Training

1 Introduction to GSM 1.1 Introduction 1.1.1 Background and Requirements At the beginning of the 1980s it was realized that the European countries were using many different, incompatible mobile phone systems. At the same time, the needs for telecommunication services were remarkably increased. Due to this, CEPT(Conférence Européenne des Postes et Télécommunications) founded a group to specify a common mobile system for Western Europe. This group was named “Group Special Mobile” and the system name GSM arose. This abbreviation has since been interpreted in other ways, but the most common expression nowadays is Global System for Mobile communications. At the beginning of the 1990s, the lack of a common mobile system was seen to be a general, world -wide problem. For this reason the GSM system has now spread also to the Eastern European countries, Africa, Asia and Australia. The USA, South America in general and Japan had made a decision to adopt other types of mobile systems which are not compatible with GSM. However, in the USA the Personal Communication System (PCS) has been adopted which uses GSM technology with a few variations. During the time the GSM system was being specified, it was foreseen that national telecommunication monopolies would be disbanded. This development set some requirements concerning the GSM system specifications and these requirements are built into the specifications as follows: •

There should be several network operators in each country. This would lead to competition in tariffs and service provisioning and it was assumed to be the best way to ensure the rapid expansion of the GSM system; the prices of the equipment would fall and the users would find the cost of calls reducing.

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The GSM system must be an open system, meaning that it should contain welldefined interfaces between different system parts. This enables the equipment from several manufacturers to coexist and hence improves the cost efficiency of the system from the operator's point of view.

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GSM networks must be built without causing any major changes to the already existing Public Switched Telephone Networks (PSTN).

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In addition to the commercial demands above, some other main objectives were defined: • The system must be Pan European. • The system must maintain a good speech quality. • The system must use radio frequencies as efficiently as possible. • The system must have high / adequate capacity. • The system must be compatible with the ISDN (Integrated Services Digital Network). • The system must be compatible with other data communication specifications. • The system must maintain good security concerning both subscriber and transmitted information.

1.1.2 Advantages of GSM Due to the requirements set for the GSM system, many advantages will be achieved. These advantages can be summarized as follows: •

GSM uses radio frequencies efficiently, and due to the digital radio path, the system tolerates more inter-cell disturbances.

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The average quality of speech achieved is better than in analogue cellular systems.

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Data transmission is supported throughout the GSM system.

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Speech is encrypted and subscriber information security is guaranteed.

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Due to the ISDN compatibility, new services are offered compared to the analogue systems.

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International roaming is technically possible within all countries using the GSM system.

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The large market increases competition and lowers the prices both for investments and usage.

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1.2 Open Interfaces of GSM The purpose behind the GSM specifications is to define several open interfaces, which then are limiting certain parts of the GSM system. Because of this interface openness, the operator maintaining the network may obtain different parts of the network from different GSM network suppliers. Also, when an interface is open it defines strictly what is happening through the interface and this in turn strictly defines what kind of functions must be implemented between the interfaces. Nowadays, GSM specifications define two truly open interfaces. The first one is between the Mobile Station and the Base Station. This open air interface is appropriately named the “Air interface”. The second one is between the Mobile Services Switching Centre – MSC (which is the switching exchange in GSM) and the Base Station Controller (BSC). This interface is called the “A interface”. The system includes more than the two defined interfaces but they are not totally open as the system specifications had not been completed when the commercial systems were launched. When operating analogue mobile networks, experience has shown that centralized intelligence generated excessive load in the system, thus decreasing the capacity. For this reason, the GSM specification, in principle, provides the means to distribute intelligence throughout the network. Referring to the interfaces, the more complicated the interfaces in use, the more intelligence is required between the interfaces in order to implement all the functions required. In a GSM network, this decentralized intelligence is implemented by dividing the whole network into three separate subsystems: • • •

Network Switching Subsystem (NSS) Base Station Subsystem (BSS) Network Management Subsystem (NMS)

The actual network needed for establishing calls is composed of the NSS and the BSS. The BSS is responsible for radio path control and every call is connected through the BSS. The NSS takes care of call control functions. Calls are always connected by and through the NSS. The NMS is the operation and maintenance related part of the network and it is needed for the control of the whole GSM network. The network operator observes and maintains network quality and service offered through the NMS. The three subsystems in a GSM network are linked by the Air, A and O&M interfaces as shown.

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Figure 1The three Subsystems of GSM and their interfaces The MS (Mobile Station) is a combination of terminal equipment and subscriber data. The terminal equipment as such is called ME (Mobile Equipment) and the subscriber's data is stored in a separate module called SIM (Subscriber Identity Module). Therefore, ME + SIM = MS.

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1.3 Traffic Management 1.3.1 Introduction A connection between two people - a caller and the called person – is the basic service of all telephone networks. To provide this service, the network must be able to set up and maintain a call, which involves a number of tasks: identifying the called person, determining his location, routing the call to him and ensuring that the connection is sustained as long as the conversation lasts. After the transaction, the connection is terminated and (normally) the calling user is charged for the service he has used. In a fixed telephone network, providing and managing connections is a relatively easy process because telephones are connected by wires to the network and their location is permanent, at least from the networks’ viewpoint. In a mobile network however, the establishment of a call is a far more complex task as the wireless (radio) connection enables the users to move at their own free will - providing they stay within the service area of the network. In practice, the network has to find solutions to three problems before it can even set up a call: • • •

Where is the subscriber Who is the subscriber What does the subscriber want

In other words, the subscriber has to be located and identified to provide him with the requested services. Let’s take an example to demonstrate these processes. A well-known professor is traveling around the world. He decides to spend the night in a hotel in, let’s say, Madrid. The first thing he will do is to contact the reception desk for registration. Basically, the reception desk is an office that supports registration. The receptionist records the registration in a database which we call the visitor’s register. The receptionist carefully checks the passport of the professor. The passport is also a database - a small one, though - and the receptionist analyses the data recorded in it. She finds the basic facts, such as citizenship, identification and the name of the professor, and also the name of the authority that has released the document. The professor appears to have his visa expiring soon, so the receptionist decides to call the office that has released the passport (presumably an embassy). This is simple as she knows the nationality and identity of the professor, and the number of his passport. The receptionist talks with the embassy secretary who recognizes the professor instantly and advises the receptionist that everything is okay. The professor is admitted into the hotel and the embassy of the professor’s home country, registers the latest information about his location.

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In other words, this is a transaction between the two offices and, as a result, IDENTIFICATION and LOCALISATION of the customer takes place in both databases. In this example, the embassy maintains a database, which contains the basic data of all the citizens who are traveling around the world and a record of their movements. When the registration is completed, the professor goes to his room. We can say that he is using a service provided by the hotel. As all the hotels in the world give this type of service, we can call it a basic service. In addition to the basic services (e.g. room and towels etc.), the hotel also provides additional services (e.g. restaurant, sauna, swimming pool etc.). These can be called supplementary services. To sum up the operations of the hotel: • • •

It provides SERVICES. It maintains a VISITOR REGISTER. It informs the HOME REGISTER of a visitor.

The purpose of the two registers is to enable the Identification, Authorization and Localization of the customer. Let’s assume that the professor checks out of Madrid and goes to Paris. He registers in another hotel and once again the receptionist informs the embassy in the home country. The registration in Madrid is cancelled, registration in Paris is made, and the location data in the Home Register is brought up-to-date. We have thus made a successful location update. Now move on and take a closer look at the mobile network. The story of the professor visiting hotels bears a striking resemblance to the users and functions of a mobile GSM network. Within the mobile network there are subscribers who move around and register into the service areas of networks in order to use the services provided by them. The visitor register of the hotel and the permanent register of the embassy also have their counterparts within the GSM network. There are fixed databases that maintain basic information about their customers including data on their current location, and temporary databases storing information about the users who are currently located in their service area.

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1.4 Mobility Functions 1.4.1 Registration and Database A new subscriber has just bought a mobile telephone and he switches it on for the first time. He can be practically anywhere in the world because, thanks to a network connection through a radio link , his telephone does not need wires. On the other hand, a connection through the mobile network is possible only if there is a point to point connection between the caller and the person who is called. Therefore, it is absolutely necessary that the network knows the subscriber’s location. The network keeps track of the subscribers’ location with the help of various databases.

1.4.2 The Subscriber Identity Module From the user’s point of view, the first and most important database is inside the mobile phone: the Subscriber Identity Module (SIM). The SIM is a small memory device mounted on a card and contains user specific identification. The SIM card can be taken out from one mobile equipment and inserted into another. In the GSM network, the SIM card identifies the user just like a traveler uses a passport to identify himself. The SIM card contains the identification numbers of the user, a list of the services that the user has subscribed to and a list of available networks. In addition, the SIM card contains tools needed for authentication and ciphering and, depending on the type of the card, there is also storage space for messages such as phone numbers, etc. A so-called “Home Operator” issues a SIM card when the user joins the network by making a service subscription. The Home Operator of the subscriber can be anywhere in the world, but for practical reasons the subscriber chooses one of the operators in the country where he spends most of his time. Now, the new subscriber switches on his phone in an area where a local operator provides network service. The area is connected through an air interface to a database known as a Visitor Location Register (VLR). The VLR is integrated into a telephone exchange known as a Mobile Services Switching Centre (MSC). The home operator of the subscriber also needs to know the location of the subscriber and so it maintains another register which is called a Home Location Register (HLR). The HLR stores the basic data of the subscriber on a permanent basis. The only variable data in the HLR is the current location (VLR address) of the subscriber. However, in the VLR, the subscriber data is stored temporarily. When the subscriber moves to another VLR area, its data is erased from the old VLR and stored in the new VLR.

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1.4.3 Location Update As an owner of a mobile phone, the subscriber does not stay in one place but keeps moving from one place to another. No matter how often or how quickly he moves, the network must be able to locate him continuously in case somebody wants to call him. The transaction that enables the network to keep track of the subscriber is called a Location Update and it happens in roughly the same way as in the example of the two hotels. The mobile phone constantly receives information sent by the network. This information includes identification (ID) of the VLR area in which the mobile is currently located. In order to keep track of its location, the mobile stores the ID of the area in which it is currently registered. Every time the network broadcasts the ID of the area, the mobile compares this information to the area ID stored in its memory. When the two IDs are no longer the same, the mobile sends the network a request, i.e. a registration inquiry to the area it has just entered. The network receives the request and registers the mobile in the new VLR area. Simultaneously, the subscriber’s HLR is informed about the new VLR location and the data concerning the subscriber is cleared from the previous VLR. The following figure gives a detailed description of the location update process.

Figure 2 Location update procedures

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1.5 Call Set-Up in a GSM Network The first is a call originating from the fixed network. Setting up a call appears to be a quick and simple operation, but it consists of a considerable number of sub operations. These operations include signalling between switching centers, identifying and locating the subscriber who is being called, making routing decisions and traffic connections etc. This section contains a step by step analysis of setting up a connection between a telephone in a fixed network and a GSM mobile station (i.e. a mobile phone). 1. A subscriber in a fixed network dials the number of a mobile station. This can be either a national or an international number. An example of a national number is: 040 2207959 As we can see there is no country code in this number. The following is an example of an international number: +358 40 2207959 The dialled number is called an MSISDN (Mobile Subscriber International ISDN Number) which contains the following elements:

• • •

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CC= Country code (33=France, 358=Finland, etc.) NDC= National Destination Code SN= Subscriber Number

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2. The PSTN exchange analyses the dialled number. The result of the analysis is the routing information required for finding the mobile network (Public Land Mobile Network, PLMN) in which the called subscriber has made his subscription. The PSTN identifies the mobile network on the basis of the NDC, after which it accesses the mobile network via the nearest Gateway Mobile Services Switching Centre (GMSC).

