

2.

Introduction to DWDM

1310 nm window The lower limit of available wavelengths in this window depends on the fiber cut-off wavelength and attenuation coefficient, while the upper limit depends on the OH absorption peak at 1385 nm. The operating wavelength range is 1260 nm - 1360 nm. The average loss is 0.3 dB/km - 0.4 dB/km. The 1310 nm window is applicable to intra-office, short-distance and long-distance communication of STM-N signals (N = 1, 4 or 16). Multi-longitudinal mode lasers (MLM) and Light Emitting Diodes (LED) can be adopted as light sources. Since the broadband optical amplifier working in 1310 nm window is not available at present, this window is not suitable for the DWDM system.

3.

1550 nm window The lower limit of available wavelengths in this window depends on the OH absorption peak at 1385 nm, while the upper limit depends on infrared absorption loss and bending loss. The operating wavelength range is 1460 nm 1625 nm. The average loss is 0.19 dB/km - 0.25 dB/km. The loss in the 1550 nm window is the lowest, so it can be applied to short-distance and long-distance communication of SDH signals. In addition, the commonly used Erbium-Doped Fiber Amplifier (EDFA) has sound gain flatness in this window, so the 1550 nm window is applicable to the DWDM system as well. The operating wavelength in the 1550 nm window is divided into three parts (S band, C band and L band), with the wavelength range shown in Fig. 1.2-4.

Fig. 1.2-4

1)

Division of Operating Wavelength in 1550 Window

S band (1460 nm - 1530 nm): Since the operating wavelength range of EDFA is in C band or L band, S band is not used in the DWDM system at present.

11

Fundamentals of DWDM Technology

2)

C band (1530 nm - 1565 nm): It is often used as the operating wavelength area of DWDM systems under 40 wavelengths (with channel spacing 100 GHz), DWDM systems under 80 wavelengths (with channel spacing 50 GHz) and SDH systems.

3)

L band (1565 nm - 1625 nm): Operating wavelength area of DWDM systems above 80 wavelengths. In this case, the channel spacing is 50 GHz.

1.2.3.2 Operating Wavelength Area of DWDM Systems Based on the quantity of multiplexing channel and frequency spacing, the system of 40 wavelengths or below, 80-wavelength system and 160-wavelength system are introduced respectively as follows. 1.

8/16/32/40-wavelength system Operating wavelength range: C band (1530 nm - 1565 nm) Frequency range: 192.1 THz - 196.0 THz Channel spacing: 100 GHz Central frequency offset: ±20 GHz (at rate lower than 2.5 Gbit/s); ±12.5 GHz (at rate 10 Gbit/s)

2.

80-wavelength system Operating wavelength range: C band (1530 nm - 1565 nm) Frequency range: C band (192.1 THz - 196.0 THz) Channel spacing: 50 GHz Central channel offset: ±5 GHz

3.

160-wavelength system Operating wavelength range: C band (1530 nm - 1565 nm) + L band (1565 nm 1625 nm) Frequency range: C band (192.1 THz - 196.0 THz) + L band (190.90 THz 186.95 THz) Channel spacing: 50 GHz Central frequency offset: ±5 GHz

12

Chapter 1

Introduction to DWDM

1.2.3.3 Operating Wavelength Allocation in DWDM Systems The operating wavelength of the DWDM system, complying with ITU-T Recommendation G.692, adopts the specific central wavelength and central frequency values in the multi-channel system. 1.

The wavelength allocation in 40-wavelength system based on C-band with wavelength spacing of 100 GHz is listed in Table 1.2-1. Table 1.2-1 Wavelength No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Wavelength Allocation of 40CH/100 GHz Spacing on C Band Central Frequency (THz) 192.1 192.2 192.3 192.4 192.5 192.6 192.7 192.8 192.9 193.0 193.1 193.2 193.3 193.4 193.5 193.6 193.7 193.8 193.9 194.0 194.1 194.2 194.3 194.4 194.5 194.6 194.7 194.8 194.9 195.0 195.1 195.2 195.3 195.4 195.5 195.6 13

Wavelength (nm) 1560.61 1559.79 1558.98 1558.17 1557.36 1556.55 1555.75 1554.94 1554.13 1553.33 1552.52 1551.72 1550.92 1550.12 1549.32 1548.51 1547.72 1546.92 1546.12 1545.32 1544.53 1543.73 1542.94 1542.14 1541.35 1540.56 1539.77 1538.98 1538.19 1537.40 1536.61 1535.82 1535.04 1534.25 1533.47 1532.68

Fundamentals of DWDM Technology

Wavelength No. 37 38 39 40

2.

Central Frequency (THz) 195.7 195.8 195.9 196.0

Wavelength (nm) 1531.90 1531.12 1530.33 1529.55

The wavelength allocation for C/C+ band in 80-wavelength system with wavelength spacing of 50 GHz is listed in Table 1.2-2. Table 1.2-2

Wavelength No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Wavelength Allocation of 80CH/50 GHz Spacing on C/C+ Band

Nominal Central Frequency (THz) 196.05 196.00 195.95 195.90 195.85 195.80 195.75 195.70 195.65 195.60 195.55 195.50 195.45 195.40 195.35 195.30 195.25 195.20 195.15 195.10 195.05 195.00 194.95 194.90 194.85 194.80 194.75 194.70 194.65 194.60 194.55 194.50

Nominal Central Wavelength (nm) 1529.16 1529.55 1529.94 1530.33 1530.72 1531.12 1531.51 1531.90 1532.29 1532.68 1533.07 1533.47 1533.86 1534.25 1534.64 1535.04 1535.43 1535.82 1536.22 1536.61 1537.00 1537.40 1537.79 1538.19 1538.58 1538.98 1539.37 1539.77 1540.16 1540.56 1540.95 1541.35 14

Wavelength No. 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72

Nominal Central Frequency (THz) 194.05 194.00 193.95 193.90 193.85 193.80 193.75 193.70 193.65 193.60 193.55 193.50 193.45 193.40 193.35 193.30 193.25 193.20 193.15 193.10 193.05 193.00 192.95 192.90 192.85 192.80 192.75 192.70 192.65 192.60 192.55 192.50

Nominal Central Wavelength (nm) 1544.92 1545.32 1545.72 1546.12 1546.52 1546.92 1547.32 1547.72 1548.11 1548.51 1548.91 1549.32 1549.72 1550.12 1550.52 1550.92 1551.32 1551.72 1552.12 1552.52 1552.93 1553.33 1553.73 1554.13 1554.54 1554.94 1555.34 1555.75 1556.15 1556.55 1556.96 1557.36

Chapter 1

Wavelength No. 33 34 35 36 37 38 39 40

3.

Nominal Central Frequency (THz) 194.45 194.40 194.35 194.30 194.25 194.20 194.15 194.10

Nominal Central Wavelength (nm) 1541.75 1542.14 1542.54 1542.94 1543.33 1543.73 1544.13 1544.53

Wavelength No. 73 74 75 76 77 78 79 80

Introduction to DWDM

Nominal Central Frequency (THz) 192.45 192.40 192.35 192.30 192.25 192.20 192.15 192.10

Nominal Central Wavelength (nm) 1557.77 1558.17 1558.58 1558.98 1559.39 1559.79 1560.20 1560.61

The wavelength allocation for L/L+ band in 80-wavelength system with wavelength spacing of 50 GHz is listed in Table 1.2-3. Table 1.2-3

Wavelength No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Wavelength Allocation of 80CH/50 GHz Spacing on L/L+ Band

Nominal Central Frequency (THz) 190.90 190.85 190.80 190.75 190.70 190.65 190.60 190.55 190.50 190.45 190.40 190.35 190.30 190.25 190.20 190.15 190.10 190.05 190.00 189.95 189.90 189.85 189.80 189.75 189.70

Nominal Central Wavelength (nm) 1570.42 1570.83 1571.24 1571.65 1572.06 1572.48 1572.89 1573.30 1573.71 1574.13 1574.54 1574.95 1575.37 1575.78 1576.20 1576.61 1577.03 1577.44 1577.86 1578.27 1578.69 1579.10 1579.52 1579.93 1580.35 15

Wavelength No. 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Nominal Central Frequency (THz) 188.90 188.85 188.80 188.75 188.70 188.65 188.60 188.55 188.50 188.45 188.40 188.35 188.30 188.25 188.20 188.15 188.10 188.05 188.00 187.95 187.90 187.85 187.80 187.75 187.70

Nominal Central Wavelength (nm) 1587.04 1587.46 1587.88 1588.30 1588.73 1589.15 1589.57 1589.99 1590.41 1590.83 1591.26 1591.68 1592.10 1592.52 1592.95 1593.37 1593.79 1594.22 1594.64 1595.06 1595.49 1595.91 1596.34 1596.76 1597.19

Fundamentals of DWDM Technology

Wavelength No. 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Nominal Central Frequency (THz) 189.65 189.60 189.55 189.50 189.45 189.40 189.35 189.30 189.25 189.20 189.15 189.10 189.05 189.00 188.95

Nominal Central Wavelength (nm) 1580.77 1581.18 1581.60 1582.02 1582.44 1582.85 1583.27 1583.69 1584.11 1584.53 1584.95 1585.36 1585.78 1586.20 1586.62

Wavelength No. 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

Nominal Central Frequency (THz) 187.65 187.60 187.55 187.50 187.45 187.40 187.35 187.30 187.25 187.20 187.15 187.10 187.05 187.00 186.95

Nominal Central Wavelength (nm) 1597.62 1598.04 1598.47 1598.89 1599.32 1599.75 1600.17 1600.60 1601.03 1601.46 1601.88 1602.31 1602.74 1602.17 1603.57

1.3 Features and Advantages 1.

Fully utilizing fiber bandwidth resources and featuring high transmission capacity The DWDM technology makes full use of the huge bandwidth (about 25 THz) resource of fibers and thus expands the transmission capacity of the system.

2.

Super-long transmission distance Through EDFA and other super-long distance transmission technologies, signals of multiple channels in the DWDM system can be amplified at the same time to support the long-distance transmission.

3.

Abundant service access types The wavelengths in the DWDM system are separated to each other, and thus are capable of transmitting different services transparently, such as SDH, GbE and ATM signals, to implement the hybrid transmission of multiple kinds of signals.

4.

Saving fiber resources The DWDM system multiplexes multiple single-channel wavelengths for transmission in one fiber, greatly saving fiber resource and reducing line construction cost.

16

Chapter 1

5.

Introduction to DWDM

Smooth upgrading and expansion Since the DWDM system transmits the data in each wavelength channel transparently and does no process the channel data, the capacity of the system can be expanded conveniently and practically only by adding more multiplexing wavelength channels.

6.

Fully utilizing mature upper layer transport network technology At present, the optical transport technologies in TDM mode, such as ATM, Ethernet and SDH, have been well developed. Through the WDM technology, the transmission capacity can be enlarged by several times or even dozens of times, with expansion cost lower than that in the TDM mode.

7.

Important base for all optical network construction The all optical network is the development trend of the optical transport network. In such network, the WDM system is connected to Optical Add/Drop Multiplexers (OADMs) and Optical Cross-connection (OXC) devices, directly adding/dropping or cross-connecting services carried by the optical wavelength signals. In this way, the all optical network with high flexibility, reliability, survivability and economical efficiency is formed to meet the requirements of information society for the broadband transport network in the future.

1.4 Future Trends of DWDM Technology 1.

Higher channel rate The channel rate of the DWDM system has developed to 10 Gbit/s from 2.5 Gbit/s, and the system at 40 Gbit/s rate is in experimentation and the technology becomes more and more mature.

2.

More wavelengths to be multiplexed The DWDM system at early phase usually adopts 8/16/32 wavelengths with channel spacing 100 GHz and the operating wavelength is in C band. Along with the constant development of DWDM technology, the operating wavelength can cover C and L bands with the spacing 50 GHz. ZTE's ZXWM M900 DWDM Optical Transmission System can support the multiplexing of 160 wavelengths at most.

17

Fundamentals of DWDM Technology

3.

Super-long all optical transmission distance The initial construction cost and operation cost for the network can be reduced through extending all optical transmission and reducing electrical regeneration nodes. Traditional DWDM systems use EDFA to extend the passive regeneration transmission distance. At present, this distance can be extended from 600 km to above 2000 km, through distributed Raman amplifier and enhanced Forward Error Correction (FEC) technology, dispersion management technology, optical equalization technology and effective modulation formats.

4.

Network developed from toll network to metro area network At earlier time, only the high-rate SDH services can be accessed to the DWDM system. With the development of DWDM technology, both SDH services at various rate and IP services can be accessed via the continuous-rate service access board, subrate convergence board and GE service access board. All these make the DWDM system satisfy the requirements of metro area networks for service access. The network element Optical Add/Drop Multiplexer (OADM) can implement the adding/dropping of services flexibly. It also supports the smooth expansion of the network. During the network construction, boards in the equipment can be configured according to current traffics. When the demand for traffics increases, the network can be expanded and upgraded smoothly by the combination of multiple OADMs in series, parallel, or series-parallel mode. The DWDM technology provides broad expansion space with lower cost. In addition, the OADM equipment can implement the protection and recovery of optical layer services by constructing chain or ring optical networks. It can implement the protection based on chain networks, such as optical multiplex section 1+1 protection, optical channel 1+1 protection and optical channel 1:N protection, and the protection based on ring networks, such as two-fiber bidirectional multiplex section protection, two-fiber bidirectional channel shared protection and channel 1+1 protection. The DWDM technology provides a perfect protection mechanism for services in metro area networks, especially for GE services, and thus improves the network reliability greatly. Therefore, more and more DWDM systems are adopted in the construction of 18

Chapter 1

Introduction to DWDM

metro area networks and local backbone networks. ZTE’s ZXMP M800 with various service interfaces and perfect network protection mechanism, which can implement the effective convergence of services and smooth expansion, is just the product of this development trend. 5.

Evolving from point-to-point WDM to full optical network Many early DWDM transmission systems adopt the networking mode of point-to-point or chain, which is mainly used for toll backbone. These toll DWDM transmission systems always use the back-to-back DWDM backbone transmission structure with 3R (Reshaping, Retiming, and Regenerating). In this structure, lots of Optical Transponder Units (OTUs) are used, which leads to high construction and maintenance cost, low network flexibility, too many Optical/Electrical/Optical (O/E/O) conversions, low circuit assignment speed, and high system fault ratio. This structure will not be adaptive to future automatic switching transmission networks. At present, in metro area network DWDM transmission networks, OADM equipment transfer optical signals of different wavelength channel to corresponding terminal. They can implement the adding/dropping and straight-through of wavelengths carrying services. The OADM is divided into the fixed wavelength add/drop multiplexer and the 100% dynamical add/drop multiplexer (Rearrangable OADM, ROADM). The fixed wavelength OADM can add/drop 20% - 40% of the input wavelengths; while some can add/drop all wavelengths. The ROADM has two kinds of structure, the broadcast/selection one and the demultipexing/cross-connect multiplexing one. The crucial parts of the ROADM include the cross-connect unit, wavelength disabler, tunable filter and tunable laser etc. The ROADM can add/drop unfixed wavelengths. The networking of OADM equipment is flexible, which can implement chain, ring, and cross networking. The Optical Cross-Connect (OXC) is the route switch of next generation optical communication. In the full optical network, it provides these functions: connection function based on wavelengths, wavelengths add/drop function of optical channels, leading the wavelength channels for the sake of best utilization of fiber infrastructure, and implementing protection and restoration on wavelength, wavelength group and fiber levels. The OXC is set at the important tandem point of the network, converging different wavelengths input from 19

Fundamentals of DWDM Technology

different directions and then output signals with proper wavelengths. Through OADM and OXC, we can construct more complicated ring network. In the next generation IP Over DWDM telecom/network architecture, the OXC is an important stage in the future development of WDM technology.

20

2 Overview of Optical Fiber Key points z

Basic knowledge of optical fiber, including structure, classification, application

frequency, types and features z

Transmission characteristics of optical fiber

2.1 Basic Knowledge of Optical Fiber 2.1.1 Structure of Optical Fiber Optical fiber is a kind of cylinder glass fiber with a good light conducting performance and a small diameter. It consists of fiber core, cladding, and coating layer, as shown in Fig. 2.1-1. Coating Cladding

n2

Fiber core

n1

n1: Refractive index of fiber core n2: Refractive index of cladding

Fig. 2.1-1

1.

Structure of Optical Fiber

Fiber core It is mainly made of SiO2 (quartz) and comprises few doped chemical, such as GeO2, to improve refractive index (n1) of the fiber core. The diameter of the fiber core usually ranges from 5 μm to 50 μm.

2.

Cladding It is made of pure SiO2, with the outer diameter of 125 μm. The refractive index 21

Fundamentals of DWDM Technology

(n2) of cladding is less than that (n1) of the fiber core. 3.

Coating It is made of macromolecule materials, such as epoxide resin and silicone rubber. The outer diameter is about 250 μm. The addition of coating improves the flexibility, mechanical strength, and aging-resistance features of the optical fiber.

2.1.2 Classification of Optical Fiber This section introduces three classification methods of optical fiber, in terms of the distribution pattern of refractive index, fiber material and transmission mode. 1.

Classification according to the distribution pattern of refractive index When a beam of light is transferred through a fiber, each incident ray arrives at the interface between the fiber core and the cladding by a proper angle. Since the refractive index of the fiber core (n1) is larger that that of the cladding (n2), the light will be reflected totally in the fiber core repeatedly on the interface if the reflection angle meets the total internal reflection condition. In this case, the light travels forward along a “Z” path, which forms the transmission wave. All the light energy is constrained within the fiber core. According to the radial distribution of refractive index on the section of fiber, optical fibers are classified into step index fiber and grade index fiber. Fig. 2.1-2 illustrates the relationship between the refractive index and the fiber structure, as well as the transmission path of light in the fibers. n2 Cladding

Fiber core

n1

a. Step Index Fiber

22

Light

Chapter 2

n2 Cladding

Fiber core

Overview of Optical Fiber

Light

n1

b. Grade Index Fiber Fig. 2.1-2

2.

Comparison Between Step Index Fiber and Grade Index Fiber

Classification according to fiber material According to the materials of fibers, optical fibers are classified into silica fiber, various glass fibers containing different ingredients, plastic-clad silica fiber with a silica-based core and a plastic cladding, and all plastic optical fiber with a plastic core as well as a plastic cladding etc. Among these fiber types, the silica fibers have less loss than the other fibers. Generally, the fibers with great loss are only employed in short-distance systems in buildings or rooms.

3.

Classification according to the transmission mode of fiber Light is a kind of electromagnetic wave. Therefore, the transmission of light through fiber should meet not only the total-reflection condition between the fiber core and cladding, but also the coherence enhancement condition for electromagnetic wave during the transmission process. For a specific fiber structure, only a series of certain electromagnetic waves can be effectively transmitted in the fiber. Such specific electromagnetic wave is called optical fiber mode. In the fiber, the conductible mode quantity depends on the structure and refractive index radial distribution of the fiber. If a fiber supports only one conduction mode (base mode), it is called Single-Mode Fiber (SMF). The core of a single-mode fiber can only carry one channel of light. If a fiber supports multiple conduction modes, it is called Multi-Mode Fiber (MMF). In a multi-mode fiber, each channel of light uses a transmission mode. Table 2.1-1 explains the differences between SMF and MMF. 23

Fundamentals of DWDM Technology

Table 2.1-1

Differences Between SMF and MMF

Fiber

SMF

Item

MMF

Transmission

Only supports the transmission in base

Supports multiple conduction

mode

mode

modes

Fiber core

Small (about 5 μm - 10 μm)

Large (about 50 μm)

The dispersion of SMF is mainly

MMF has great mode dispersion

caused by the transmission rates of

due to different transmission rates

different frequency elements in optical

of different modes, which directly

signal, which increases along with the

affects the transmission bandwidth

spectral width of the optical signal.

and transmission distance.

Dispersion

Ordinary SMF, Dispersion Shifted Type

Fiber (DSF) and Dispersion

Ordinary MMF

Compensation Fiber (DCF) Working

1310 nm and 1550 nm

window

850 nm and 1310 nm Short-distance fiber

Long-distance fiber communication

Applications

communication systems at low

systems with large capacity

rate

2.1.3 Working Frequency of Optical Fiber With the improvement of fiber manufacturing techniques, the fiber transmission loss keeps decreasing. At present, there are five low-loss windows, as shown in Fig. 2.1-3. 3.0 ~ 2.5

Loss (dB/km)

140 THz ~

OH-Absorption peak

2.0 1.5

I

OHAbsorption peak OH- II Absorption peak

1.0

50 THz

V

III IV

0.5 O 0

O: Original Band

80 0

100 0

E: Extended Band

120 0

S: Short Band

Fig. 2.1-3 24

E 140 0

S C L 160 0 Wavelength (nm)

C: Conventional Band

Division of Low-Loss Windows

L: Long Band

Chapter 2

Overview of Optical Fiber

Table 2.1-2 lists the optical signal mark, wavelength range, applied fiber types and application occasions of the five low-loss windows: I, II, III, IV and V. Table 2.1-2 Comparison Between Low-Loss Windows Item Window

Wavelength

Mark (nm)

Fiber Type

Range (nm)

Application Scope

I

850

600-900

MMF

Short distance

II

1310 (O band)

600-900

MMF/G.652/G.653

and low rate

III

1550 (C band)

1530-1565

G.652/G.653/G.655

IV

1600 (L band)

1565-1625

G.652/G.653/G.655

V

1360-1530 (E+S band)

Long distance and high rate

1360-1530

Full-wave fiber

2.1.4 Types and Features of Common SMFs This section briefly introduces the features and functions of three kinds of SMFs, G.652, G.653 and G.655. The fiber types applied in the DWDM systems are also involved. 1.

G.652 (ordinary SMF) It is also called dispersion non-shifted SMF, applied in 1310 nm and 1550 nm windows. In the 1310 nm window, it has dispersion close to zero. In the 1550nm window, its loss is the smallest with the dispersion of 17 ps/km·nm. When the G.652 fiber is used in the 1310 nm window, it is only applicable to SDH systems; while it is applicable to both SDH systems and DWDM systems when it is used in the 1550 nm window. The dispersion compensation is needed when the single channel rate is over 2.5 Gbit/s.

2.

G.653 (dispersion shifted SMF) The G.653 fiber has the smallest loss and the smallest dispersion in the 1550 nm window. Therefore, it mainly works in the 1550 nm window. It is applicable to high-rate and long-distance single-wavelength communication systems. When the DWDM technology is used, the serious non-linear Four Wave Mixing (FWM) problem will occur in the zero-dispersion wavelength area, which leads to signal attenuation in multiplexing channels and channel crosstalk. 25

Fundamentals of DWDM Technology

3.

G.655 (non-zero dispersion shifted SMF) In the 1550 nm window, the absolute dispersion of G.655 fiber is within a certain range instead of zero. It ensures the smallest loss and small dispersion in this window. It is applicable to high-rate and long-distance optical communication systems. In addition, because the non-zero dispersion suppresses the influence of non-linear FWM over DWDM system, this kind of fiber is usually used in DWDM systems.