Figure 3 PSTN originates the call 3. The GMSC analyses the MSISDN in the same way as the PSTN exchange did. As a result of the analysis, it obtains the HLR address in which the subscriber is permanently registered. The GMSC itself does not have any information about the location of the called subscriber. The subscriber’s location can only be determined by the two databases, the HLR and VLR. At this stage however, the GMSC only knows the HLR address and so it sends a message (containing the MSISDN) to the HLR. In practice this message is a request for locating the called subscriber in order to set up a call. This is called an “HLR Enquiry”.

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4. The HLR analyses the message. It identifies the called subscriber on the basis of MSISDN and then checks its database to determine the subscribers location. As, the HLR is informed every time the subscriber moves from one VLR area to another, i.e. the HLR knows in which VLR area the subscriber is currently registered. A traffic connection requires two network elements that are able to provide speech connections. A speech connection is a network service and it can be handled only by an MSC. Therefore, to enable the traffic connection, maybe two MSC’s will have to be connected. The first MSC is the Gateway MSC which is contacted by the PSTN exchange. The HLR acts as a co-ordinator to set up the connection between the GMSC and the destination MSC (which could of course be the GMSC itself). Let’s have a look at the contents of an HLR database to discover how it locates the called subscriber. We will use an Italian subscriber as an example:

As we can see the first field contains the identity numbers of the subscriber. There is also another identification number involved in the process known as the International Mobile Subscriber Identity (IMSI). The purpose of IMSI is to identify the subscriber in the mobile network. The total length of the IMSI is 15 digits and it consists of the following elements:

• • •

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MCC = Mobile Country Code (three digits) MNC = Mobile Network Code (two digits) MSIN = Mobile Subscriber Identification Number (ten digits)

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The IMSI number is used for registering a user in the Public Land Mobile Network (PLMN). To locate the subscriber and to enable the traffic connection, the HLR has to associate the MSISDN with the IMSI of the mobile subscriber. But why do we need the IMSI? Why not simply use the MSISDN both for network registration and for setting up a call? The reason for this can be explained with an example: Suppose that three subscribers from three countries (Finland, Italy and the USA) are in the same location and their mobile stations try to register with the same VLR. Also assume that they try to register using their MSISDN (which is actually not the case): • • •

John, from the USA, MSISDN = + 1 XYZ 1234567 Ilkka, from Finland, MSISDN = + 358 AB 6543210 Claudio, from Italy, MSISDN = + 39 GHI 1256890

Notice that the length of, for example, the country code is different for each number. If the MSISDN numbers were used in registering the subscribers, we would also need a length indicator for each field to prevent the various parts of the number from getting mixed up with each other - and that would be too complicated. If the length of the fields are the same for all countries, no extra information is needed and the identification process is relatively simple. Another reason for using the IMSI is that the MSISDN identifies the service used such as speech, data, fax, etc. Therefore one subscriber may need several MSISDNs depending on the type of services he uses, whereas he has only one IMSI. To get back to the HLR database: one data field is reserved for the address of the MSC/VLR where the called subscriber is currently registered. (Normally the VLR and the MSC have the same address.) This is needed in the next phase of establishing the connection. 5. Now the HLR interrogates the MSC/VLR that is currently serving the called subscriber. But why do we need to interrogate instead of connecting right away? First of all, the current status of the mobile station is stored in the VLR database and we need to know the status to avoid setting up a call to a subscriber whose phone is switched off. Secondly, we need to have some sort of information that enables the GMSC to route the call to the target MSC, wherever in the world it may be. 6. In terms of routing the call, the serving MSC/VLR is the destination of the call. This means that we must direct the call to it by using the following procedure: after receiving the message from the HLR, the serving MSC/VLR generates a temporary Mobile Station Roaming Number (MSRN) and associates it with the IMSI. The roaming number is used in initiating the connection and it has the following structure:

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• • •

CC = Country Code (of the visited country) NDC = National Destination Code (of the serving network) SN = Subscriber Number

By comparing the MSRN and the MSISDN, we notice that they have the same structure, though they are used for different purposes. The MSISDN is used to interrogate the HLR, whereas the MSRN is the response given by the serving MSC/VLR and it is used for routing the call. The SN field of the MSRN is actually an internal number that is temporarily associated with the IMSI. The MSRN does not merely identify the subscriber, it also points to the exchange itself so that all intermediate exchanges, if there are any, know where the call is to be routed. Since the roaming number is temporary, it is available for establishing another traffic connections after the call has been set up. In essence, the SN field in the MSISDN points to a database entry in the HLR, and the SN field in the MSRN points to a database entry in the VLR. 7. The MSC/VLR sends the roaming number to the HLR. The HLR does not analyse it because the MSRN is used for traffic transactions only and the HLR does not handle traffic, it is only a database that helps in locating subscribers and co-ordinates call set-up. Therefore, the HLR simply sends the MSRN forward to the GMSC that originally initiated the process. 8. When the GMSC receives the message containing the MSRN, it analyses the message. The roaming number identifies the location of the called subscriber, so the result of this analysis is a routing process which identifies the destination of the call - the serving MSC/VLR. 9. The final phase of the routing process is taken care of by the serving MSC/VLR. In fact, the serving MSC/VLR also has to receive the roaming number so that it knows that this is not a new call, but one that is going to terminate here - i.e. a call to which it has already allocated an MSRN. By checking the VLR, it recognises the number and so it is able to trace the called subscriber.

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1.5.1 Network Switching Subsystem (NSS) The GSM network is divided into three subsystems: Network Switching Subsystem (NSS), Base Station Subsystem (BSS), and Network Management Subsystem (NMS). The elements of Network Switching Subsystem that are: • • •

MSC (Mobile Services Switching Centre) VLR (Visitor Location Register) HLR (Home Location Register)

The MSC is responsible for controlling calls in the mobile network. It identifies the origin and destination of a call (either a mobile station or a fixed telephone in both cases), as well as the type of a call. An MSC acting as a bridge between a mobile network and a fixed network is called a Gateway MSC. An MSC is normally integrated with a VLR, which maintains information related to the subscribers who are currently in the service area of the MSC. The VLR carries out location registrations and updates. The MSC associated with it initiates the paging process. A VLR database is always temporary (in the sense that the data is held as long as the subscriber is within its service area), whereas the HLR maintains a permanent register of the subscribers. In addition to the fixed data, the HLR also maintains a temporary database which contains the current location of its customers. This data is required for routing calls. In addition, there are two more elements in the NSS: the Authentication Centre (AC) and the Equipment Identity Register (EIR). They are usually implemented as part of HLR and they deal with the security functions To sum up, the main functions of NSS are: Call Control --This identifies the subscriber, establishes a call and clears the connection after the conversation is over. Charging-- This collects the charging information about a call such as the numbers of the caller and the called subscriber, the time and type of the transaction, etc., and transfers it to the Billing Centre. Mobility management --This maintains information about the location of the subscriber. Signalling with other networks and the BSS --This applies to interfaces with the BSS and PSTN. Subscriber data handling-- This is the permanent data storage in the HLR and temporary storage of relevant data in the VLR. Locating the subscriber-- This locates a subscriber before establishing a call. DDU

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1.5.2 Locating the Subscriber The GMSC/VLR and the MSC/VLR has now been connected via a traffic and signalling channel and the call set up has almost been completed. The caller is connected to the PSTN exchange, the PSTN is connected to the GMSC, the GMSC is connected to the MSC/VLR that is serving the called subscriber but we have not yet established a connection to the called subscriber. In order to set up the connection, we first have to understand how the subscriber is located. As we do not know the exact location of the subscriber, it seems inevitable that we have to search for him in the entire VLR service area. This could be a wide geographical area and so finding the subscriber requires a lot of work for the MSC/VLR. Things will be easier to grasp if we go back for a moment and follow the famous professor in the hotel. The hotel is an establishment that provides services. If we want to find our professor, we need to search the entire area of the hotel, which can be really frustrating. He might be in his room, in the sauna, in the swimming pool, in the restaurant, in fact practically anywhere. The only thing we know is that he is in the hotel, because he has not checked out. In order to simplify the search, we can register his movements by setting up a registration routine for the various parts of the hotel. In other words, the hotel service area is divided into location areas. This simplifies the search for the professor. Something similar happens in the cellular network. When we want to find the subscriber, it would be necessary to conduct a search throughout the entire MSC/VLR area unless this area is divided into smaller areas. Therefore, the MSC/VLR area is divided into smaller areas. These are called Location Areas (LA) and they are managed by the MSC/VLR. Each MSC/VLR contains several Location Areas. We can define a LA as an area in which w e search for the subscriber in case there is a call addressed to a mobile station.

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Each LA is identified by a Location Area Identity (LAI). Its structure is as follows:

• • •

MCC= Mobile Country Code (of the visited country) MNC= Mobile Network Code (of the serving PLMN) LAC= Location Area Code

10. Now that we know the LA of the subscriber, we can start searching for him. To locate the subscriber, a Paging process is initiated in the Location Area.

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1.5.3 Base Station Subsystem (BSS) To understand the paging process, we must analyse the functions of the BSS. The Base Station Subsystem consists of the following elements: •

BSC Base Station Controller

•

BTS Base Transceiver Station

•

TC Transcoder

The Base Station Controller (BSC) is the central network element of the BSS and it controls the radio network. This means that the main responsibilities of the BSC are: Connection establishment between MS and NSS, Mobility management, Statistical raw data collection, Air and A interface signalling support. The Base Transceiver Station (BTS) is a network element maintaining the Air interface. It takes care of Air interface signalling, Air interface ciphering and speech processing. In this context, speech processing refers to all the functions the BTS performs in order to guarantee an error-free connection between the MS and the BTS. The TransCoder (TC) is a BSS element taking care of speech transcoding, i.e. it is capable of converting speech from one digital coding format to another and vice versa.

Figure 4 The Base Station Subsystem (BSS)

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The BTS, BSC and TC together form the Base Station Subsystem (BSS) which is a part of the GSM network taking care of the following major functions: Radio Path Control : In the GSM network, the Base Station Subsystem (BSS) is the part of the network taking care of Radio Resources, i.e. radio channel allocation and quality of the radio connection. BTS and TC Control: Inside the BSS, all the BTSs and TCs are connected to the BSC(s). The BSC maintains the BTSs. In other words, the BSC is capable of separating (barring) a BTS from the network and collecting alarm information. Transcoders are also maintained by the BSC, i.e. the BSC collects alarms related to the Transcoders. Synchronisation: The BSS uses hierarchical synchronisation which means that the MSC synchronises the BSC and the BSC further synchronises the BTSs associated with that particular BSC. Inside the BSS, synchronisation is controlled by the BSC. Synchronisation is a critical issue in the GSM network due to the nature of the information transferred. If the synchronisation chain is not working correctly, calls may be cut or the call quality may not be the best possible. Ultimately, it may even be impossible to establish a call. Air & A Interface Signalling: In order to establish a call, the MS must have a connection through the BSS. This connection requires several signalling protocols Connection Establishment between MS and NSS: The BSS is located between two interfaces, the Air and the A interface. From the call establishment point of view, the MS must have a connection through these two interfaces before a call can be established. Generally speaking, this connection may be either a signalling type of connection or a traffic (speech, data) type of connection. Mobility Management and speech transcoding: BSS Mobility Management mainly covers the different cases of handovers. Collection of Statistical Data The BSS collects a lot of short-term statistical data that is further sent to the NMS for post processing purposes. By using the tools located in the NMS the operator is able to create statistical "views" and thus observe the network quality.