2.1.5 New Optical Fiber Types The features and applications of some new-type fibers are introduced below. 1.

G.654 (lowest attenuation SMF) The G.654 fiber works in the 1550 nm window with the average loss of 0.15dB/km - 0.19dB/km, which is less than that of other fibers. Its zero-dispersion point is also in the 1310 nm window. It is mainly applicable to optical transmission systems with long regeneration distance. The G.654 fiber with lowest attenuation meets the requirements of long-haul communication through submarine optical fiber cables. The dispersion of G654 fiber in 1.3 μm wavelength area is zero, while the dispersion in 1.55 μm is larger (17 ps/km·nm - 20 ps/km·nm).

2.

Full-wave fiber The full-wave fiber, water peak free fiber, eliminates the appended water peak attenuation caused by the OH- ions by eliminating OH- ions near the 1385 nm wavelength. In this way, the fiber attenuation is only determined by the internal scattering loss of the silicon glass. Full-wave fiber is numbered as G.652 C&D in ITU-T Recommendations. It is one kind of G.652 fiber. Its full name is wavelength-expanded dispersion non-shifted single-mode fiber.

The attenuation of the full-wave fiber becomes flat at the band of 1310 nm1600 nm. As internal OH- ions are already eliminated, no water peak attenuation will occur even when the fiber is exposed to hydrogen gas. It has the long-term attenuation reliability. 26

Chapter 2

Overview of Optical Fiber

The full-wave optical fiber can provide a complete transmission band from 1280 nm to 1625 nm. The available wavelength range is about 1.5 times of the wavelength range of ordinary fibers. 3.

Real-wave fiber The real-wave fiber is a kind of non-zero dispersion shifted single-mode fiber (G.655 fiber) widely used at present. Its fiber characteristics are similar to those of G.655 fiber. The zero dispersion point is in short-wavelength area below 1530 nm. In 1549 nm - 1561 nm band, the dispersion coefficient is 2.0ps/nm·km 3.0ps/nm·km. The real-wave fiber has small dispersion slope and dispersion coefficient with the capability of tolerating higher non-linear effect. It is applicable to large-capacity optical transmission systems, and thus reducing the network construction cost.

4.

Fiber with large effective fiber core area It also belongs to non-zero dispersion shifted single-mode fiber (G.655 fiber). Essentially, it improves non-linear resistance capability of the system. The main performance of super-speed system is limited by dispersion and non-linear effect. Usually, dispersion can be eliminated through dispersion compensation. But the non-linear effect cannot be eliminated through linear compensation. The effective area of the fiber determines the fiber non-linear effect. Larger effective area means higher optical power affordable, that is, better resistance to non-linear effect.

2.2 Transmission Characteristics of Optical Fiber 2.2.1 Loss The loss of power during transmission is one of the basic and important parameters of optical fibers. Due to the existence of loss, the optical power transmitted in fibers attenuates by index with the increase of transmission distance. 1.

Cause of optical fiber loss and low-loss window The loss of optical fiber mainly comes from the following two causes:

1)

Loss coming from the optical fiber itself, including the inherent absorption loss 27

Fundamentals of DWDM Technology

of fiber materials, absorption loss of material impurity (especially the loss caused by the remained OH- component in the optical fiber), Rayleigh scattering loss, as well as the scattering loss caused by inperfect fiber structure. 2)

Since optical cables are made of a bundle of clustered optical fibers, the layout of optical cables, connection of optical fibers, and coupling and connection of the transmission system may all cause the additional loss of fibers, including bending loss, microbending loss, coupling loss in the optical fiber line, and coupling loss between optical components. The fiber attenuation spectrum is shown in Fig. 2.1-3. As shown in the figure, the average loss of window I is 2 dB/km, that of window II is 0.3dB/km to 0.4dB/km, and that of window III is 0.19dB/km to 0.25dB/km. The 1380 nm point in window V is an OH- absorption peak.

2.

The line losses of common SMFs are listed in Table 2.2-1. Table 2.2-1 Loss of Common SMFs Fiber Type

Typical loss (1310 nm) Typical loss (1550 nm) Working window

3.

G.652

G.653

G.655

0.3dB/km-0.4dB/km

-

-

0.15dB/km-0.25dB/km

0.19dB/km-0.25dB/km

0.19dB/km-0.25dB/km

1310 nm and1550 nm

1550 nm

1550 nm

Relationship between loss and Optical Signal-to-Noise Ratio (OSNR) OSNR is the ratio between optical signal power and noise power. It is a very important parameter for estimating and measuring the system bit error performance, engineering design and maintenance. Take the OSNR at the receiving end of a DWDM system for example. The calculation formula is: OSNR= Pout - 10 log M - L + 58 - NF - 10 log N Where, Pout: the input optical power (dBm) M: Number of multiplexing channels of the DWDM system 28

Chapter 2

Overview of Optical Fiber

L: Loss between any two optical amplifiers, that is, section loss (dB) NF: Noise figure of EDFA (dB) N: Number of optical amplifiers between optical multiplexer and optical demultiplexer of the DWDM system. The formula shows that when the other parameters keep unchanged, greater line loss leads to lower OSNR, which means decreased transmission quality of the optical line. In the initial design of a DWDM system, besides the loss limit and dispersion limit, the OSNR at the receiving end, Q value and Bit Error Ratio (BER) should also be considered. The design is qualified only when all these three factors satisfy the requirements.

2.2.2 Dispersion After the incidence optical pulse signal has been transmitted through a long distance, time spreading occurs on the optical pulse waveform at the output end of the fiber. This phenomenon is called dispersion. Fig. 2.2-1 illustrates the dispersion in a Single-Mode Fiber (SMF) for example. Optical power

SMF

Time

Optical power

Time Emergent optical pulse waveform

ncident optical pulse waveform

Fig. 2.2-1

Dispersion in SMF

Dispersion will cause inter-symbol interference, affect the correct judgment of optical pulse signal at the receiving end, deteriorate the BER performance and severely affect the information transmission. Dispersion in the SMF is mainly caused by different transmission rates of different frequency components in the optical signal. This kind of dispersion is called chromatic dispersion. In the area where the chromatic dispersion is negligible, the polarization mode dispersion is the major part of SMF dispersion.

29

Fundamentals of DWDM Technology

2.2.2.1 Chromatic Dispersion 1.

Brief introduction to chromatic dispersion Chromatic dispersion includes material dispersion and wave-guide dispersion.

1)

Material dispersion: The fiber material of quartz glass has different refractive indexes for different optical wavelengths; while light source has certain spectral width, and different wavelengths result in different group rates. Therefore, the optical pulse spreading occurs.

2)

Wave-guide dispersion: For a transmission mode of the fiber, it is the pulse spreading caused by different group rates in different optical wavelengths. This dispersion is related to the wave-guide effect of fiber structure, so it is also called structure dispersion. Material dispersion is greater than wave-guide dispersion. According to the dispersion calculation formula, the material dispersion at a specific wavelength may be zero, and this wavelength is called the zero dispersion wavelength of the material. Fortunately, this wavelength is in the low-loss window near 1310 nm. For example, G.652 fiber is the zero dispersion fiber. Although the optical components are much affected by dispersion, there is a tolerable maximum dispersion value (dispersion tolerance). As long as the generated dispersion is within the tolerance, normal transmission can be ensured.

2.

Influence of chromatic dispersion Chromatic dispersion will result in pulse spreading and chirp effect.

1)

Pulse spreading: It is the major influence of fiber dispersion on the system performance. When the transmission distance is longer than the fiber dispersion length, the pulse spreading is too large. At this time, the system will generate serious inter-symbol interference and bit errors.

2)

Chirp effect: Dispersion not only results in pulse spreading but also makes pulse to generate phase modulation. Such phase modulation makes different parts of the pulse to generate different offsets from the central frequency, so that different parts have different frequencies, which is called Chirp effect of pulse. Due to chirp effect, the fiber is divided into normal dispersion fiber and abnormal dispersion fiber. In the normal dispersion fiber, the high-frequency 30

Chapter 2

Overview of Optical Fiber

component of the pulse is located at the rear edge of the pulse, and the low-frequency component is located at the front edge of the pulse. In the abnormal dispersion fiber, the low-frequency component of the pulse is located at the rear edge of the pulse, and the high-frequency component is located at the front edge of the pulse. In the transmission line, proper usage of these two fibers can offset the chirp effect and remove the pulse dispersion spreading. Since the DWDM system mostly works in the 1550 nm window, if G.652 fiber is used, it is required to use the DCF fiber with negative wavelength dispersion to compensate the dispersion and reduce the total dispersion value of the whole transmission line. 2.2.2.2 Polarization Mode Dispersion (PMD) PMD is a kind of physical phenomenon existing in the fields of optical fiber and optical component. The basic mode in SMF has two polarization modes that are orthogonal. In ideal cases, two polarization modes should have the same feature curve and transmission characteristics. However, due to geometrical and pressure asymmetry, two polarization modes have different transmission rates, resulting in delay and PMD, as shown in Fig. 2.2-2. Usually, the unit of PMD is ps/km1/2. Optical fiber Emergent light

Incident light

Delay

Fig. 2.2-2 PMD in SMF

In the digital transmission system, PMD results in pulse separation and pulse spreading, degrades transmission signal, and limits the transmission rate of carriers. Compared with other dispersions, PMD can almost be omitted but cannot be totally eliminated. Instead, it can only be minimized through optical components. The narrower the pulse in the ultra-high speed system is, the greater the PMD influence is. 31

Fundamentals of DWDM Technology

2.2.3 Non-Linear Effect In common fiber communication systems, the transmitting optical power is low and the fiber exhibits a linear transmission feature. For the DWDM system, however, the fiber exhibits the non-linear effect after EDFA is used. The non-linear effect of the fiber results in a serious cross-talk between multi-wavelength channels in DWDM system; leads to additional attenuation of the fiber optic transmission system; restricts the light-emitting power, EDFA amplification performance, and current-free regenerative relay distance. Four non-linear effects are introduced in this section, including Self-Phase Modulation (SPM), Cross-Phase Modulation (XPM), Four Wave Mixing (FWM), Stimulated Raman Scattering (SRS), and Stimulated Brillouin Scattering (SBC). 1.

Self-Phase Modulation (SPM) Due to the dependency between refractive index and light intensity, the refractive index changes during optical pulse continuance, and the pulse peak phase is delayed for both front and rear edges. With the transmission distance increasing, the phase shift keeps accumulating and represents a large phase modulation upon reaching a certain distance, resulting in the spectrum spreading which causes pulse spreading. This process is called SPM, as shown in Fig. 2.2-3. Strength

Spectrum before transmission

λ

Spectrum after transmission

λ

Pulse width before transmission Strength

Pulse width after transmission

Fig. 2.2-3

32

Self-Phase Modulation (SPM)

Chapter 2

Overview of Optical Fiber

When the system works in the fiber working area with negative dispersion index (such as the short wavelength area of G.653 fiber, or working area with negative dispersion of G.655 fiber), SPM will result in smaller dispersion limit distance. When the system works in the fiber working area with positive dispersion index (such as the long wavelength area of G.652 or G.653 fiber, or working area with positive dispersion of G.655 fiber), SPM will result in greater dispersion limit distance. The SPM effect mainly occurs within a certain distance from the transmitter. In addition, the low-dispersion fiber can also reduce such SPM effect on system performance. 2.

Cross Phase Modulation (XPM) When two or more light waves with different frequencies are simultaneously transmitted in non-linear media, the amplitude modulation of each frequency light wave will result in the corresponding changes of the fiber refractive index, resulting in non-linear phase modulation of the light wave with other frequency. This is called XPM. XPM often occurs along with SPM. XPM may cause a series of non-linear effects, such as signal interference between DWDM system paths, and non-linear dual-refraction of fiber, leading to unstable polarization of the fiber transmission. Meanwhile, XPM also affects waveform and spectrum of pulse. Increasing the dispersion properly can reduce the XPM influence.

3.

Four Wave Mixing (FWM) FWM refers to a physical process of energy exchange between multiple optical carriers caused by non-linear effect of the fiber, when multiple optical carriers of different frequencies with high power are simultaneously transmitted in one fiber. FWM results in optical signal energy attenuation in multiplexing channels and channel cross-talk. As shown in Fig. 2.2-4, a new optical wave is generated at another wavelength due to FWM effect.

33

Fundamentals of DWDM Technology

Incident light

Emergent light

New light

Fig. 2.2-4

Four Wave Mixing (FWM)

The generation of FWM is related to fiber dispersion. The mixing efficiency is the highest when the dispersion is zero. With the increase of dispersion, the mixing efficiency reduces rapidly. The DWDM system uses the G.655 optical fiber to avoid the FWM effect in the 1550 nm zero-dispersion wavelength area. 4.

Stimulated Raman Scattering (SRS) SRS belongs to the stimulated non-elastic scattering process caused by the non-linear effect. It originates from the mutual action and energy exchange between photon and optical phonon (molecular vibration status). The SRS effect results in the attenuation of the short-wavelength signals and reinforcement of the long-wavelength signals, as shown in Fig. 2.2-5. Incident light

Emergent light

Power

Power ...

λ1

...

λ1

λ2 λ3 λ Fig. 2.2-5

λ2 λ3

λ

SRS phenomenon

SRS effect is widely applied in the fiber optic transmission, for example, distributed Raman amplifier can be made based on the Raman gain to provide distributed broadband amplification for optical signals, the DRA board of ZTE DWDM equipment implements the optical amplification function through the SRS effect. On the other hand, SRS exerts certain negative influence on the transmission system. In the DWDM system, lights in the short-wavelength channel serve as the pump light to transfer energy to the long-wavelength channel, resulting in Raman crosstalk between channels. 5.

Stimulated Brillouin Scattering (SBS) SBS belongs to the stimulated non-elastic scattering process caused by the 34

Chapter 2

Overview of Optical Fiber

non-linear effect. It originates from the mutual action and energy exchange between photon and acoustical phonon (crystal vibration status). The SBS effect can be used to make the fiber Brillouin laser and amplifier. On the other hand, SBS results in unstable signal light source and crosstalk between reverse transmission channels. However, along with the increase of system transmission rate, the SBS peak gain obviously reduces. Therefore SBS will not greatly affect the high-speed fiber transmission system.

35

3 Key Technologies of DWDM System Key points z

Basic structure of DWDM systems

z

Light source technology

z

Optical wavelength division multiplexing and de-multiplexing technologies

z

Optical transponder technology

z

Optical amplifying technology

z

Supervision technology

3.1 Basic Structure of DWDM System The DWDM system multiplexes several or dozens of optical channel signals with different nominal wavelengths to one fiber for transmission, with each optical channel carrying one service signal. The basic structure of a unidirectional DWDM system is shown in Fig. 3.1-1. Optical transmitter TX1 OTU

TX2 TX3

TXn

OTU OTU

.. .

OTU

Optical receiver

Optical regenerating amplifier

G.69 2 1 2 3

OM

OBA

OP A

OLA

OD

1

OTU

2

OTU

3

OTU

n

n

Receiver/transmitter of optical supervision channel Transmitter of optical supervision channel

Receiver of optical supervision channel

OTU = Optical Transponder Unit,

OM = Optical Multiplexer

OBA = Optical Booster Amplifier,

OLA = Optical Line Amplifier

OPA = Optical Pre-Amplifier,

Fig. 3.1-1

OD = Optical Demultiplexer

Basic Structure of DWDM System 36

.. .

OTU

RX1 RX2 RX3

RXn

Chapter 3

1.

Key Technologies of DWDM System

Optical transmitter end TX1…TXn, the optical transmitters of all the multiplexing channels, respectively transmit the optical signals (λ1, λ2 …λn, with the corresponding frequencies as f1, f2…fn) with different nominal wavelengths. Each optical channel carrys different service signals, such as standard SDH signal, ATM signal and Ethernet signal. The optical multiplexer (OM) combines these signals into one beam of optical wave, which will be output by the OBA to the fiber for transmission.

2.

Optical receiver end After the optical wave in the line fiber being amplified through the OPA, it is de-multiplexed by the optical de-multiplexer (OD) and then the signals of different wavelengths are respectively input to the corresponding multiplexing channel optical receivers, RX1…RXn.

3.

Optical regenerating amplifier end It is located in the middle of the optical transmission section, on which the optical signals are amplified by optical amplifiers (OAL, or OBA+OPA).

4.

Optical supervisory channel In the DWDM system shown in Fig. 3.1-1, an independent wavelength (1510 nm) is used as the optical supervisory channel for transmitting optical supervision signals. The optical supervision signals carry NE management and monitoring information of the DWDM system, so as to manage the DWDM system effectively with the network management system.

5.

Network management system This module is omitted in Fig. 3.1-1. The DWDM NMS is capable of managing optical amplifying units (such as OBA, OLA and OPA), wavelength division multiplexers, OTUs and the performance of supervisory channel on one platform. In addition, it can manage the equipment in terms of performance, fault, configuration and security. The information of the NMS is carried by the supervision signals in the optical supervisory channel.

The transmission in the DWDM system with 40 wavelengths or below adopts the C band and the spacing 100 GHz. Fig. 3.1-2 illustrates the principle diagram of the DWDM system. 37

Fundamentals of DWDM Technology

Fig. 3.1-2

40-Wavelength DWDM Optical Transmission System Principle (Bidirectional)

The transmission in the DWDM system with 80 wavelengths or below adopts the C band and the spacing 50 GHz with the application of Interleave technology. Fig. 3.1-3 illustrates the principle diagram of the DWDM system.

38

Chapter 3

Fig. 3.1-3

Key Technologies of DWDM System

40-Wavelength DWDM Optical Transmission System Principle (Bidirectional)

In the 160-wavelength DWDM transmission system, 80 wavelengths are transferred in C band and L band respectively with the spacing of 50 GHz. Fig. 3.1-4 illustrates the principle diagram of the 160-wavelength DWDM system.

Fig. 3.1-4

160-Wavelength DWDM Optical Transmission System Principle (Unidirectional)

39

Fundamentals of DWDM Technology

3.2 Light Source Technology 1.

Type of optical sources At present, the semi-conductor optical sources widely used are Laser Diode (LD) and Light Emitting Diode (LED). LD is coherence light source, with large in-fiber power, narrow spectral line width and high modulation rate. It is applicable to the long-distance high-speed system. The LED is non-coherence light source, with small in-fiber power, broad spectral line width and low modulation rate. It is applicable to short-distance low-speed system. The light source of the DWDM system adopts the semi-conductor laser diode.

2.

Features of DWDM system light source

1)

Providing standard and stable wavelength The DWDM system has very strict requirements for the operating wavelength of each multiplexing channel. Wavelength drift will cause unstable and unreliable operation of the system. The common wavelength stabilization measures are temperature feedback control method and wavelength feedback control method.

2)

Providing rather large dispersion tolerance Fiber transmission may be limited by system loss and dispersion. With increased transmission rate, the dispersion influence is larger. The dispersion limit can be solved by using optical fiber cables with small dispersion coefficient or semi-conductor laser with narrow spectral width. For the optical cables have been laid, minimizing spectral width of light source devices is an effective measure to solve the dispersion limit problem.

3.

Modulation modes of DWDM system laser There are two methods of light source intensity modulation: Direct modulation and indirect modulation (that is, external modulation).

1)

Direct modulation Direct modulation means controlling the working current of semi-conductor laser directly with the electrical pulse code stream, and thus making it generate

40

Chapter 3

Key Technologies of DWDM System

the optical pulse stream corresponding to the electrical signal pulse. For example, when the electrical pulse signal is "1", the working current of the laser is larger than its current threshold; and then it generates an optical pulse. When the electrical pulse signal is "0", the working current of the laser is smaller than its current threshold; therefore it does not generate optical pulse. The direct modulation mode is simple, with low loss and low cost. But, the super-speed change of working current will result in modulation chirp easily. And the chirp will limit transmission rate and distance. The direct modulation mode is often used in the transmission system composed of G.652 fiber, with transmission distance shorter than 100 km and rate lower than 2.5 Gbit/s. 2)

Indirect modulation (external modulation) The external modulation mode refers to indirectly control (modulate) the continuous light generated by the laser, which is in the continuous light emitting status, and thus obtaining optical pulse stream. Therefore, in external modulation case, the laser generates stable high-power light, which is modulated in low chirp. And the external modulation can obtain the maximum dispersion value much greater than that in direct modulation.It is applicable to the long-distance transmission system at rate over 2.5 Gbit/s. At present, common external modulators include Electrical Absorption modulator (EA) and waveguide Mach-Zehnder (M-Z) modulator.

·

EA modulator It uses absorber controlled by electrical pulse signals to absorb or not absorb the optical wave transmitted by the continuous-wave semi-conductor laser (CW), and thus control optical pulse stream indirectly with the electrical pulse signal stream. The EA light source features small size, high integration, low driving power and low power consumption. The maximum dispersion can reach 12 000 ps/nm.

·

Waveguide M-Z modulator At the input end, the CW is in continuous wave working status. The optical wave emitted by it is divided into two equal signal channels by the optical

41

Fundamentals of DWDM Technology

de-multiplexer, which will respectively enter two optical tributaries of the modulator. Under the control of electrical pulse stream, the modulator performs phase modulation to the optical signals. At the output end, two optical tributaries are combined by the optical multiplexer. When the signal phases in two optical tributaries are reverse to each other, the optical multiplexer has no optical signal output; when the signal phases in two optical tributaries are the same, the optical multiplexer has optical signal output. In this way, the optical pulse stream is controlled by the electrical pulse stream. The M-Z light source features high modulation rate, large maximum dispersion value, and large extinction ratio. Its chirp coefficient can be zero in theory. However, its disadvantage is that polarization maintaining fiber must be used to connect the laser and the modulator, because modulation status is related to light polarization status.

3.3 Wavelength Division Multiplexing/Demultiplexing Technology 3.3.1 Overview The optical wavelength division multiplexer and de-multiplexer, also called optical multiplexer and de-multiplexer, is actually a kind of optical filter. At the transmitting end, the Optical Multiplexer Unit (OMU) combines the optical signals with nominal wavelength in each multiplexing channel into a beam of optical wave, and then transmits it into the fiber for transmission, that is, multiplexing optical wave. At the receiving end, the Optical De-multiplexer Unit (ODU) divides the optical wave in the fiber into optical signals with formal nominal wavelength of each multiplexing channel, and then inputs them into corresponding optical channel receivers, that is, de-multiplexing optical wave. Since the performance of OMU and ODU determine the system transmission quality, the attenuation, offset and channel crosstalk of them must be small.

3.3.2 Introduction to Optical Multiplexer Four types of common OMs are briefly introduced below, as well as the OM types often used in the DWDM systems with different wavelength numbers. 42

Chapter 3

1.

Brief introduction to common OMs

1)

Grating OM

Key Technologies of DWDM System

The grating type of OM is an angular dispersion type of device. Since the optical signals with different wavelengths have different refractive angles on the grating, it divides and combines the optical signals with different wavelengths. Its working principle is shown in Fig. 3.3-1. λ 1,2,3,...n

λ1 λ2 λ3

Fig. 3.3-1

λ4

λn

Working Principle of Grating OM

It has sound wavelength selection performance, and is capable of narrowing wavelength spacing to about 0.5 nm. However, the manufacture of grating should be very precise which make it not suitable for large-batch manufacture. So it is just often used for research in the laboratory. 2)

Dielectric thin film OM It is composed of Thin Film Filter (TFF). TFF consists of dozens layers of dielectric films with different materials, different refractive indexes and different thickness. One layer features high refractive index and the other layer features low refractive index; therefore TFF emerges a passband within certain wavelength range while a stopband within other wavelength ranges. In this way, the desired filtering performance is got. The working principle is shown in Fig. 3.3-2.