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A Base Station Subsystem is controlled by an MSC. Typically, one MSC contains several BSSs. A BSS itself may cover a considerably large geographical area consisting of many cells. (A cell refers to an area covered by one or more frequency resources). Each cell is identified by an identification number called Cell Global Identity (CGI) which comprises the following elements:

• • • •

MCC Mobile Country Code MNC Mobile Network Code LAC Location Area Code CI Cell Identity

As an example of two adjacent BTSs. One serves an industrial area and the other a nightlife area. It is obvious that traffic is not handled the same way and at the same time in these BTSs. One has traffic peaks during weekdays and especially during working hours, and the other during the evenings and weekends. There is one 2Mbit/s PCM line reserved for each BTS to provide the connection to NSS. But as we can see, the BTS’s are used at different times and on different days. Why not use the same line for both of the two BTSs? It can be done, but in this case there has to be a concentrator between MSC and BTS. The BSC acts as a concentrator (in addition to being the radio network controller). One BSC is capable of serving several BTSs.

Figure 5 BTS-BSC- BTS connections

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There is no relation between a BSC area and Location Area and the serves as a concentrator in addition to its major role of radio network control. The purpose of the location area is to facilitate the paging process (searching for the subscriber), whereas a BSC area is related to traffic connections and radio resources.

1.5.4 A Mobile Terminated Call and Paging Let us go back to our professor. We know that he is within the hotel area. Thanks to the registration system of the hotel, we also know that he went to the restaurant and registered his presence there. Somebody calls him and the receptionist answers. The receptionist checks the registration system of the hotel and discovers that the professor is in the restaurant. A message about an incoming call is sent to the restaurant and one of the waiters starts looking for the professor. If the waiter does not know the right table, he uses the public address system and "pages" the professor as follows: “There is a telephone call for Mr. So and So. Could you please come forward?” Once the professor raises his hand, the search is complete and the call is set up.

Figure 6 The paging process

Again, a similar process is used in the cellular network. Paging is a signal that is transmitted by all the cells in the Location Area (LA). It contains the identification of the subscriber. All the mobile stations in the LA receive the paging signal, but only one of them recognises the identification and answers to it. As a consequence of this answer, a point to point connection is established. Now the two subscribers are connected, and traffic can be carried through the network. Let’s sum up the entire process:

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Figure 7 Simplified steps in setting up a call • • • • • • • • • •

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A subscriber in a fixed network dials a number of a mobile phone. The dialled number is the MSISDN. The Public Switched Telephone Network (PSTN) exchange analyses the number and contacts the Gateway Mobile Services Switching Centre (GMSC). The Gateway MSC analyses the MSISDN and sends a message to the Home Location Register (HLR). The HLR checks its database to determine the current location of the called subscriber. The HLR interrogates the MSC/VLR (Visitor Location Register) that is currently serving the called subscriber. The serving MSC/VLR generates a temporary MSRN (Mobile Subscriber Roaming Number). MSC/VLR sends MSRN to HLR and the HLR forwards the MSRN to the GMSC. The GMSC identifies the serving MSC/VLR as the destination for routing the call. Destination MSC/VLR receives MSRN. It identifies the number that is called and traces the called subscriber. The destination MSC/VLR initiates a paging process in the Location Area to locate the called subscriber. The mobile phone of the called subscriber recognises the paging signal and answers it.

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1.5.5 Mobile Originated Call How connection is established when the call is initiated by a mobile subscriber instead of a fixed one? The mobile subscriber dials a number. In other words, the subscriber issues a service request to the network in which he is currently registered as a visitor. After receiving the request, the network analyses the data of the calling subscriber in order to do three things: • • •

Authorise or deny the use of the network. Activate the requested service. Route the call.

The call may have two types of destinations: a mobile station or a telephone in a fixed network. If the call is addressed to a telephone in a fixed telephone network, it is routed to the PSTN, which in turn routes it to the destination. If the called number is another mobile station in the same network, the MSC starts the HLR Enquiry procedure which is processed in the same way as in the example of a PSTN originated call.

Figure 8 Mobile Originated Call procedure. DDU

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Identifying and locating the called subscriber are the two key preconditions of setting up a point to point connection. The MSISDN fulfils the purpose of identification, but locating requires a quick and comprehensive system for keeping track of the subscriber. If the network does not have up-to-date information about the subscriber’s current location, setting up a call would mean paging large network areas in order to find the subscriber and that would be a complex and time -consuming task. To avoid this, the GSM network monitors and records the movements of the subscribers all the time. This process is called Location Update.

1.6 Location Update 1.6.1 Types of Location Update In practice, there are three types of location updates: • • •

Location Registration Generic Periodic

Location registration takes place when a mobile station is turned on. This is also known as IMSI Attach because as soon as the mobile station is switched on it informs the Visitor Location Register (VLR) that it is now back in service and is able to receive calls. As a result of a successful registration, the network sends the mobile station two numbers that are stored in the SIM (Subscriber Identity Module) card of the mobile station. These two numbers are the Location Area Identity (LAI) and the Temporary Mobile Subscriber Identity (TMSI). The network, via the control channels of the air interface, sends the LAI. The TMSI is used for security purposes, so that the IMSI of a subscriber does not have to be transmitted over the air interface. The TMSI is a temporary identity, which regularly gets changed. A Location Area Identity (LAI) is a globally unique number. A Location Area Code (LAC) is only unique in a particular network. Every time the mobile receives data through the control channels, it reads the LAI and compares it with the LAI stored in its SIM card. A generic location update is performed if they are different. The mobile starts a Location Update process by accessing the MSC/VLR that sent the location data.

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A channel request message is sent that contains the subscriber identity (i.e. IMSI/TMSI) and the LAI stored in the SIM card. When the target MSC/VLR receives the request, it reads the old LAI which identifies the MSC/VLR that has served the mobile phone up to this point. A signalling connection is established between the two MSC/VLRs and the subscriber’s IMSI is transferred from the old MSC to the new MSC. Using this IMSI, the new MSC requests the subscriber data from the HLR and then updates the VLR and HLR after successful authentication. Periodic location update is carried out when the network does not receive any location update request from the mobile in a specified time. Such a situation is created when a mobile is switched on but no traffic is carried, in which case the mobile is only reading and measuring the information sent by the network. If the subscriber is moving within a single location area, there is no need to send a location update request. A timer controls the periodic updates and the operator of the VLR sets the timer value. The network broadcasts this timer value so that a mobile station knows the periodic location update timer values. Therefore, when the set time is up, the mobile station initiates a registration process by sending a location update request signal. The VLR receives the request and confirms the registration of the mobile in the same location area. If the mobile station does not follow this procedure, it could be that the batteries of the mobile are exhausted or the subscriber is in an area where there is no network coverage. In such a case, the VLR changes the location data of the mobile station to “unknown”.

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1.7 Handover In a mobile communications network, the subscriber can move around. Maintaining the traffic connection with a moving subscriber is made possible with the help of the handover function. The basic concept is simple: when the subscriber moves from the coverage area of one cell to another, a new connection with the target cell has to be set up and the connection with the old cell has to be released. There are two reasons for performing a handover: •

•

Handover due to measurements occurs when the quality or the strength of the radio signal falls below certain parameters specified in the BSC. The deterioration of the signal is detected by the constant signal measurements carried out by both the mobile station and the BTS. As a consequence, the connection is handed over to a cell with a stronger signal. Handover due to traffic reasons occurs when the traffic capacity of a cell has reached its maximum or is approaching it. In such a case, the mobile stations near the edges of the cell may be handed over to neighbouring cells with less traffic load.

The decision to perform a handover is always made by the BSC that is currently serving the subscriber, except for the handover for traffic reasons. In the latter case the MSC makes the decision. There are four different types of handover and the best way to analyse them is to follow the subscriber as he moves: Intra cell - Intra BSC handover: The smallest of the handovers is the intra cell handover where the subscriber is handed over to another traffic channel (generally in another frequency) within the same cell. In this case the BSC controlling the cell makes the decision to perform handover. Inter cell - Intra BSC handover: The subscriber moves from cell 1 to cell 2. In this case the handover process is controlled by BSC. The traffic connection with cell 1 is released when the connection with cell 2 is set up successfully. Inter cell - Inter BSC handover: The subscriber moves from cell 2 to cell 3, which is served by another BSC. In this case the handover process is carried out by the MSC, but, the decision to make the handover is still done by the first BSC. The connection with the first BSC (and BTS) is released when the connection with the new BSC (and BTS) is set up successfully.

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Inter MSC handover: The subscriber moves from a cell controlled by one MSC/VLR to a cell in the domain of another MSC/VLR. This case is a bit more complicated. Considering that the first MSC/VLR is connected to the GMSC via a link that passes through PSTN lines, it is evident that the second MSC/VLR can not take over the first one just like that. The MSC/VLR currently serving the subscriber (also known as the anchor MSC), contacts the target MSC/VLR and the traffic connection is transferred to the target MSC/VLR. As both MSCs are part of the same network, the connection is established smoothly. It is important to notice, however, that the target MSC and the source MSC are two telephone exchanges. The call can be transferred between two exchanges only if there is a telephone number identifying the target MSC. Such a situation makes it necessary to generate a new number, the Handover Number (HON). The anchor MSC/VLR receives the handover information from the BSS. It recognises that the destination is within the domain of another MSC and sends a Handover Request to the target MSC via the signalling network. The target MSC answers by generating a HON and sends it to the anchor MSC/VLR, which performs a digit analysis in order to obtain the necessary routing information. This information allows the serving MSC/VLR to connect the target MSC/VLR. When the two MSCs are connected, the call is transferred to a new route. In practice, the handover number is similar to the roaming number. Moreover, the roaming number and the handover number have a similar purpose, that is connecting two MSCs. The structure of the handover number is shown below:

• • •

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CC= Country Code NDC= National Destination Code SN= Subscriber Number

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Figure 9 Inter MSC handover procedure

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1.8 Transmission In a mobile communications network, part of the transmission connection uses a radio link and another part uses 2Mbit/s PCM links. Radio transmission is used between the Mobile Station and the Base Transceiver Station and the information must to be adapted to be carried over 2Mbit/s PCM transmission through the remainder of the network. The radio link is the most vulnerable part of the connection and a great deal of work is needed to ensure its high quality and reliable operation. The frequency ranges of GSM 900 and GSM 1800 are indicated below:

Figure 10 Frequency Allocations for GSM Note that the uplink refers to a signal flow from Mobile Station (MS) to Base Transceiver Station (BTS) and the downlink refers to a signal flow from Base Transceiver Station (BTS) to Mobile Station (MS). The simultaneous use of separate uplink and downlink Frequencies enables communication in both the transmit (TX) and the receive (RX) directions. The radio carrier frequencies are arranged in pairs and the difference between these two frequencies (uplink downlink) is called the Duplex Frequency. The frequency ranges are divided into carrier frequencies spaced at 200kHz. As an example, the following table shows the distribution of frequencies in GSM 900:

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In GSM 900 the duplex frequency (the difference between uplink and downlink frequencies) is 45 MHz. In GSM 1800 it is 95 MHz. The lowest and highest channels are not used to avoid interference with services using neighboring frequencies, both in GSM 900 and GSM 1800. The total number of carriers in GSM 900 is 124, whereas in GSM 1800 the number of carriers is 374. The devices in the Base Transceiver Station (BTS) that transmit and receive the radio signals in each of the GSM channels (uplink and downlink together) are known as Transceivers (TRX). The radio transmission in GSM networks is based on digital technology. Digital transmission in GSM is implemented using two methods known as Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). Frequency Division Multiple Access (FDMA) refers to the fact that each Base Transceiver Station is allocated different radio frequency channels. Mobile phones in adjacent cells (or in the same cell) can operate at the same time but are separated according to frequency. The FDMA method is employed by using multiple carrier frequencies, 124 in GSM 900 and 374 in GSM 1800. Time Division Multiple Access (TDMA), as the name suggests, is a method of sharing a resource (in this case a radio frequency) between multiple users, by allocating a specific time (known as a time slot) for each user. This is in contrast to the analogue mobile systems where one radio frequency is used by a single user for the duration of the conversation. In Time Division Multiple Access (TDMA) systems each user either receives or transmits bursts of information only in the allocated time slot. These time slots are allocated for speech only when a user has set up the call however, some timeslots are used to provide signalling and location updates etc. between calls.