43

Fundamentals of DWDM Technology λ 1,2,3,...n

λ1 λ3

λ2

λ5 λ4

λ7 λ6

Fig. 3.3-2

Working Principle of Dielectric Thin Film OM

The dielectric thin film OM is a kind of compact passive optical device with stable structure, featuring flat signal passband, low insertion loss and sound channel isolation. 3)

Array Waveguide OM (AWG) AWG OM is the flat waveguide device based on optical integration technology. Its working principle is shown in Fig. 3.3-3.

Fig. 3.3-3

Working Principle of AWG OM

Due to compact structure and low insertion loss, it is the best scheme for optical wavelength division multiplexing/de-multiplexing in the optical transport network. 4)

Coupling OM It is a kind of surface interactive device with two or more fibers close to each other and properly melted, which is mainly used as OM. The working principle is shown in Fig. 3.3-4.

44

Chapter 3

Key Technologies of DWDM System

λ1 λ2 λ3 λ4 λ5

λ1，2，3……

λ6 λ7 λ8

Fig. 3.3-4

Working Principle of Coupling OM

The coupling OM can only implement the multiplexing function, with low cost but large insertion loss. 2.

Multiplexer/de-multiplexer in DWDM systems The relationship between systems with different wavelengths and their corresponding optical wavelength division multiplexers used is shown in Table 3.3-1.

Table 3.3-1

Relationship between DWDM Systems and Corresponding Optical Wavelength Division Multiplexers

OMD &Wavelength Below 32 wavelengths

OM 40 wavelengths

Above 80 wavelengths

Below 32 wavelengths

OD 40 wavelengths

Above 80 wavelengths

Coupling type

√

-

-

-

-

-

Array waveguide type

√

√

-

√

√

-

Dielectric thin film type

√

√

-

√

√

-

Grating type

-

-

√

-

-

√

Type

3.3.3 Main Performance Indices 1.

Multiplexing channel quantity It represents the quantity of optical channels to be multiplexed by the optical wavelength multiplexer. The channel quantity is closely related to the resolution and isolation of the device.

2.

Insertion loss The insertion loss is the attenuation effect of wavelength division multiplexer itself on optical signals and it will affect the transmission distance directly. 45

Fundamentals of DWDM Technology

Different types of wavelength division multiplexers have different insertion loss. The multiplexer with smaller insertion loss is preferable. 3.

Isolation It represents the isolation degree between multiplexing optical channels in the optical device. The higher the channel isolation is, the better is the frequency selection performance of the wavelength division multiplexer. Consequently, the crosstalk suppression ratio becomes higher and the mutual interference between multiplexing optical channels becomes lower. It is meaningful only for the wavelength sensitive devices (TFF type and AWG type devices). It is not meaningless for coupling devices.

4.

Reflection coefficient It is the ratio between the reflection optical power and incidence optical power at the input end of the wavelength division multiplexer. Smaller coefficient is preferable.

5.

Polarization Dependent Loss (PDL) It represents the maximum change value of the insertion loss caused by the change of optical wave polarization status. Light is the electromagnetic wave with extremely high frequency, therefore, there is the problem of wave vibration direction (polarization). For the optical signals input to the wavelength division multiplexer, their polarization statuses will not be totally consistent. And the same wavelength division multiplexer has different attenuation effects on the optical waves in different polarization statuses. Smaller PDL value is preferable.

6.

Temperature coefficient It represents the central working frequency offset of the multiplexing channel caused by the ambient temperature change. The wavelength division multiplexer with smaller temperature coefficient is preferable. Smaller coefficient means more stable central working frequency of the multiplexing channels.

7.

Bandwidth The bandwidth is a parameter of the wavelength sensitive devices (TFF type and AWG type devices). It is meaningless for coupling multiplexers. 46

Chapter 3

Key Technologies of DWDM System

The bandwidth is divided into channel bandwidth @-0.5 dB and channel bandwidth @-20 dB. ·

Channel width @ -0.5 dB It refers to the corresponding operating wavelength change when the OD insertion loss decreases by 0.5 dB. It describes the bandpass feature of the OD. A sound bandpass feature curve should be flat and wide. Greater bandpass value is preferable.

·

Channel width @ -20 dB It refers to the corresponding operating wavelength change when the OD insertion loss decreases by 20 dB. It describes the stopband feature of the OD. The stopband feature curve should be sharp. Smaller bandwidth value is preferable.

3.4 OTU Technology 3.4.1 Overview The main purpose of Optical Wavelength Transponder Unit (OTU) is wavelength conversion. It converts the non-nominal wavelength of the optical channel signal into the nominal optical wavelength compliant with ITU-T G.692, which is accessed to the DWDM system. OTU also provides other functions: 1.

Standard and stable light source The DWDM system needs to multiplex multiple wavelengths in a low-loss window, with small wavelength spacing, so the central frequency of the DWDM light source must work stably in the nominal central frequency sequence specified in ITU-T recommendations.

2.

Light source with rather large dispersion tolerance The increase of passive regeneration distance in the DWDM system requires the greater dispersion tolerance distance of the light source, and the capability of solving non-linear effect of the fiber.

3.

Capable of being used as a regenerator 47

Fundamentals of DWDM Technology

When the transponder unit serves as a regenerator, it has the data regeneration function, which is an optional function of the OTU.

3.4.2 Working Principle and Performance Indices 1.

Working principle The working principle of the OTU is shown in Fig. 3.4-1.

G.957

Shaping, timing, (regeneration)

O/E

Optical input

Fig. 3.4-1

G.692

E/O

Optical output

Working Principle of OTU

The OTU converts the multiplexing optical channel signals which accord with the ITU-T G.957 to electrical signals through O/E conversion, and implements shaping, timing extraction and data regeneration (whether perform regeneration depends on actual situations) for the electrical signals, and then performs E/O conversion to output optical signals whose wavelength, dispersion and optical transmitting power accord with G.692 specifications. If only shaping and timing processing (that is, 2R functions) are implemented after the O/E conversion, this OTU only implements the function of wavelength conversion, and thus the transmission distance supported by it will be short. If shaping, timing processing and regeneration (that is, 3R functions) are implemented after O/E conversion, then this OTU has the additional function of regenerator (REG) actually. 2.

Main performance indices

1)

System operating wavelength area It is located in the 1550 nm low-loss window, being divided into C band and L band.

·

C band (conventional band) Wavelength range: 1530 nm - 1565 nm Working frequency: 196.05 THz - 192.10 THz (1 THz = 1000 GHz)

·

L band (long-wavelength band)

48

Chapter 3

Key Technologies of DWDM System

Wavelength range: 1565 nm - 1625 nm Working frequency: 190.90 THz - 186.95 THz

y Note Generally, the working range of the DWDM system is represented by frequency.

2)

Channel spacing Channel spacing means the nominal frequency difference between two adjacent multiplexing channels, covering uniform channel spacing and non-uniform channel spacing. At present, uniform channel spacing is used mostly. The minimum channel spacing of the DWDM system is the integer times of 50 GHz.

·

When the multiplexing channels are 8 wavelengths, the channel spacing is 200 GHz.

·

When the multiplexing channels are 16/32/40 wavelengths, the channel spacing is 100 GHz.

·

When the multiplexing channels are above 80 wavelengths, the channel spacing is 50 GHz. Smaller channel spacing requires higher resolution of the OD and means more multiplexing channels.

3)

Nominal central frequency It refers to the central wavelength (frequency) corresponding to each multiplexing channel in the DWDM system. For example, when the multiplexing channels are 16/32/40 wavelengths, the central frequency of the first wavelength is 192.1 THz and the channel spacing is 100 GHz. The frequency increases in ascending order.

4)

Central frequency offset It is also called frequency offset. It refers to the offset between the actual working central frequency of the multiplexing optical channel and nominal 49

Fundamentals of DWDM Technology

central frequency. According to the national standards, in the system with frequency spacing of 100 GHz, the maximum central frequency offset is ±20 GHz (about ±0.16 nm) when the rate is below 2.5 Gbit/s, and it is ±12.5 GHz when the rate is 10 Gbit/s. For the system with frequency spacing as 50 GHz, the maximum central frequency offset is ±5 GHz. The maximum central frequency offset is the value which can still be met when the designed life cycle of the system expires, with temperature, humidity and other factors taken into consideration. 5)

Dispersion tolerance Dispersion reflects the spreading of the optical pulse during the transmission in the fiber. The pulse spreading will result in decreased extinction ratio of signal pulse at the receiving end, that is, the electrical level of bit “1” and bit “0” are close to each other, which may lead to mistaken judgment of the receiver. To avoid bit errors, it is required to take proper measures to compensate the optical pulse spreading in the fiber transmission process, for the pulse spreading will be more and more serious with the increasing of transmission distance. The requirement of DWDM system for the fiber chromatic dispersion coefficient is basically that of a single multiplexing channel signal for fiber chromatic dispersion coefficient. In addition, since the passive regenerating distance of the DWDM system is much greater than that of a single SDH system, the dispersion tolerance distance of the system light source must be prolonged.

6)

Receiver sensitivity The receiver sensitivity refers to the minimum average receiving optical power on the OTU input port when the input signals are located in the 1550 nm window and the bit error rate reaches 10-12.

7)

Overloaded optical power The overloaded optical power refers to the maximum average receiving optical power on the OTU input port when the input signals are located in the 1550 nm window and the bit error rate reaches 10-12.

50

Chapter 3

Key Technologies of DWDM System

3.4.3 Classification and Application of OTU According to the location in DWDM systems, OTU can be classified into transmitter OTU (OTUT), receiver OTU (OTUR) and regenerator OTU (OTUG). Fig. 3.4-2 illustrates the application of three kinds of OTUs in a DWDM transmission system. λ1

OTUT

λ2

OTUT

Line fiber

OTUG O M

O A

O A

O D

OTUG

O M

O A

O A

O D

OTUR

λ1

OTUR

λ2

Internal fiber

OM: Optical Multiplexer OD: Optical Demultiplexer OA: Optical Amplifier

Fig. 3.4-2

1.

Application of OTU

OTUT The OTUT is applied between the client-side equipment at the transmit end and OM. It forwards the optical signal that meets the specification of G.692 to the OM. Besides the Optical/Electrical/Optical (O/E/O) conversion function, the OTUT also has the functions of reshaping and retiming (2R) as well as the checking of the byte B1.

2.

OTUR The OTUR is applied between the OD and the client-side equipment at the receive end. The optical signal output by the OD meets the specification of G.692. The OTUR has similar functions as the OTUT, including wavelength conversion, 2R and B1 byte check.

3.

OTUG The OTUG is applied between OM and OD. Both the input and the output optical signal of it meet the specification of G.692. Besides the O/E and E/O conversion functions, the OTUG also has the functions of reshaping, retiming and regenerating (3R), as well as the checking of the byte B1. With the 3R function, the OTUG is equivalent to a general regenerator (REG).

51

Fundamentals of DWDM Technology

3.5 Optical Amplifying Technology For the long-distance optical transmission, optical power gradually decreases with the increasing of transmission distance. The output of the light source laser usually is not more than 3 dBm, otherwise, the laser life cycle may be unqualified. In addition, in order to ensure correct signal receiving, the receiving power at the receiving end must always be a certain value, for example, -28 dBm. Therefore, the optical power becomes the major factor determining the transmission distance. Optical amplifier is the technology to solve the problem of optical power limit. Without the O/E/O conversion, it directly amplifies the optical signals. The classification of optical amplifier is shown in Fig. 3.5-1.

Fig. 3.5-1 Optical Amplifier Classification

In this section, the EDFA and Raman fiber amplifier are introduced.