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1.8.1 Transmission Through the Air Interface Time Division Multiple Access (TDMA) divides one radio frequency channel into consecutive periods of time, each one called a "TDMA Frame". Each TDMA Frame contains eight shorter periods of time known as "Timeslots". The TDMA timeslots are called "Physical Channels" as they are used to physically move information from one place to another. The radio carrier signal between the Mobile Station and the BTS is divided into a continuous stream of timeslots which in turn are transmitted in a continuous stream of TDMA frames - like a long line of vehicles with eight seats in each. If the time slots of the TDMA frame represent the physical channels, what about the contents? The contents of the physical channels - i.e. the soldiers and officers travelling in the eight seats of the vehicle, according to their roles, are called "logical channels". In the example of the army, the soldiers are one type of logical channel and the officers are other types of logical channels and they exercise some kind of control depending on their responsibilities. In GSM the logical channels can be divided into two types: • •

Dedicated Channels Common Channels

Let’s look at things in a more practical way: a subscriber switches on his mobile phone and receives a call. This simple act of switching on the phone involves the following steps: • • •

The mobile scans all the radio frequencies and measures them. It selects the frequency with the best quality and tunes to it. With the help of a synchronization signal in a TDMA frame, the mobile synchronizes itself to the network.

The synchronization information required by this process is broadcast by the network and analyzed by the mobile. Registration and authentication are the next steps and these consist of the following operations: 1. A point to point connection must be set up. The mobile station makes a request for a channel to establish the connection. 2. The network acknowledges the request and allocates a channel. The mobile receives and reads this information. 3. The mobile then moves to the allocated (dedicated) channel for further transactions with the network. The next steps are registration and authentication.

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Once the subscriber is registered in the network and the authentication is successful, calls can be set-up. In the case of a mobile terminated call, the subscriber has to be paged. This process is described below: 1. The network sends a Paging message to all the Base Transceiver Stations (BTS) within the Location Area (LA) where the subscriber is registered. 2. The mobile station answers the paging message by sending a service/channel request. 3. The network acknowledges this request and again the authentication is needed. A dedicated signaling channel is assigned in order to transmit the data related to the call. 4. A Traffic Channel is assigned for the conversation. During the conversation, the mobile measures the signal strength of adjacent carriers and sends measurement reports to the Base Station Controller (BSC). A channel must be dedicated also for this function. This is a simplified description of the process, but it conveys the idea that there are many functions involved in the air interface to enable a mobile user to have conversation. Each one of these functions requires a separate "logical channel" as the data contents are different. Some of them are Uplink , others are Downlink and some are Bi-directional.

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1.8.2 Logical channels There are twelve different types of Logical Channels which are mapped into Physical Channels in the radio path. Logical channels comprise of Common Channels and Dedicated Channels. Common Channels are those which are used for broadcasting different information to mobile stations and setting up of signaling channels between the MSC/VLR and the mobile station. Over the radio path, different type of signaling channels are used to facilitate the discussions between the mobile station and the BTS, BSC and MSC/VLR. All these signaling channels are called Dedicated Control Channels. Traffic channels are also Dedicated Channels as each channel is dedicated to only one user to carry speech or data.

Figure 11 TDMA frames with Common and Dedicated Channels

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1.8.3 Broadcast Channels Base Stations can use several TRXs but there is always only one TRX which can carry Common Channels. Broadcast channels are downlink point to multipoint channels. They contain general information about the network and the broadcasting cell. There are three types of broadcast channels: 1. Frequency Correction Channel (FCCH)

FCCH bursts consist of all "0"s which are transmitted as a pure sine wave. This acts like a flag for the mobile stations which enables them to find the TRX among several TRXs, which contains the Broadcast transmission. The MS scans for this signal after it has been switched on since it has no information as to which frequency to use. 2. Synchronisation Channel (SCH) The SCH contains the Base Station Identity Code (BSIC) and a reduced TDMA frame number. The BSIC is needed to identify that the frequency strength being measured by the mobile station is coming from a particular base station. In some cases, a distant base station broadcasting the same frequency can also be detected by the mobile station. The TDMA frame number is required for speech encryption. 3. Broadcast Control Channel (BCCH) The BCCH contains detailed network and cell specific information such as: · Frequencies used in the particular cell and neighboring cells. · Channel combination. As we mentioned previously, there are a total of twelve logical channels. All the logical channels except Traffic Channels are mapped into Timeslot 0 or Timeslot 1 of the broadcasting TRX. Channel combination informs the mobile station about the mapping method used in the particular cell. · Paging groups . Normally in one cell there is more than one paging channel. To prevent a mobile from listening to all the paging channels for a paging message, the paging channels are divided in such a way that only a group of mobile stations listen to a particular paging channel. These are referred to as paging groups. · Information on surrounding cells . A mobile station has to know what are the cells surrounding the present cell and what frequencies are being broadcast on them. This is necessary if, for example, the user initiates a conversation in the current cell, and then decides to move on. The mobile station has to measure the signal strength and quality of the surrounding cells and report this information to the base station controller.

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1.8.4 Common Control Channels Common Control Channels comprise the second set of logical channels. They are used to set up a point to point connection. There are three types of common control channels: 1. Paging Channel (PCH) The PCH is a downlink channel which is broadcast by all the BTSs of a Location Area in the case of a mobile terminated call. 2. Random Access Channel (RACH) The RACH is the only uplink and the first point to point channel in the common control channels. It is used by the mobile station in order to initiate a transaction, or as a response to a PCH. 3. Access Grant Channel (AGCH) The AGCH is the answer to the RACH. It is used to assign a mobile a Stand-alone Dedicated Control Channel (SDCCH). An additional information in the AGCH is the frequency hopping sequence. It is a downlink, point to point channel. Frequency hopping is designed to reduce the negative effects of the air interface, which sometimes results in the loss of information transmitted, the mobile station may transmit information on different frequencies within one cell. The order in which the mobile station should change the frequencies is called the "frequency hopping sequence".

1.8.5 Dedicated Control Channels Dedicated Control Channels compose the third group of channels. Once again, there are three dedicated channels. They are used for call set-up, sending measurement reports and handover. They are all bidirectional and point to point channels. There are three dedicated control channels: 1. Stand-alone Dedicated Control Channel (SDCCH) The SDCCH is used for system signalling: call set-up, authentication, location update, assignment of traffic channels and transmission of short messages. 2. Slow Associated Control Channel (SACCH) An SACCH is associated with each SDCCH and Traffic Channel (TCH). It transmits measurement reports and is also used for power control, time alignment and in some cases to transmit short messages. 3. Fast Associated Control Channel (FACCH) The FACCH is used when a handover is required. It is mapped onto a TCH, and it replaces 20 ms of speech and therefore it is said to work in "stealing" mode.

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1.8.6 Traffic Channels (TCH) Traffic Channels are logical channels that transfer user speech or data, which can be either in the form of Half rate traffic (5.6 kbits/s) or Full rate traffic (13 kbits/s). Another form of traffic channel is the Enhanced Full Rate (EFR) Traffic Channel. The speech coding in EFR is still done at 13Kbits/s, but the coding mechanism is different than that used for normal full rate traffic. EFR coding gives better speech quality at the same bit rate than normal full rate. Traffic channels can transmit both speech and data and are bi-directional channels.

1.8.7 Time Slots And Bursts Consider the example of a 2Mbit/s PCM signal which can carry 30 speech channels with each channel occupying 64Kbits/s. The speech signals from the mobile stations must be placed into a 2Mbit/s signal that connects the BTS and the BSC. It is very important that all the mobile stations in the same cell send the digital information at the correct time to enable the BTS to place this information into the correct position in the 2Mbit/s signal. How do we manage the timing between multiple mobile stations in one cell? The aim is that each mobile sends its information at a precise time, so that when the information arrives at the Base Transceiver Station, it fits into the allocated time slot in the 2Mbit/s signal. Each Mobile Station must send a burst (a burst occupies one TDMA timeslot) of data at a different time to all the other Mobile Stations in the same cell. The mobile the n falls silent for the next seven timeslots and then again sends the next burst and so on. In the air interface a TDMA timeslot is a time interval of approximately 576. 9 ms which corresponds to the duration of 156.25 bit times. All bursts occupy this period of time, but the actual arrangement of bits in the burst will depend on the burst type. Two examples of burst types are : • •

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Normal Burst is used to send the traffic channels, stand alone dedicated channels, broadcast control channel, paging channel, access grant channel, slow and fast associated control channels. Access Burst which is used to send information on the Random Access Channel (RACH). This burst contains the lowest number of bits. The purpose of this “extra free space” is to measure the distance between the Mobile Station to Base Transceiver Station at the beginning of a connection. This process determines a parameter called "timing advance" which ensures that the bursts from different mobile stations arrive at the correct time, even if the distances between the various MSs and the BTS are different. This process is carried out in connection with the first access request and after a handover. In GSM a maximum the oretical distance of about 35 km is allowed between the base transceiver station and mobile station.

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Figure 12 Normal Bursts and Access Bursts

1.9 Problems and Solutions of the Air Interface It has already been pointed out that the radio air interface link is the most vulnerable part of the GSM connection. There are three major sources of problems in the air interface, which can lead to loss of data. These are: • • •

Multi path propagation Shadowing Propagation delay

Multipath propagation Whenever a mobile station is in contact with the GSM network, it is quite rare that there is a direct "line of sight" transmission between the mobile station and the base transceiver station. In the majority of cases, the signals arriving at the mobile station have been reflected from various surfaces. Thus a mobile station (and the base transceiver station) receives the same signal more than once. Depending on the distance that the reflected signals have traveled, they may affect the same information bit or corrupt successive bits. In the worst case an entire burst might get lost. Depending on whether the reflected signal comes from near or far, the effect is slightly different. A reflected signal that has travelled some distance causes "inter symbol interference" whereas near reflections cause "frequency dips". There are a number of solutions that have been designed to overcome these problems:

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• • • • •

Viterbi Equalisation Channel Coding Interleaving Frequency Hopping Antenna Diversity

Viterbi Equalisation This is generally applicable for signals that have been reflected from far away objects. When either the base transceiver station or mobile station transmits user information, the information contained in the burst is not all user data. There are 26 bits which are designated for a "training sequence" included in each TDMA burst transmitted. Both the mobile station and base transceiver station know these bits and by analyzing the effect the radio propagation on these training bits, the air interface is mathematically modelled as a filter. Using this mathematical model, the transmitted bits are estimated based on the received bits. The mathematical algorithm used for this purpose is called "Viterbi equalisation". Channel Coding Channel coding (and the following solutions) is normally used for overcoming the problem caused by fading dips. In channel coding, the user data is coded using standard algorithms. This coding is not for encryption but for error detection and correction purposes and requires extra information to be added to the user data. In the case of speech, the amount of bits is increased from 260 per 20 ms to 456 bits per 20 ms. This gives the possibility to regenerate up to 12.5% of data loss. Interleaving Interleaving is the spreading of the coded speech into many bursts. By spreading the information onto many bursts, we will be able to recover the data even if one burst is lost. Frequency Hopping With Frequency Hopping, the frequency on which the information is transmitted is changed for every burst. Frequency hopping generally does not significantly improve the performance if there are less than four frequencies in the cell. Antenna Receiver Diversity In this case two physically separated antennas receive and process the same signal. This helps to eliminate fading dips. If a fading dip occurs at the position of one antenna, the other antenna will still be able to receive the signal. Since the distance between two antennas is a few meters, it can only be implemented at the Base Transceiver Station.

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Figure 13Antenna Receiver Diversity Shadowing Hills, buildings and other obstacles between antennas cause shadowing (also called Log Normal Fading). Instead of reflecting the signal these obstacles attenuate the signal. Base Transceiver Station transmits information at a much higher power compared to that from the mobile station. The solution adopted to overcome this problem is known as adaptive power control. Based on quality and strength of the received signal, the base station informs the mobile station to increase or decrease the power as required. This information is sent in the Slow Associated Control Channel (SACCH).

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Propagation Delay Information is sent in bursts from the mobile station to the Base Transceiver Station (BTS). These bursts have to arrive at the base transceiver station such that they have to map exactly into their allocated time slots. However, the further away the mobile station is from the BTS then the longer it will take for the radio signal to travel over the air interface. This means that if the mobile station or base station transmits a burst only when the time slot appears, then when the burst arrives at the other end, it will cross onto the time domain of the next timeslot, thereby corrupting data from both sources. The solution used to overcome this problem is called "adaptive frame alignment". The Base Transceiver Station measures the time delay from the received signal compared to the delay that would come from a mobile station that was transmitting at zero distance from the Base Transceiver Station. Based on this delay value, the Base Transceiver Station informs the mobile station to either advance or retard the time alignment by sending the burst slightly before the actual time slot. The base station also adopts this time alignment in the down link direction.

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2 Nokia Implementation 2.1 Introduction The preceding chapter have discussed the GSM network and it’s various concepts in considerable detail. These concepts and ideas are generic in nature and are based on the standard specifications of the GSM defined by ETSI (European Telecommunication Standard Institute). All the GSM networks operate in the same way as explained in the preceding chapter. However, the implementation of the network may differ between different organisations. The end result will of course be the same as specified in the standards, but the equipment and the implementation of it can vary.

2.2 Network Architecture Consider the generic GSM network architecture shown below, which has all the elements discussed so far. In this diagram all the network elements and the interfaces are shown. Then compare this with the Nokia Implementation diagram of the same GSM network on other diagram and note the differences.

Figure 14 Generic GSM Network Architecture

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Figure 15 Nokia Implementation of GSM Network. The differences are quite obvious. The main highlight seems to be a certain piece of equipment called DX 200 which has been implemented in BSC, MSC, TC, VLR, HLR, AC and EIR.

2.2.1 Integration of Elements The picture shows that different GSM network elements are integrated into one DX 200 system. The first of these is the DX 200 MSC/VLR in which the MSC and the VLR are integrated. In the standard GSM specifications the MSC and VLR are two different network entities with functions of their own. However the amount of signalling between MSC and VLR is quite heavy and their tasks are so related with each other that it makes good sense to integrate them together. The second integration is the DX 200 HLR/AC/EIR. All these three elements are basically databases which hold various types type of information. The HLR keeps the subscribers’ subscription data, AC holds the authentication data and the EIR, equipment data. In Nokia implementation these three elements are integrated and implemented as parts of the DX 200 HLR.

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2.2.2 DX 200 Platform In the early days of the telecommunication history, switching exchanges were huge, cumbersome and not so effective, requiring manual switching. As technology evolved, switches also started becoming automatic and more efficient. The cross bar exchange was considered one of the major developments in telecommunication, however, the development of the transistor after the Second World War has revolutionised telecommunications and other industries. The computer industry changed dramatically and microprocessors started to evolve and develop. To make use of these developments, exchanges became heavily computerised. The various tasks required in a telephone exchange could be executed using the power of microprocessors. We can see that there are different tasks for a CPU (Central Processing Unit) of a switching exchange. For one processor to carry out all the functions, a very powerful processor would be required. However, what if these tasks were to be distributed among many different computers? Would we still need very powerful processors? The answer is no. We could use commercially available processors and write software for it to execute. This is the principle on which Nokia has built switching equipment. All the various tasks of the exchange are distributed to different functional units which have a CPU of their own. Since these functional units have to communicate with each other, a message bus is used for communication. The next figure shows the distributed architecture of Nokia’s DX platform.

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This kind of architecture has a number of advantages which make this a very reliable platform to implement high fault tolerant systems. Distributed Processing is the first one. Since the tasks are distributed among different computer units, it makes it easier and faster to track down and solve problems. Modularity is another advantage as only those units which are necessary need to be installed in an exchange. If the units are required at a later date they can be installed later. One does not have to buy everything to have just a fraction of its resources. Alternatively, if a single unit performing certain task is not enough, then the same type of unit can be “added-on.” Reliability is another significant advantage. A functional unit, depending on its vulnerability to disturbances is either duplicated, or a spare unit for a number of units is installed. These are called the 2N redundancy and N+1 redundancy. In the former, there is always a unit in “hot stand-by” mode for each working one, in the latter there is one spare unit for N number of units. Based on this DX platform, Nokia has built different types of switching exchanges. DX210 and DX220 are the fixed telephone network (PSTN) switches. The DX 200 is the platform used for GSM elements BSC, MSC/VLR, HLR/AC/EIR

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3 Nokia BTS Implementation Base Transceiver Station, BTS, is a part of the Base Station subsystem, BSS. The BTS provides the radio part for the BSS and is located between the BSC and the Mobile Stations, MS. The Nokia BTS is not based on the DX switching platform. It has it’s own platform. BTS handles the signalling as well as traffic in the air interface. It also detects the MS. A base transceiver station is a physical site from where the radio transmission in both the downlink and uplink direction takes place. The radio resources are the frequencies allocated to the Base Station. The particular hardware element inside the Base Transceiver Station (BTS) responsible for transmitting and receiving these radio frequencies is appropriately named "Transceiver (TRX)". A Base Station site might have any number of TRXs from one to twelve. These TRXs are then configured into one, two or three cells. If a BTS is configured as one cell it is called an "Omnidirectional BTS" and if it is configured as either two or three cells it is called a "Sectorised BTS". In an omnidirectional BTS the maximum number of TRXs is ten, and in a sectorised BTS the maximum number of TRXs is four per sector. Frequency Hopping is another function which is implemented using the Frequency Hopping Unit between the Frame Units and Carrier Units. Hopping can be cyclic or pseudo-random as defined in the GSM specifications. The BSC provides the Frequency Hopping parameters for the BTS that operates according to the parameters. Frequency hopping is optional. BTS is also responsible for power control in down link direction. LAP-D signalling is used between the BSC and BTS and LAP-Dm between MS and BTS.

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3.1 Introduction to Nokia UltraSite EDGE Base Station Nokia UltraSite EDGE BTS is part of the Nokia UltraSite Macrocellular Solution that delivers high-capacity and wide-coverage macrocellular BTS products complete with transmission and auxiliary equipment. Nokia UltraSite EDGE BTS supports both omnidirectional and sectorised configurations for traditional voice and future data applications. The BTS can be used in GSM 800, 900, 1800, or 1900 systems, or as a GSM 800/1900 or GSM 900/1800 dual-band BTS. With the addition of EDGE/EGPRS, the BTS offers a maximum data rate of over 400 kbit/s with multiple timeslots, as compared to over 100 kbit/s with multiple timeslots for GSM/GPRS. Nokia UltraSite EDGE BTS is available in different cabinets for outdoor and indoor applications: • Nokia UltraSite EDGE BTS Outdoor • Nokia UltraSite EDGE BTS Indoor

3.1.1 Features and benefits Nokia UltraSite EDGE BTS has many features and benefits, including: •

Nokia UltraSite EDGE BTS is lightweight and compact and, with its full-frontal accessibility, can be installed just about anywhere.

•

The modular design of Nokia UltraSite EDGE BTS guarantees smooth expansion and upgrades of base station equipment from GSM to GSM/ EDGE and Wideband Code Division Multiple Access (WCDMA) with minimal disturbance to network operation. The BTS supports hot insertion of plug-in units, which means that most units can be replaced during operation without disrupting the BTS functions.

•

Both the outdoor and indoor cabinets can hold an Integrated Battery Backup (IBBU). However, the IBBU reduces the maximum number of Transceiver (TRX) units in the cabinet from 12 to 6.

•

Nokia UltraSite EDGE BTS cabinets can be installed side by side and in corners, which means less space is required.

•

Nokia UltraSite EDGE BTS fits into the corresponding Nokia Talk-family BTS footprints. The operator does not need to alter any previous plans for expansion.

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3.2 Additional features Following features of the Nokia UltraSite EDGE BTS: •

Macrocellular network

•

Cell capacity and coverage area

•

Cost-effective, multi-purpose platform

3.2.1 Macrocellular network Nokia UltraSite EDGE BTS is designed to be a high-capacity macrocellular network that supports multimedia services. The BTS offers High Speed Circuit Switched Data (HSCSD) that allows the operator to provide: • •

data rates up to 57.6 kbit/s over four channels. real-time services to the end user.

Nokia UltraSite EDGE BTS also uses General Packet Radio Service (GPRS), which allows the operator to: • Maintain connection. • Experience increased data transmission speeds of over 100 kbits/s with multiple timeslots. • Extend the Internet connection all the way to mobile terminals. The data capacity of each Transceiver for HSCSD and GPRS can be increased by using EDGE in the network. The EDGE enhanced data speed rate is over 400 kbit/s with multiple timeslots for EGPRS. Nokia UltraSite EDGE BTS supports effective network capacity building and highcall quality in a dual-band configuration in a single cabinet. Common antennas and antenna feeder cables for both the GSM/EDGE 800, 900, 1800, and 1900 TRXs are used. GSM/EDGE 800/1900 or 900/1800 dual-band configurations are also supported.