3.5.1 EDFA Technology 3.5.1.1 Technical Principle of EDFA 1.

Amplifying principle Erbium (Er) is a kind of lanthanon. In the fiber manufacture process, certain quantity of Er3+ ions is doped to form Erbium Doped Fiber (EDF). The Er3+ ions in such fiber will absorb photon energy to make it own energy level change, which is called stimulation. The light source for stimulation is called pump light source, and the corresponding transmitting stimulation optical wave is called as pump light. The working principle is shown in Fig. 3.5-2.

52

Chapter 3

Key Technologies of DWDM System

N3~0 1550 nm signal light

1480nm

980 nm pump light

N2 1550 nm stimulated emission

N1

Fig. 3.5-2

Working Principle of EDFA

The Er3+ ion free from stimulation is at the lowest energy level. When the pump light is shot in, the Er3+ ion absorbs energy from the pump light and transits itself to the higher energy level. At the higher energy level, the Er3+ ions are in instable status, therefore they continuously converge to metastable energy level in non-radiant transition format, and thus implementing population inversion distribution. When the optical signals with 1550 nm wavelength pass this segment of EDF, the metastable particles are transited to the ground status in stimulated emission format, and then photons which are the same as those in the incoming signal light are generated. In this way, the optical signals are amplified. 2.

Composition The EDFA consists of the EDF, bump light source, coupler and isolator, as shown in Fig. 3.5-3.

λ λ

λ λ

λ

λ

Fig. 3.5-3 EDFA Composition

The coupler is used to combine the signal light with pump light. The isolator is used to suppress the light reflection, to ensure stable working of the optical amplifier. The pump laser generates pump light source. 53

Fundamentals of DWDM Technology

3.

Main performance indices

1)

Gain (G) It is the ratio between output optical signal power and input optical signal power. Greater gain means more powerful amplifying capability.

2)

Noise Figure (NF) It is the ratio between the Signal-Noise Ratio (SNR) at the EDFA input end and SNR at output end. EDFA noise comes from many ways, such as signal shot noise, internal reflection noise and Amplified Spontaneous Emission (ASE) noise, which is the major part of EDFA noise.

y Note ASE is the emission noise caused by the EDFA’s own factors, such as the unbalance between optical transmitting area and absorption area, the different population inversion degrees (quantity of ions in stable energy level E2 and the quantity of ions in ground energy level E1 are different), the gain and working status of the EDFA.

Since the EDFA can amplify both optical signal and noise, the parameter NF appears which is closely related to the ASE noise of the EDFA. It greatly affects the system performance, especially the OSNR of the whole system. Smaller NF is preferable, for example, below 5.0 dB. 3)

Bandwidth The operating wavelength range of the DWDM system covers C band and L band. The optical amplifier needs to amplify all the multiplexing channel signals of the system, so its bandwidth should be wide enough.

4)

Gain flatness (Gp-p) It represents the allowed fluctuation of EDFA gain within the specified working band range. In order to get sound flatness, the aluminum doped technology is usually used in the EDF. In the DWDM system, smaller EDFA gain flatness is preferable so as to 54

Chapter 3

Key Technologies of DWDM System

minimize the difference between output optical power signals of different multiplexing channels and facilitate optical power estimation,. 5)

Total input/output power range It is the optical power range at input/output end of the EDFA. In WDM systems, an EDFA is responsible for amplifying all the multiplexing optical channel signals in the system. Therefore, its input/output optical power range should be large enough, especially for WDM systems with lots of multiplexing channels. On the other hand, to ensure the gain flatness and low noise performance, the EDFA should work in small signal working range, that is, the input/output power range of the EDFA cannot be too large. It is more important that the EDFA output power cannot be too large in order to avoid fiber non-linear effect. For this purpose, the optical power of a signal channel cannot be too large. The proper power should be determined according to the signal rate and the type of transmission fiber.

6)

Polarization Dependent Gain (PDG) Since the EDFA generates different gains for optical waves in different polarization statuses. So, the maximum EDFA gain change caused by the polarization status change of the optical wave is called as PDG. Smaller PDG value is preferable.

7)

Pump light leakage Although optical isolators are configured at the input and output ends of the EDFA, a few pump light leakage occurs. Smaller leakage is preferable. Pump light leakage represents the ratio between the pump light leakage power and the input/output pump light power.

8)

Input/output optical reflectance It is the ratio between the optical power at the EDFA input/output end and the reflection optical power. Greater value is preferable.

4.

Importance of EDFA for DWDM system To ensure the transmission quality of DWDM systems, the EDFAs used in the DWDM system must have sufficient bandwidth, flat gain, low NF and high 55

Fundamentals of DWDM Technology

output power. Proper gain flatness is especially important, which is special requirement of DWDM system for EDFA. 3.5.1.2 Classification of EDFA Depending on the location of EDFA in the DWDM system and pump source types, two EDFA classification modes are introduced below. 1.

Classification by location The EDFA is divided into Optical Booster Amplifier (OBA), Optical Line Amplifier (OLA) and Optical Pre-Amplifier (OPA).

1)

OBA: It is located behind the OTM or the transmitting light source of regenerator device, being in the front of the regeneration segment. The OBA is mainly used to boost the transmitting power so as to extend transmission distance.

2)

OLA: It is located in the middle of the regeneration segment, with the EDFA inserted directly into the fiber transmission link for amplifying signals. Multiple OLAs can be equipped in the regeneration segment as required.

3)

OPA: It is located between the end of the regeneration segment and the optical receiving device. The OPA is mainly used to pre-amplify small signals going through line attenuation, and boost the power of optical signals before entering the receiver so as to meet the sensitivity requirements of the receiver. The locations of these three kinds of amplifiers in the optical line are shown in Fig. 3.5-4. Regeneration segment OTM

OBA

OLA

OLA

OPA

OTM

Fig. 3.5-4 Locations of Amplifiers in Regeneration Segment

2.

Classification by pump source The pump sources often used now cover 980 nm and 1480 nm, for these two types of pump sources have high pump efficiency. The 980 nm pump light source has lower NF; while the 1480 nm one has higher pumping efficiency and therefore a larger output power is obtainable (about 56

Chapter 3

Key Technologies of DWDM System

3 dB higher than that of the 980 nm pump light source). In actual applications of line amplifier, most 8-channel WDM systems use the 980 nm pump source, because the WDM system of G.652 fiber mostly features dispersion limit other than loss limit. If such WDM system uses the 1480 nm pump source, the system power attenuation will increase and it is unnecessary to boost EDFA output power. WDM systems of more than 16 channels use the 1480 nm pump source instead, because enormous tributaries decrease the available power range and the pump source with higher power is necessary. A two-level pump can also be used to improve the NF and increase the output power. 3.5.1.3 Main Problems of EDFA to Be Solved The EDFA also introduces in some new problems while solving some problems of fiber transmission system. 1.

Non-linear effect EDFA amplifies the optical power through increasing the optical power shot into the fiber. However, it does not mean the greater optical power is surely the best. When the optical power is increased to certain degree, fiber non-linear effect will occur. Therefore, in the usage of fiber amplifier, it is required to control the value of the in-fiber optical power in a single channel.

2.

Bandwidth Bandwidth refers to the range of the optical wavelength which can be amplified flatly. The operating wavelength range of the EDFA in C band is 1530 nm - 1561 nm, and the one in L band is 1565 nm - 1625 nm. The gain flatness filter is used inside the EDFA, so that the EDFA has almost the same gain to each multiplexing optical channel signal within corresponding wavelength range. The gain fluctuation should be limited within the allowed range, for example, ±1 dB. Therefore, the bandwidth is closely related to the gain flatness.

3.

Optical surge When the optical line is normal, the erbium ions stimulated by the pump light are carried off by the signal light, thus implementing the amplification of the 57

Fundamentals of DWDM Technology

signal light. If the input light is interrupted, the metastable erbium ions still converge continuously, and finally the energy transient occurs, leading to optical surge. The solution of optical surge is to implement Automatic Power Reduction (APR) or Automatic Power ShutDown (APSD) function in the EDFA. In other words, the EDFA should automatically reduce power or shut down power upon no input light, and thus suppressing surge. 4.

Dispersion With the extended transmission distance, the total dispersion increases correspondingly. Therefore, the passive regeneration segment in the WDM system cannot be prolonged limitlessly. The dispersion compensation measure can be taken to prolong the passive regeneration distance of the multiplexing section.

3.5.2 Raman Amplification Technology 1.

Working principle The Raman amplification technology bases on the non-linear effect -Stimulated Raman Scattering (SRS), that is, when a light wave strong enough is transmitted on a line fiber, its energy can be translated to other wavelength section. There are two kinds of SRS: upward frequency shifted scattering and downward frequency shifted scattering. In other words, the energy is translated to short wavelength (upward) and long wavelength (downward). The downward SRS is the basis of Raman amplification. The Raman effect of fiber can be adopted in the manufacturing of broadband Raman amplifier and tunable Raman laser. The signal light is amplified by translating the energy of shortwave pump light to the long wave signal light. In the non-linear medium, the incident photons interact with phonons generated by molecule oscillation of the medium. The incident photons are scattered by the medium molecules to low-frequency Stocks photons, and other energies are translated to the phonons at the same time. Then the molecules implement the transition between oscillation states, as shown in Fig. 3.5-5. This procedure is called as the simulated Raman scattering or the Raman effect.

58

Chapter 3

Fig. 3.5-5

Key Technologies of DWDM System

Working Principle of Raman Amplifier

If the pump light is used as the incident light in the fiber, the frequency shift light of Stocks wave will be generated after the scattering effect of molecules. When the frequency of the input optical signal is same as that of the Stocks wave, the optical signal will be amplified, while the frequency downward offset is determined by the oscillating mode of the medium and the incident pump light. Therefore, different pump light can be selected to implement the amplification or oscillation for optical signals as required. The application of multiple pumps with different wavelength can provide ultra broadband amplification. The fiber Raman amplifier mainly consists of gain medium fiber and pump source. It has various types and structures according to different fiber types, pump types and modes and different amplification modes. General transmission fiber can be used; however, fibers with higher non-linear characteristic are better to achieve higher amplification efficiency. Many kinds of pump sources can be chosen, such as single pump, dual pumps and multiple pumps. The pump wavelength and power should be designed elaborately for each pump source. 2)

Features of Raman fiber amplifier

·

Based on dozens of kilometers of line fibers, it implements distributed amplification, with low NF and effective improvement of system SNR.

·

With the same SNR, it can reduce the optical power at the transmitting end and minimize the non-linear effect.

·

It can generate gain for all the wavelengths, serving as full-band amplifier (however, it should be divided into C band amplifier and L band amplifier).

·

It has flat gain. The gain wavelength range depends on the pump wavelength.

·

Since the noise of Raman fiber amplifier reduces with fiber distance increase, the fiber should be long enough. There is no requirement for the fiber type.

·

The pump conversion efficiency is low, so the high-power pump laser source is 59

Fundamentals of DWDM Technology

required. ·

The amplifying gain is low, so it needs to cooperate with the EDFA to form combined amplifier, in order to compensate the line attenuation and node insertion loss.