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3.2.2 Cell capacity and coverage area The RF performance of Nokia UltraSite EDGE BTS is also optimised for macrocellular applications. The BTS offers a broad voice and data coverage area due to an improved link budget. Flexible combiner options are available to create highoutput power with a minimum number of antennas. To increase the cell capacity and maintain large cell coverage, Nokia UltraSite EDGE BTS uses the following features: • • •

Nokia Intelligent Coverage Enhancement (ICE) Nokia UltraSite Masthead Amplifiers (MHAs)with multiport RX diversity reception schemes (2- and 4-way) Nokia Smart Radio Concept (SRC) for GSM/EDGE

These features further maximise the extremely high BTS receive sensitivity and push the cell coverage and capacity to the maximum. For example, the standard GSM cell range is 35 km; the extended cell range feature allows cell ranges of up to 67 km. Using the extended cell range feature requires EDGE-capable TRX baseband units (which will be available in a future software release). These features also assist in meeting the coverage and capacity needs for many different applications. The high TRX density (with 12 TRXs per cabinet) of Nokia UltraSite EDGE BTS has the following benefits: • reduces the total network investment (less sites and equipment per site). • supports efficient capacity growth with the modular design. • allows synchronised handovers and shared frequencies in sectorised sites between cabinets. • permits the combination of TRXs from different cabinets into the same cell. For capacity enhancement and network quality assurance, Nokia UltraSite EDGE BTS uses Intelligent Frequency Hopping and Adaptive Multi-Rate (AMR) speech coding. Adaptive Multi Rate Code (AMR) introduces a new set of codes and adaptive algorithm for code changes and thus can provide significantly better speech quality and more capacity on the air interface. With AMR we can achieve very good speech quality in full rate mode even we in increase the speech capacity by using the half rate mode and still maintain the quality level of current FR calls.

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3.2.3 Cost-effective, multi-purpose platform Nokia UltraSite EDGE BTS provides a smooth evolution from GSM to GSM/ EDGE and WCDMA. The BTS has the capacity to contain both GSM/EDGE. Transceiver units and WCDMA Transmitter and Receiver units in a single cabinet. The BTS can be upgraded to GSM/EDGE simply by adding the EDGE capable BB2 and TRX plug-in units. Triple-mode Nokia UltraSite EDGE BTS supports full 3G capability with sufficient voice and data capacity to meet future demands. The platform supports various combinations of GSM, GSM/EDGE, and WCDMA simultaneously. With Nokia UltraSite EDGE BTS, the same equipment can be used to build coverage, capacity, or both. The cabinets can be chained to provide a dense, highcapacity site, such as 36 GSM/EDGE TRXs using only three BTS cabinets or a maximum of 108 TRXs using nine cabinets. Nokia UltraSite EDGE BTS can be configured to provide coverage to the widest rural area. With established networks, sites can be expanded by using Nokia UltraSite EDGE BTS as an upgrade cabinet at existing Nokia Talk-family sites. Nokia UltraSite EDGE BTS and Nokia Talk-family cabinets can be placed side by side. Planning for Nokia UltraSite and Talk-family co-sites is easy, because the Nokia UltraSite EDGE BTS cabinet fits into the corresponding Nokia Talk-family footprint, and the existing antennas and feeder cables can be used.

3.3 Configurations The actual installation and commissioning of Nokia UltraSite EDGE BTS is fast and easy because of the simplified internal cabling and uniformity of unit placement in all cabinet types. In addition, BTS Manager provides easy, step-by step commissioning instructions and quick integration into the network, which ensures that Nokia UltraSite EDGE BTS is up and running soon after installation. Installation costs and required skill sets are kept to a minimum, and revenue flow starts immediately. Nokia UltraSite EDGE BTS is delivered to the site with the necessary plug-in units. To add to the ease of installation, all plug-in units are delivered with the required cables for the configuration. In addition, Nokia UltraSite EDGE BTS: • • •

can be sectored and expanded easily. supports multiple sectors in each cabinet. can be used with various flexible antenna configurations, including multiband and cross-polarised antennas.

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As a standard, TRXs come with built-in support for 2-way diversity, and support for 4way diversity will be available in the CX3.0 software release. 4-way RX diversity requires two TRXs. In addition, the Nokia SRC for GSM/EDGE with a combination of diversity solutions can be applied in both uplink and downlink directions and used for enhancing radio-link performance.

3.3.1 Configurations for GSM/EDGE 800, 900, 1800, and 1900 To minimise the number of antennas and optimise the coverage range, various antenna combining options are available for Nokia UltraSite EDGE BTS: •

Combining bypass – no signal combining is used.

•

2-way wideband combining – signals from two TRXs are combined into one antenna using one WBC unit.

•

4-way wideband combining – signals from four TRXs are combined into one antenna using two parallel WBC units in a series with a single WBC unit.

•

Remote Tune Combining – signals from six TRXs are combined into one antenna with one Remote Tune Combiner (RTC) unit. With two RTC units, 12 TRXs in one sector can be connected to two antennas. A separate antenna for RX diversity is unnecessary.

Combining bypass

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2-way wideband combining

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4-way wideband combining

Remote Tune Combining

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3.4 Nokia Smart Radio Concept for GSM/EDGE A novel feature for obtaining the maximum benefit for EDGE is the Nokia SRC. The Nokia SRC enhances radio-link performance in both GSM and EDGE mode to ensure maximum coverage, improved data capacity, and high service quality. The Nokia SRC includes a combination of diversity solutions applied in both uplink and downlink directions. In addition, the Nokia SRC supports synthesized frequency hopping. The BTS reception is enhanced when Interference Rejection Combining and MHAs are used together with 2- and 4-way uplink diversity. Antenna diversity is also used in downlink enhancement with a feature called Intelligent Downlink Diversity (IDD). IDD extends the cell coverage by sending the same downlink signal through two transmitters simultaneously. IDD boosts downlink performance by up to 5 dB in all radio timeslots. The typical configurations in one cabinet are: • •

1+1+1 with combiner bypass 2+2+2 with 4-way diversity

Nokia Smart Radio Concept for EDGE with IDD and 2-way diversity (one TRX capacity)

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Nokia Smart Radio Concept for EDGE with IDD and 4-way diversity (two TRX capacity)

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3.5 GSM/EDGE units

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3.6 Transceiver units Following two units:

I. Transceiver RF unit (TRX RF) II. Transceiver Baseband unit (TRX BB2)

3.6.1 Transceiver RF unit (0 to 12 units)

The TSxx unit of the Nokia UltraSite EDGE BTS performs RF modulation/ demodulation and amplification for one RF carrier. The TSxx unit handles uplink signals from the Mobile Station (MS) to the BTS and downlink signals from the BTS to the MS. Three versions of the TSxx unit are available for different frequency bands: • TSGx for GSM 900 • TSDx for GSM 1800 • TSPx for GSM 1900

Main modules: The TSxx unit consists of the following modules: 1. Transceiver (TRX) 2. Frequency Hopping Synthesiser (FHS) 3. Power Amplifier (PA) 4. Power Supply (PSU)

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1.TRX module: The TRX module provides the main RF functions for the Nokia UltraSite EDGE BTS. The TRX module has the following functional sections: • Interface • Transmitter (TX) • Receiver (RX) • TRX loop These functional sections communicate with the Transceiver Baseband (BB2x) and Base Operations and Interfaces (BOIx) units through the RFU backplane, which is connected to the common backplane. The functional sections process the following signals: • • •

data signals between the TSxx and BB2x units initialisation and control signals from the BB2x unit to the TSxx unit status and alarm signals from the TSxx unit to the BB2x unit

The TX and RX sections each have one Application Specific Integrated Circuit (ASIC). The TX ASIC incorporates Direct Digital Synthesis (DDS), generating a Gaussian Minimum Shift Keying (GMSK) signal for the TX. The Interface converts the baseband (BB) data stream to GMSK modulation for the TX. It also converts the analogue RX frequency signal from the main and diversity branches to the data stream. The Interface controls all synthesisers and the TRX loop. It also handles clock distribution from the BB2x unit and alarm functions. The intermediate frequency (IF) sections in the TX raise the signal to the carrier frequency. Thereafter, the RF section amplifies the signal to the desired output signal amplitude. The RF section also handles the signal power control. The TSxx unit supports 16 power levels with 2 dB steps, with a maximum range of 30 dB. Power levels from 0 to 6 are static; power levels from 7 to 15 are dynamic. The RF section of the RX converts the carrier frequency signal to the IF frequency. The IF sections of the RX perform channel filtering and prevent interfering frequencies from distorting the signal. The IF sections also provide automatic gain control. The TRX loop supports the self-testing of the TSxx unit. The tests are carried out by converting the frequency of the TX signal to the RX band. The signal is coupled from the TX output, and the resulting low-level signal is routed back through the RX path. The signal can be routed to the main branch or to the diversity branch.

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2. Frequency Hopping Synthesiser module The Frequency Hopping Synthesiser (FHS) module employs two FHSs — RX FHS and TX FHS—for RF frequency hopping. The RX FHS serves as the first local oscillator in the receiver. The TX FHS serves as the second local oscillator in the transmitter. Both the RX FHS and the TX FHS have two Phased Locked Loop (PLL) circuits. Each circuit contains a PLL, an amplification chain, and a switching network. The output buffer is common for both circuits. Each circuit has a voltage-supply regulator. Both FHSs work according to the Ping-Pong principle: The output frequency is taken alternately from the two PLLs. 3. Power Amplifier module The Power Amplifier (PA) module receives a GMSK-modulated signal from the TRX module and amplifies that signal to the appropriate level. 4.Power Supply module The Power Supply (PSU) module consists of a commercial DC/DC converter, input and output filters, and connectors. The PSU module converts the 48 VDC / ±12 VDC supply voltage to the 26.0 VDC required by the PA module.

3.6.2 Transceiver Baseband unit (1 to 6 units) The TRX BB2 is a digital signal processing board, consisting of two independent baseband modules. Each module functions independently for its own GSM/ EDGE TRX RF unit. One of the Transceiver Baseband units also controls frequency hopping. The BB2x unit of the Nokia UltraSite EDGE BTS has the following main functions: • performs digital signal processing for speech and data channels • manages signalling for speech functions The BB2x also: • uses software downloaded from the Base Operations and Interfaces (BOIx) unit • sets its timing according to references from the BOIx unit • supports synthesised (RF) and baseband (BB) frequency hopping Operation The BB2x communicates with the following units: • BOIx • Transceiver (TSxx) • Transmission (VXxx) These units send and/or receive signals according to the uplink and downlink paths.