3)

Application If the DWDM system above 40 G only uses EDFA for amplifying, spontaneous emission will accumulate, restricting the overall system performance. Compared with EDFA, the SRA has such advantages as low noise, not introducing in additional loss upon removal of pump light, and no transient effect. Therefore, the combination of EDFA and SRA can form the important optical amplifying technology for the ultra long-haul transmission system above 40 G. Besides the reverse pumping distributed Raman amplification, other kinds of Raman amplification technologies also emerges, such as forward pump and bidirectional pump Raman amplification, which can provide higher gain and lower noise figure, achieving the flatness of gain and NF at the same time. The discrete Raman amplifier taking the Dispersion Compensation Fiber (DCF) as the gain medium can compensate the dispersion on the transmission links and implement the total ultra broadband integrated amplification of optical signals. In addition, it has the potential to adjust the gain slope. In the overall Raman transmission system, in which the distributed and discrete Raman amplifiers are used, the continuous gain bandwidth can reach 100 nm. Such system supports the ultra broadband transmission including the S band and xL band. Of course, the Raman amplification has its inherent shortcomings. The forward pump and bidirectional pump Raman amplification has the problem of pump light Relative Intensity Noise (RIN) transition, which has evident influence on the noise characteristic of the Raman amplifier. Especially in the transmission fibers with small dispersion coefficient, such as G.655 fiber, this RIN transition problem becomes more serious, and it degrades the noise figure of Raman amplifier greatly. To sum up, the discrete Raman amplifier is not better than the EDFA on the aspect of economic efficiency and noise figure.

3.6 Supervision Technology Detection, control and management are basic requirements of all the network 60

Chapter 3

Key Technologies of DWDM System

operations. To ensure the secure operation of DWDM systems, the supervision system is designed as an independent system separated from working channels and devices physically. For example, ZTE’s DWDM system uses an independent wavelength (1510 nm) and depends on no service channel, to ensure that no active amplification is required for the long distance transmission and improve the reliability. In this way, the supervision system can monitor all the NE equipment in the system.

3.6.1 Functions of Optical Supervisory Channel (OSC) Different from the conventional SDH system, the DWDM system with optical amplifier can supervise and manage EDFAs in the system additionally. Since the EDFA only amplifies optical signals without electrical signal input. Especially when it is used as an optical amplifier regenerator, it has no electrical interface connection because no service signal will be added or dropped on it. This makes it difficult for supervision. In addition, there is no special byte in the SDH overhead for monitoring the EDFA, so an electrical signal must be added to monitor the status of EDFA. The OSC is used to transmit the NE management and supervision information related to the DWDM system through a wavelength. The information involves the fault alarm, fault location, quality parameter supervision during operation, the control over backup line upon line interruption and the EDFA supervision etc. In this way, the network operator can effectively manage the DWDM system.

3.6.2 Requirements for OSC The DWDM system has the following requirements for the OSC: 1.

The OSC should not restrict the optical wavelengths (980 nm and 1480 nm) of the pump light source in the optical amplifier.

2.

The OSC should not restrict the transmission distance between two OLAs.

3.

The OSC should not restrict the services on the 1310 wavelength in the future.

4.

The OSC can still be available upon failure of the OLA. The supervision information transmitted on the OSC includes the information related to all kinds of optical amplifiers, such as the input/output optical power and the operating wavelength of pump light source etc. Therefore, the supervision would be meaningless if the OSC can not work normally when the 61

Fundamentals of DWDM Technology

optical amplifier fails. 5.

The OSC transmission is bidirectional, which ensures the supervision information can still be received by the line terminal when one fiber is broken.

6.

The segmenting of OSC transmission enables dropping supervision information or adding new supervision information on each optical amplifier regeneration station and DWDM system office station.

3.6.3 Implementation of OSC The implementation principle of OSC is shown in Fig. 3.6-1. λ

λ

λ

λ λ

λ

OM: Optical Multiplexer OD: Optical Demultiplexer OBA/OPA: Optical amplifier OTUT/OTUR: Optical transponder

Fig. 3.6-1 Implementation Principle of OSC

1.

Dropping and adding of OSC information As shown in Fig. 3.6-1, to ensure that the supervision information transmitted on the OSC can be dropped or added on each optical amplifier regeneration station and DWDM system office station without influences from optical amplifier, it is required to use a 2-wavelength OM (OM2) behind the OBA at the transmitting end to add the OSC information into the main channel; and use a 2-wavelength OD (OD2) ahead of the OPA at the receiving end to drop the OSC information.

2.

Operating wavelength of OSC For the DWDM system with line amplifiers, an additional OSC is required, which should be able to perform adding/dropping with BER as low as possible in each optical regenerator/amplifier. 62

Chapter 3

Key Technologies of DWDM System

According to ITU-T recommendations, a specific wavelength can be used as the OSC. Such wavelength can be 1,310 nm, 1,480 nm or 1,510 nm when it is out of the service transmission band, among which the 1,510 nm is preferable. Since this channel is out of the gain bandwidth of the EDFA (also called as outband OSC), the supervision signals must be dropped (from optical channel) ahead of EDFA and be added (to optical channel) behind the EDFA. As shown in Fig. 3.6-1, the OSC is added behind the OBA and dropped ahead of the OPA. 3.

Transmission rate of OSC In actual DWDM systems, most of the information really needing supervision is the working status of EDFA. So the amount of supervision information is not huge. In addition, to ensure normal operation of the OSC upon optical amplifier failure, the receiving sensitivity should be high in order to enable the supervision channel signals without being amplified covering the maximum transmission distance of major service signals. Therefore, the working rate of the OSC is set to 2 Mbit/s. With the continuous technology development, the OSC rate improves as well. For example, ZTE’s DWDM equipment can provide supervision rate of 10 Mbit/s or 100 Mbit/s.

4.

Frame structure of OSC information For the supervision system at working rate of 2 Mbit/s, thirty-two 64 kbit/s bytes are used to carry supervision information, which is transmitted and exchanged in PCM32 frame format. For the system at supervision rate of 10 Mbit/s or 100 Mbit/s, taking ZTE’s DWDM equipment as example, the supervision channel adopts 10/100 M Ethernet technology to encapsulate supervision data in IP packets. Then the supervision information is transmitted and exchanged in Ethernet data frames.

5.

Line coding The 2Mbit/s supervisory channel adopts Code Mark Inversion (CMI) as the line code type. The 10/100 Mbit/s supervisory channel adopts 4B/5B code.

6.

OSC protection

63

Fundamentals of DWDM Technology

If the OSC bidirectional transmission is interrupted because of the total break-off of the fiber, the NE management system cannot obtain the supervision information normally. At this time, the backup route, such as the Data Communication Network (DCN), should be used to transmit supervision information so as to protect the OSC.

64

4 Protection Principles of DWDM System Key points z

Principle of 1+1 protection

z

Principle of 1:N protection

z

Principle of Optical CHannel (OCH) protection

z

Principle of Optical Multiplex Section (OMS) protection

4.1 Introduction to DWDM System Hierarchy

y Note In this chapter, we take ZTE’s DWDM equipment as an example for introduction.

The DWDM system protection involves the protection of optical channel layer and optical Multiplex Section (MS) layer. First of all, we will introduce the location of each layer in the system. The DWDM system is divided into Optical Multiplex Section (OMS) layer, Optical Transport (OTS) layer, Optical Channel (OCH) layer and Optical Access (OAC) layer. The locations of these layers in the system are shown in Fig. 4.1-1, and their functions are listed in Table 4.1-1

65

Course Code Course Name

OTM

OTM OLA

G.692

TX1 TX2 TX3

OTU

1

1

OTU

OTU

2

2

OTU

OTU

3

3

OTU

.. . OTU

TXn

OM

OBA

OP A

OLA

OD

n

n OTS

RX1 RX2 RX3

.. . OTU

RXn

OTS OMS

OAC

OAC

OCH

Fig. 4.1-1 DWDM System Hierarchy

Table 4.1-1 Layer

Explanation of DWDM System Hierarchy

Location

Function Multiplexing optical channel signals and de-multiplexing multiplexed optical channel signals

OMS

Between OTMs

OTS

Between OTM and OLA, or between OLAs

Transmitting optical signals on all kinds of fibers

OCH

At the line side of optical transponder platform

Supporting OAC to convert customer signals into optical signals compliant with G.692 specifications for transmission

OAC

At the client side of optical transponder platform

Accessing customer signals

4.2 1+1 Protection In the 1+1 protection switching, optical signals are simultaneously transmitted on working line and protection line. In other words, the signals are permanently connected (bridged) with working line and protection line at the transmitting end. At the receiving end, the statuses of the signals received from these two lines are monitored and selectively connected to the line with better signal quality. So this protection mode is called "concurrent transmitting and priority receiving". For the protection of chain networks, the OMS line 1+1 protection or the OCH 1+1 protection can be implemented according to different configurations. In the case of OMS protection, the “concurrent transmitting and priority receiving” is located after the optical power amplifier and the pre-amplifier of the optical terminal. In the case of OCH protection, it is located before the OTU board at the transmitting end and after the OTU board at the receiving end of the optical terminal. For ZTE’s DWDM equipment, the 1+1 protection is implemented by the OP board. 66

Chapter 4

Protection Principles of DWDM System

4.2.1 1+1 Protection in Chain Network Depending on the location, the OP board can implement the 1+1 protection of OCH and OMS. 1.

OCH 1+1 protection One OP board is used to protect a pair of bidirectional traffic. In the case of channel 1+1 protection, the number of OP boards configured must be consistent with that of the channels to be protected. The protection channel and protected channel are transmitted in the same fiber. The channel 1+1 protection in chain networks can only protect equipment other than routes, as shown in Fig. 4.2-1.

Fig. 4.2-1

2.

OCH 1+1 Protection

OMS 1 +1 protection The 1+1 protection of OMS is in segment-by-segment 1+1 protection mode, as shown in Fig. 4.2-2.

67

Course Code Course Name

OTU OTU OTU OTU OTU OTU OTU OTU

λ1 λ2 λ3

O M D

OBA

O P

λ1

OPA

O D U

λ2 λ3 λn

O P

λ1

Line 1 in direction B

λ2

O D D

OPA

Line 2 in direction A

λn

λ3

λ1

Line 1 in direction A

OBA

Line 2 in direction B

O M U

λn

λ2 λ3

λn

OTU OTU OTU OTU OTU OTU OTU OTU

Line 1 is working channel and line 2 is protection channel.

Fig. 4.2-2

1 +1 Protection of OMS

The OP board supervises the main optical path. The switching is implemented through the optical switch inside the board when the switching conditions are met.

4.2.2 1+1 Protection in Ring Network The 1+1 protection in ring networks can also be divided into the 1+1 protection of OCH and 1+1 protection of OMS. In the ring network, the protection channel and protected channel reaches the receiving end through different routes. 1.

1+1 protection of OCH The 1+1 protection of OCH can protect not only routes but also devices. Suppose there is a ring network as shown in Fig. 4.2-3.

C Protection channel B

D

Working channel

A

Fig. 4.2-3

Ring Network

Fig. 4.2-4 illustrates the optical connection between Node A and Node B.

68

Chapter 4

Protection Principles of DWDM System

O M U OTU O P OTU

Fig. 4.2-4

2.

Working channel

O D U

O M U

O M U

O D U

O D U

Site A

O D U

Protection channe

OTU O P OTU

O M U

Site B

1+1 Protection of OCH (Ring Network)

1+1 protection of OMS In the ring network, the 1+1 protection of OMS protects the multiplexed signals. When the fiber is broken, two nodes adjacent to the broken points implement the “loop-back” function, and thus protecting all the services. It is similar to the protection mode shown in Fig. 4.2-2.

4.2.3 Features of 1+1 Protection 1.

The protection line is special, which can not be shared with other working lines.

2.

It does not need the support of signaling, being easy for implementation.

3.

Capable of being used in networks of any structure, such as point-to-point, ring and mesh networking.

4.

It is still a kind of revertive protection even having no signaling support.

5.

Low bandwidth utilization ratio and high cost.

4.3 1:N Protection 4.3.1 Working Principle In the 1:N protection switching, multiplex working lines share one protection line. Both ends of N working lines are bridged to the protection line. The protection function monitors and judges the status of received signal, and switch the services on this working line to the protection line upon detecting any deterioration or failure of service 69

Course Code Course Name

signals on the working line. This mode is called as "transmitting-receiving switching". Its working principle is shown in Fig. 4.3-1.