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Uplink signal processing In the uplink path, the TSxx unit sends a demodulated digital signal to the BB2x unit in High Level Data Link (HDLC) format. The BB2x unit samples the signal and extracts and decodes the information bits. The BB2x unit then sends the processed signal to the VXxx unit, which passes the signal through the Abis interface to the Base Station Controller (BSC). Downlink signal processing In the downlink path, the BSC sends a signal through the Abis interface to the VXxx unit, which passes the signal to the BB2x unit. The BB2x unit encodes the signal and reformats it as a GSM TDMA burst. The BB2x unit then sends the signal to the TSxx unit. The TSxx unit modulates and amplifies the signal and sends it to the RF filter units. From those units, the signal goes to the antenna, which passes the signal through the Air interface to the Mobile Station (MS). Main blocks The BB2x unit has two independent BB sections. Each section communicates with the TRX module of one TSxx unit. Therefore, one BB2x unit can process two TSxx units, each with eight received and transmitted logical channels. Each BB section consists of the following blocks: • Interface • D-bus interface • Control block • Digital Signal Processor (DSP) block • F-bus interface

BB2x main blocks

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Interface The interface converts the BB data stream to TX. The interface also handles synthesiser control, clock distribution from the BB module, alarm functions, and TRX loop control. In the downlink direction (BTS to MS), the BB2x unit sends transmission, initialisation, and synthesiser-control data to RF through a serial point-to-point line using HDLC protocol. In the uplink direction (MS to BTS), the BB2x unit receives: • I (In Phase) and Q (Quadrature) components of the normal and diversity branch data samples • RF alarms and status information through a serial point-to-point line D-bus Interface The D-bus interface synchronises the signals transmitted and received through the Dbus. The D-bus interface also synchronises data between the D-bus and Unit Controller (UC) processor and between the D-bus and Channel Digital Signal Processor (CHDSP). The D-bus consists of the following buses: • D1-bus, which transfers traffic and signalling data among the BB2x, VXxx, and BOIx units • D2-bus, which transfers internal O&M communications (including software downloads) among the BOIx, BB2x, and Remote Tune Combiner (RTxx) units Control Block The Control block handles the following functions: • clock generation and synchronisation • interrupt • alarm management The Control block contains the UC processor, which runs the BTS software. Digital Signal Processor Each DSP block has an Equaliser DSP processor (EQDSP) and a Channel DSP processor (CHDSP). Equaliser DSP processor The EQDSP handles the following functions: • sample reception from RF • bit detection • channel equalization

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Channel DSP processor The CHDSP handles the following functions: • sample transmission to RF • channel decoding and encoding • ciphering and deciphering F-bus Interface The frequency-hopping bus (F-bus) between the BB2x units is used for BB hopping (moving TX and RX bursts between the BB2x units).

3.7 Duplex units The DVxx performs duplex operation of TX and RX signals through a common antenna. It also provides filtering and amplification for main and diversity receive signals before they are fed to the TRX RF units using the Receiver Multicoupler. The unit contains a variable gain LNA for optimal amplification of the receive signal. The high-gain LNA is fixed and used without the optional MHA unit. The low-gain LNA is variable and used only with the MHA unit; the low-gain LNA is set according to antenna cable attenuation values. The DVxx is sub-banded for GSM/EDGE 1800 to increase TX/RX separation and achieve better performance. Each unit has 50 MHz passband width, which provides the needed overlap to fit most operator spectrum allocations. A full-band DDU for GSM/EDGE 1800 will also be available but with slightly lower performance.

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3.8 Combiners The following two units: • •

Wideband Combiner (WBC) Remote Tune Combiner (RTC)

3.8.1 Wideband Combiner (0 to 9 units)

The WBC combines two TRX RF transmitter outputs into one. The WBC unit consists of the following components:

• • • •

one 2-way combiner two isolators one 50ohm termination one heatsink for thermal dissipation

3.8.2 Remote Tune Combiner (0 to 2 units)

The RTC combines up to six TRX outputs into one antenna. It also: •

• •

uses the Receiver Multicoupler to filter and amplify the main and diversity receive (RX) signals before they are fed to the TRX RF units. contains a variable-gain LNA that is used for optimal amplification of the receive signal. The high-gain LNA is fixed and used without the optional MHA unit. The low-gain LNA is variable and used only with the MHA unit. The low-gain LNA is set according to antenna cable attenuation values. has a built-in duplexer. has sub-banded and full-band versions available like the DDU.

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•

3.9 Receiver Multicouplers

(0 to 6 units for 2-way or 0 to 2 units for 6-way) The Receive Multicoupler unit (RMU) distributes RX signals to the TRX RF units. The 2-way unit is used in most WBC or combining bypass configurations. The 6-way unit is always used with the RTC unit and someWBC configurations. One unit performs signal splitting for both main and diversity branches.

3.10 Masthead Amplifier and Bias Tee The following two units: • Masthead Amplifier • Bias Tee/VSWR unit

3.10.1 Masthead Amplifier (0 to 12 units)

Nokia has an MHA unit specifically designed for Nokia UltraSite EDGE BTS to deliver:

• • •

33 dB RX gain in GSM/EDGE 1800 and 1900 units and 32 dB RX gain in GSM/EDGE 900 units low RX noise figure (improved RX sensitivity and signal-to-noise ratio) low TX loss in a compact, low-volume, lightweight, sealed enclosure

The Bias Tee unit is required when using the MHA.

3.10.2 Bias Tee/VSWR unit (0 to 12 units)

The Bias Tee provides DC power to the MHA unit using an RF cable. The Bias Tee is available in two versions:

• •

Bias Tee with VSWR antenna monitoring Bias Tee without VSWR antenna monitoring

VSWR monitoring systematically checks the condition of the antenna line and gives an alarm if the VSWR (Voltage Standing Wave Ratio) value exceeds 2.6. The Bias Tee without VSWR is used solely with the MHA unit, while the Bias Tee with VSWR can be used with or without the MHA unit.

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3.11 Transmission units The Transmission unit connects Nokia UltraSite EDGE BTS with other BTSs and to the rest of the network (such as BSC, Nokia Talk-family, and Nokia MetroSite sites) through the Abis interface. Nokia UltraSite EDGE BTS supports 16 kbit/s, 32 kbit/s, and 64 kbit/s Abis TRX signalling. The O&M signalling speed can be 16 kbit/s or 64 kbit/s. The Transmission unit is available for the following media: • •

radio-link wireline

3.11.1 Radio-link transmission

The FXC RRI unit is the radio-link transmission unit for Nokia UltraSite EDGE BTS. The FXC RRI supports the following:

•

two Flexbus connections (coaxial cable), each at 16 x 2 Mbit/s

•

branching

•

loop protection

•

cross-connection at 8 kbit/s level

A Flexbus connects FXC RRI transmission units to Nokia FlexiHopper Microwave Radio or Nokia MetroHopper Radio. Multiple BTS cabinets at the same site can also be connected using a Flexbus between FXC RRI units in each cabinet. The FXC RRI transmission unit operates as a repeater and interconnects Nokia UltraSite EDGE BTS and the BSC using loop, chain, star, and point-to-point network configurations.

3.11.2 Wireline transmission

The wireline transmission units for Nokia UltraSite EDGE BTS are: •

FC E1/T1 – 1 x 2 Mbit/s (E1) or 1 x 1.5 Mbit/s (T1) PCM connection, one coaxial 75-ohm TX and one coaxial 75-ohm RX connector for E1 use, one twisted pair 120-/100-ohm TX/RX interface connector for either E1 or T1 use

•

FXC E1 – 4 x 2 Mbit/s (E1) PCM connections, four coaxial 75-ohm TX and four coaxial 75-ohm RX connectors for E1 use, grooming, branching, and loop protection support, cross-connection down to 8 kbit/s level

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•

FXC E1/T1 – 4 x 2 Mbit/s (E1) or 4 x 1.5 Mbit/s (T1) PCM connections, four twisted pair 120-/100-ohm TX/RX interface connectors for either E1 or T1 use, grooming, branching, and loop protection support, crossconnection down to 8 kbit/s level. Interfaces can be configured independently either E1 or T1 mode

The FC E1/T1 transmission unit operates as the termination point in a chain, star, and point-to-point topology network. The FXC E1 and FXC E1/T1 transmission units operate as branching points and interconnect Nokia UltraSite EDGE BTS cabinets and the BSC using loop, chain, star, and point-to-point network configurations.

3.12 Base Operations and Interfaces unit

The BOIx unit handles the control functions common to all other units in the BTS. These functions include: • BTS initialisation and self-testing •

configuration

•

O&M signalling

•

software download

•

main clock functions

•

collection and management of external and internal alarms

3.12.1 Main blocks

The BOIx unit consists of the following functional blocks: • Unit Controller (UC) • Master Clock Generator (MCLG) • D-bus • Field Programmable Gate Array (FPGA)

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Unit Controller The UC is the main BTS processor. It performs O&M communication through the network and Nokia BTS Manager, and it controls the BTS units through the D bus. The BSC or Nokia BTSManager downloads software to the UC, which stores the software in non-volatile memory. The UC downloads that software to the other BTS units through the D2-bus. One tri-colour LED that is controlled by the main processor indicates the current status of the BOIx unit. Master Clock Generator The MCLG generates the accurate clock for the BTS. Other common clock signals are also derived from this clock reference and distributed to other units. D-bus The D-bus consists of two separate serial buses — D1-bus and D2-bus. D1-bus The BOIx unit, BB2x unit, and Transmission (VXxx) unit use the D1-bus to transfer the following data over the Abis interface between the BTS and BSC: • • •

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D2-bus The BOIx, BB2x, and RTxx units use the D2-bus for internal O&M signaling and for internal communications, such as downloading software. Field Programmable Gate Array The FPGA handles the following functions: • controls and monitors the status of the fans in the BTS • checks the external frame clock • allows flexible connections that are set by software between any TSxx unit and any BB2x unit

3.13 BTS Power Supply The following power supply units: •

-48 VDC Power Supply

•

+24 VDC Power Supply

•

AC Power Supply

•

AC Filter unit

-48 VDC Power Supply unit (0 to 3 units) The -48 VDC Power Supply unit converts -48 VDC input voltage to the different DC voltages required for the various BTS units. It distributes the appropriate voltages through the backplanes to the units — except for the optional Heater unit, which receives its voltage from the AC mains through the AC Filter unit. The PWS unit also supplies power for theMHAunit. Nokia UltraSite EDGE BTS holds up to three -48 VDC Power Supply units, which provide full redundancy for up to twelve TRX units. +24 VDC Power Supply unit (0 to 2 units) The +24 VDC Power Supply unit converts +24 VDC input voltage to the different DC voltages required for the various BTS units. It distributes the appropriate voltages through the backplanes to the units — except for the optional Heater unit, which receives its voltage from the AC mains through the AC Filter unit. The PWS unit also supplies power for theMHA unit. Nokia UltraSite EDGE BTS holds up to two +24 VDC Power Supply units, which provide full redundancy for up to six TRX units.

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AC Power Supply unit (0 to 2 units) The AC Power Supply unit converts input AC voltage to the different DC voltages required for the various BTS units. It distributes the appropriate voltages through the backplanes to the units—except for the optional Heater unit, which receives its voltage from the AC mains through the AC filter unit. The PWS unit also supplies power for the MHA unit. Nokia UltraSite EDGE BTS holds up to two AC power supplies, which provide full redundancy for up to six TRX units. AC Filter unit (0 to 1 unit) An AC Filter unit is an optional unit that is necessary when AC Power Supply, IBBU, or Heater units are used.