Fig. 4.3-1

Working Principle of 1:N Protection

4.3.2 Implementation of 1:N Protection ZTE’s DWDM equipment can provide the 1:N protection of OCH. We introduces the implementation of the 1:N protection with the application of the Electrical Switching Board (SWE). The SWE board implements the switching in an electrical cross-connect mode. At the transmitting end, N channels of service signals are input to the input ports 1 - N of the SWE board, and then are output to the OTU through the output ports 1 - N. At the receiving end, the input ports 1 - N of the SWE board receive the signals from OTU respectively, and the output ports 1 - N of the SWE board output the signals to the user terminal. The protection function is shown in Fig. 4.3-2.

Fig. 4.3-2

Functional Block Diagram of 1+1 Protection of OCH

70

Chapter 4

Protection Principles of DWDM System

If any channel of the N channels fails, once the receiving end detects the faulty service, it notifies the SWE boards at the transmitting end and receiving end through protocols, and then the receiving/transmitting end switches this channel of service to the port N + 1 to protect the service. When multiple channels of services are faulty at the same time, the service of higher priority will be protected first. The protection priority is set in the network management system.

4.3.3 Features of Channel 1:N Protection 1.

The protection line is shared by multiple working lines.

2.

Signaling support is required. The implementation process is relatively complicated.

3.

It can be used in ring and grid networks.

4.

The protection is restorable.

5.

The bandwidth utilization ratio is high but the protection reliability is low.

4.4 Two-Fiber Bidirectional Channel Shared Protection 1.

Working principle In the two-fiber bidirectional channel shared protection ring, the λ1 of the external ring forms the working channel, while the λ1 of the internal ring forms the protection channel. The working channel allows wavelengths of multiple unidirectional services being used repeatedly, and the protection channel shares protection of all services on the working channel. As shown in Fig. 4.4-1, when a cross-segment fiber fails (the symbol “×” means failure in the figure), the service passing this span is damaged, which leads to the switching operation at the transmitting end of the service. Then the service is transmitted along the protection route. Meanwhile the two switches at the receiving end act, and then the service are received from the protection route. In this way, the service protection is implemented.

71

Course Code Course Name

F ig. 4.4-1

2.

Principle of Two-Fiber Bidirectional Channel Shared Protection

Implementation of OPCS board The ZTE’s DWDM equipment ZXMP M800 implements the bidirectional channel shared protection through the Optical Channel Shared Protection (OPCS) board. Besides the channel protection of the ring network, the OPCS board also controls the status of the added protection wavelength through connecting to the optical switch, to avoid conflict of multiple services that use the same operating wavelength on the protection ring. Fig. 4.4-2 illustrates a networking example.

H

A

λ21(B A) B λ21(B A)

B

λ22(A B) λ22(A B) G

F

λ22(E F)

λ22(E F) E

λ21(F E)

λ21(F E)

Fig. 4.4-2

Wavelength Configuration of Channel Shared Protection

72

D

Chapter 4

Protection Principles of DWDM System

Suppose that a pair of bidirectional services between Site A and Site B need protection. The OPCS board should be configured firstly at Sites A and B. Then connect fibers between them. In the configuration, the services should be transmitted through different wavelengths. The service from A to B is carried by λ21 (external ring), while the service from B to A is carried by λ22 (internal ring). In this way, the operating wavelength formed by λ21 and λ22 can be repeatedly used between other nodes on the ring network. The λ21 of internal ring serves as the protection wavelength of external ring λ21. Similarly, the λ22 wavelength serves as the protection wavelength of internal ring λ22. The shared protection of multiple services in the ring network is implemented. The wavelength allocation can be flexibly adjusted. But it should be ensured that the services are bidirectional and the operating wavelengths are different. For the convenience of project debugging and maintenance, the adjacent odd and even wavelengths are allocated by default. 3.

Application features

·

It is used for loop protection.

·

The service protection is based on channels. The switching depends on the quality of signals in a channel dropped from the loop.

·

In the loop, the directions of receiving information and transmitting information on the node are reverse. The resource utilization ratio is high.

·

The switching is implemented in the adding channel node and the dropping channel node of the service.

·

The wavelength allocation is flexible.

4.5 Two-Fiber Bidirectional MS Shared Protection 1.

Working principle In the two-fiber bidirectional MS protection, the system uses the same wavelength in internal ring and external ring for mutual protection. For example, in a 32-wavelength system, the first 16 wavelengths of the internal ring serve as operating wavelengths, and the last 16 wavelengths serve as protection

73

Course Code Course Name

wavelengths. The first 16 wavelengths of the external ring serve as protection wavelengths, and the last 16 wavelengths serve as operating wavelengths. The wavelengths are complementarily distributed. The scheme can also be adopted to protect only eight wavelengths in the 32-wavelength system. In this case, eight wavelengths on the internal ring and external ring protect each other, and the other 24 wavelengths are the actual operating wavelengths. The operating wavelengths usually transmit services while the protection wavelengths usually not. Fig. 4.5-1 shows the principle diagram of MS protection for mutual protection of the wavelengths in the internal and external rings, with 16 operating wavelengths. The solid lines indicate working routes, while the dotted lines indicate protection routes of the external ring when fault occurs between D and E. B

A

H

λ17 C

Add

D

Drop

Fig. 4.5-1

2.

λ1

G

λ17 E

F

λ1

Add

Drop

Principle of Two-Fiber Bidirectional MS Shared Protection

Implementation of OPMS board The ZTE’s DWDM equipment ZXMP M800 implements the bidirectional OMS shared protection through the Optical MS Shared Protection (OPMS) board. Fig. 4.5-2 illustrates a networking example.

74

Chapter 4

H

A

Protection Principles of DWDM System

λ21(B→A) B

λ21(B→A)

B

λ43(A→B) λ43(A→B)

G

F

λ43(E→F)

λ43(E→F) E

D

λ21(F→E)

λ21(F→E)

Fig. 4.5-2

Wavelength Configuration of MS Shared Protection

Suppose that a pair of bidirectional services between Site A and Site B need protection. The OPMS board should be installed at the site A and B. Then connect fibers between them. In the configuration, the services should be transmitted through different wavelengths. Both working bands and protection bands of internal/external ring are distributed symmetrically. For example, 16 wavelengths (192.1 THz - 193.8 THz) of the external ring serve as the operating wavelengths of external ring, and 16 wavelengths (194.3 THz - 196.0 THz) of the internal ring serve as operating wavelengths of internal ring. The service from A to B is carried by λ21 (external ring), while the service from B to A is carried by λ43 (internal ring). In this way, the operating wavelength formed by λ21 and λ43 can be repeatedly used between other nodes on the ring network. The λ21 of internal ring serves as the protection wavelength of external ring λ21, while the λ43 wavelength of external ring serves as the protection wavelength of internal ring λ43. Then the shared protection of multiple services in the ring network is implemented. 3.

Application features

·

It is applicable to the loop protection.

·

The service protection is based on the multiplex section. The switching depends on the quality of the MS signals between adjacent nodes.

·

In the loop, the directions of node receiving information and node transmitting 75

Course Code Course Name

information are reverse. The resource utilization ratio is high. ·

The switching is executed between adjacent nodes of the faulty span when a fault occurs.

·

While configuring the MS shared protection, at least one Optical MS Shared Protection board with a band elimination optical switch (OPMSS) should be configured in the loop to avoid self-stimulation.

76

Appendix A Abbreviations Abbreviation

Full Name

AFR

Absolute Frequency Reference

AFEC

Advanced FEC

AIS

Alarm Indication Signal

APR

Automatic Power Reduction

APS

Automatic Protection Switching

APSD

Automatic Power Shutdown

APSF

Automatic Protection Switching for Fast Ethernet

ASE

Amplified Spontaneous Emission

AWG

Array Waveguide Grating

BER

Bit Error Ratio

BLSR

Bidirectional Line Switching Ring

BSHR

Bidirectional Self-Healing Ring

CDR

Clock and Data Recovery

CMI

Code Mark Inversion

CODEC

Code and Decode

CPU

Center Process Unit

CRC

Cyclic Redundancy Check

DBMS

Database Management System

DCC

Data Communications Channel

DCF

Dispersion Compensation Fiber

DCG

Dispersion Compensation Grating

DCN

Data Communications Network

DCM

Dispersion Compensation Module

DCF

Dispersion Compensating Fiber

DDI

Double Defect Indication

DFB-LD

Distributed Feedback Laser Diode

DSF

Dispersion Shifted Fiber

DGD

Differential Group Delay

DTMF

Dual Tone Multi-Frequence

DWDM

Dense Wavelength Division Multiplexing

DXC

Digital Cross-connect

EAM

Electrical Absorption Modulation

ECC

Embedded Control Channel

EDFA

Erbium Doped Fiber Amplifier 77

Course Code Course Name

Abbreviation

Full Name

EFEC

Enhanced FEC

EX

Extinction Ratio

FDI

Forward Defection Indication

FEC

Forward Error Correction

FPDC

Fiber Passive Dispersion Compensator

FWM

Four Wave Mixing

GbE

Gigabits Ethernet

GUI

Graphical User Interfaces

IP

Internet Protocol

LD

Laser Diode

MDI

Multiple Document Interface

MCU

Management and Control Unit

MOADM

Metro Optical Add/Drop Multiplexer Equipment

MBOTU

Sub-rack backplane for OTU

MQW

Multiple Quantum Well

MSP

Multiplex Section Protection

MST

Multiplex Section Termination

NCP

Net Control Processor

NDSF

None Dispersion Shift Fiber

NE

Network Element

NNI

Network Node Interface

NMCC

Network Manage Control Center

NRZ

Non Return to Zero

NT

Network Termination

NZDSF

Non-Zero Dispersion Shifted Fiber

OA

Optical Amplifier

OADM

Optical Add/Drop Multiplexer

OBA

Optical Booster Amplifier

Och

Optical Channel

ODF

Optical fiber Distribution Frame

ODU

Optical Demultiplexer Unit

OGMD

Optical Group Mux/DeMux Board

OHP

Order wire

OHPF

Overhead Processing Board for Fast Ethernet

OLA

Optical Line Amplifier

OLT

Optical Line Termination

OMU

Optical Multiplexer Unit

ONU

Optical Network Unit

OP

Optical Protection Unit 78

Appendix A

Abbreviation

Abbreviations

Full Name

OPA

Optical Preamplifier Amplifier

OPM

Optical Performance Monitor

OPMSN

Optical Protect for Mux Section (without preventing resonance switch)

OPMSS

Optical Protect for Mux Section (with preventing resonance switch）

OSC

Optical Supervisory Channel

OSCF

Optical Supervision channel for Fast Ethernet

OSNR

Optical Signal-Noise Ratio

OTM

Optical Terminal

OTN

Optical Transport Network

OTU

Optical Transponder Unit

OXC

Optical Cross-connect

PDC

Passive Dispersion Compensator

PMD

Polarization Mode Dispersion

PDL

Polarization Dependent Loss

RZ

Return to Zero

SBS

Stimulated Brillouin Scattering

SDH

Synchronous Digital Hierarchy

SDM

Supervision add/drop multiplexing board

SEF

Severely Error Frame

SES

Severely Error Block Second

SFP

Small Form Factor Pluggable

SLIC

Subscriber Line Interface Circuit

SMCC

Sub-network Management Control Center

SMT

Surface Mount

SNMP

Simple Network Management Protocol

SPM

Self-Phase Modulation

SRS

Stimulated Raman Scattering

STM

Synchronous Transfer Mode

SWE

Electrical Switching Board

TCP

Transmission Control Protocol

TFF

Thin Film Filter

TMN

Telecommunications Management Network

VOA

Variable Optical Attenuator

WDM

Wavelength Division Multiplexing

XPM

Cross-Phase Modulation

79