3.14 Operation

The Nokia UltraSite EDGE BTS performs the radio functions of the Base Station Subsystem (BSS). The BTS receives and sends signals through: • •

Air interface — frequencies that connect the BTS to the Mobile Station (MS) Abis interface — cable or radio link that connects the BTS to the Base Station Controller (BSC), which is the central element of the BSS

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3.14.1 Uplink and downlink signaling In the uplink path, the BTS receives signals from the MS; in the downlink path, the BTS sends signals to the MS. Uplink and downlink signals travel through the Air interface on different frequencies, the higher frequency carrying downlink signals. Uplink path The antenna picks up a signal from the MS through the Air interface. The antenna passes the signal to the optional Masthead Amplifier (MNxx) and Bias Tee (BPxx) units or to the optional Dual Band Diplex Filter (DU2A) unit. The signal then goes through either the Dual Variable Gain Duplex Filter (DVxx) unit or the Remote Tune Combiner (RTxx) unit to the Receiver Multicoupler (M2xx and M6xx) and TSxx units. The Transceiver module (TRX) on the TSxx unit converts the received signal to Intermediate Frequency (IF) levels and filters the signal. The TSxx then sends the signal to the Transceiver Baseband (BB2x) unit for digital signal processing. The BB2x sends the processed signal to the Transmission (VXxx) unit, which passes the signal through the Abis interface to the BSC. Downlink path The BSC receives a signal from the network and sends the signal to the VXxx unit through the Abis interface. The VXxx passes the signal to the BB2x unit for digital signal processing. The BB2x sends the processed signal to the TSxx unit. The TRX module on the TSxx unit filters the signal, raises it to the carrier frequency, and amplifies it. The TSxx then sends the signal either to the RTxx unit or through the optional Wideband Combiner unit (WCxx) to the DVxx unit. The DVxx or RTxx unit sends the signal through either the optional DU2A unit or the BPxx and MNxx units to the antenna, which passes the signal through the Air interface to the MS.

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4

Nokia FlexiHopper

4.1 Introduction A nokia flexiHopper network element consist of an outdoor (OU) unit and an indoor unit (IU). The units are connected together with single coaxial cable, Flexibus. The flexibus cable carries power and digital baseband data and can be upto 300m long. FlexiHopper microwave radios are available for the 7,8,13,15,18,23,26 &38 Ghz freq bands. The radio transmission capacity of all FlexiHopper model is 2x2, 4x2,8x2,16x2 Mbps. This can be selected using the node manager without any hardware changes. The higher the transmission capacity selected, the wider the freq. channel. All the freq bands use the same technical concept and a similar mechanical construction. The only difference between outdoor units is the length of the collar, which houses the antenna filter, the lower the freq, the longer the filter and collar. The transmitter uses pi/4-DQPSK modulation, which has the advantages of a narrow spectrum and good output power efficiency. Typical maximum output power is 16-23dbm.

4.2 Indoor Units Different indoor units for FlexiHopper are available to provide optimal features for different environment. All the freq bands use the same indoor units. FIU19 the interface capacity of fiu19 can be from 4x2 upto 16x2 Mbps. And con be expanded easily with the help of plug in units. The 2Mbps cross connect function is integrated. It enables connection for two or four outdoor units and supports hot standby and diversity protection method. FIU19E supports Ethernet interface. RRI transmission integrated into the BTS. Bypass capacity to another RRI. Cross connections are integrated. FlexiHopper can be fully controlled and managed locally or remotely.

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4.2.1 Versatile maintenance and troubleshooting • • •

The quality of the transmission can be monitored with the built in BER measurement. Far-end and near end loops used for troubleshooting. Alarms with troubleshooting Information.

4.2.2 Integrated Radio and Cross connect 2 Mbps cross connection is integrated into the indoor units and is freely programmable between different flexibuses and 2Mbps interfaces. The indoor units has two or four totally independent framing defaming sections, which can be cross connected to external or internal flexbus interfaces.

The bidirectional flexbus cable connects all system elements together. Flexbus carries digital 2-16x2 Mbps signals and control data between the units of the network elements from indoor units to the outdoor as well as from indoor units to another indoor units. Flexibus also feeds DC power to outdoor unit. Several different logical signals can be carried by flexbus and all on site cabling is made by internal electrical cross connections. The traffic routes via the network elements are defined by using the crossconnection functions available in the network elements. Thus, traffic routing means managing the cross-connections in the network elements.

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In a conventional setup the system elements are connected using 2M cables, all this replaced with a single flexbus cable. In this ,in a conventional 16x2Mbps 75 ohm system there could be upto 96 cables.

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4.3 FlexiHopper Outdoor Unit Outdoor unit includes five functional units. • • • • •

Power supply unit (PSU) Modem board Intermediate freq unit(IFU) Microwave unit(MWO) Duplex filter

The outdoor unit is connected to the indoor unit by using a single coaxial cable which carries the baseband data between the indoor and outdoor unit in full duplex mode. The cable is connected to the power supply unit in which the data traffic is filtered and transferred to the modem board. The needed DC voltages are generated in the PSU and delivered to the modem board. Unlike the traditional outdoor units design the modem board is located in the outdoor unit, which makes it possible to use more advance control loops between modem board and RF parts (IFU and MWU). The main component of the modem board is the custom design ASIC. The ASIC contains a digital modulator and demodulator with Reed-Solomon Forward Error Correction (FEC). The interface between the modem board and RF part is analog I and Q signals. The modem board includes also embedded micro processor system, which is used to control all units inside the outdoor unit as well as the far-end unit when needed. The RF functions are divided between two units: IF units and microwave unit. MWU includes all microwave circuits, most of which are MMICs, while IFU includes required intermediate frequency circuits.

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In the transmission side direct conversion architecture is implemented to enable use of single microwave local oscillator. Since the I/Q up converter operates at the end frequency, a digital feedback loop is required to correct the amplitude and phase errors of the modulator. After the up-conversion the signal is amplified enough in order to obtain the required maximum output power levels. A temperature compensated power detector is used to monitor the power level after the high power amplifier (HPA), and thus, to drive the voltage variable attenuator (VVA) in order to obtain the required output power level. In the receiver side single TF conversion architecture is used. After the low-noise amplifier (LNA), the received signal is down converted to IF. Automatic gain control (AGC) with dynamic range of about 100db is used to obtain a constant rms power level for the IQ- demodulator. The outdoor unit contains two separate phase locked oscillator circuits. In the MWU the fundamental oscillator frequency is multiplied in order to obtain the low phase noise VCO signal for the transmitter (Tx) and receiver (Rx) up- and down converters. Due to the common VCO frequency at Tx and Rx, the IF frequency is always equal to the duplex spacing. The waveguide duplex filter is used to separate the transmitter and receiver and to provide at the same time low loss connection to the antenna port.

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4.3.1 Forward Error Correction and Interleaving FlexiHopper radio use forward error correction (FEC) and interleaving to improve transmission quality. The FEC is continuously on and the interleaving is selectable between off, 2-depth, 4-depth modes. FEC uses Reed-Solomon coding. The code uses 4 redundancy symbols for every 59 data symbol and redundancy of the code is 6.4%. Together with the interleaving also errors of burst can be corrected. Maximum error correction effectiveness is achieved with 4-depth interleaving. When interleaving is in use, transmission delay increases slightly. This is not a problem but in long chains of radio links the delay accumulates. And it might necessary to turn the interleaving off.

4.3.2 ALCQ (Adaptive Level Control with Quality measure) ALCQ is a method for Automatic Transmit Power Control (ATPC). This feature enables the radio transmitter to increase or decrease the transmit power automatically, according to the response received from the other end of the hop. This approach achieves more efficient utilization of radio frequencies than the constant power level approach. The controlled use of transmit power reduces interference between systems, which in turn allows tighter packing of radio links within the same geographical area or at network star points. The maximum transmit power is set with nokia hopper manager. But when ALCQ is in use the radio always tries to transmit at minimum power. The common idea behind the ALCQ is to monitor the received signal level together with the bit error ratio (BER) of the receiver. And to adjust the far end transmitter output power to adapt the fading conditions. If the fading increases rapidly, the radio reacts immediately by increasing the power, but not higher the set maximum value.

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4.4 FIU 19 (E) Indoor Units One FIU can support up to four outdoor units. FIU can feed power to two outdoor units via the flexbus connections. When more than two outdoor units are used, a flexbus plug in units with its own power supply is required. When four outdoor units are connected, the OUs connected through the main flexbus interface(FB1 FB2) must be configured to protected mode. So, transmission to a maximum of three direction can be achieve with one FIU. Full 1+1 (2IU/2OU) protection an be implemented via FIU.

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4.5 FXC-RRI Indoor unit FXC RRI indoor unit is fully integrated with Nokia UltraSite EDGE BTS. FXC RRI supports two outdoor units and provides up to 16x2 Mbits/s add/drop capacity. FXC RRI has an integrated 8Kbit/s cross connection function, enabling hot standby and loop protection. FXC RRI transmission unit features the cross-connection function, allowing the traffic to be groomed so that transmission paths are fully utilised. This will help to reduce transmission costs. If the total flexbus interface traffic in one FXC RRI is more than 16x2Mbit/s, the extra traffic can be by passed from one flexbus interface to another in a separate 2Mbit/s cross connection field.

4.5.1 Features • • • • • • • •

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The main features of FXC RRI transmission unit are: two Flexbus interfaces, which give support for two radio outdoor units or any other network elements with Flexbus interfaces A separate short circuit protection in both Flexbus interfaces. This ensures that a short circuit in either of the interfaces does not affect the other Flexbus interface. capacity bypassing possibility at 2M level from one Flexbus interface to another up to 16 x 2 Mbit/s add/drop capacity cross-connections with the following granularities: 8k, 16k, 32k, 64k, n x 64k, and 2M grooming, branching, and loop protection support easy management of settings and transmission configurations both remotely and locally advanced testing features: the transmission unit's internal tests and loopbacks can be started with the node manager.

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4.6 Application Nokia FlexiHopper is the optimal solution to a wide range of different access needs in various network environments both in cellular and fixed applications. Typical applications which can use Nokis FlexiHopper radio includes: •

As an access node for Nokia UltraSite EDGE BTS.

•

In cellular transmission application - BTS to BTS - BTS to BSC - BSC to MSC

•

In dedicated networks for - railway companies - electrical utilities - Oil and gas companies - Defense institutions

•

In providing temporary voice or data links.

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5

Architecture of the BSC3i

The Nokia BSC3i is based on a modular software (SW) and hardware (HW) Structure. Because there are exact specifications for the interfaces between different modules, new functions can easily be added without changing the architecture of the system. Thus, the BSC3i can have a long operational lifespan and still always have up-to-date features. The distributed architecture of the BSC3i is implemented by a multiprocessor system. In a multiprocessor system the data processing capacity is divided among several computer units, each of which has a microcomputer of its own. Call handling capacity depends on the number of Call Control Computer Units. Adding more Call Control Computer Units to the BSC can easily increase the capacity of the BSC. Important functional units of the BSC are: •

Bit oriented Group Switch (GSWB), which is used for switching speech and data, and connecting signaling circuits

•

Base Station Controller Signaling Unit (BCSU), which handles the BSC Signaling functions. The optional Packet Control Unit (PCU) is included in every BCSU, when (E)GPRS service is implemented. It handles the (E)GPRS packet control functions in the BSC.

•

Marker and Cellular Management Unit (MCMU), which controls and Supervises the GSWB and implements radio resource management (RRM) Functions, being responsible for cells and radio channels. Two integrated LAN switch units are configured in both MCMU units. The MCMU also Acts as a BSC SYM (system maintenance) unit in case of OMU failure.

•

Operation and Maintenance Unit (OMU), which serves as an interface Between the user and the BSC, but also works as a SYM unit of BSC and Automatically supervises the BSC

•

The high-speed Message Bus (MB), which interconnects the functional Units of BSC

•

The Ethernet Bus, which interconnects the Ethernet interfaces to the Integrated LAN switch units

•

Exchange Terminals (ET), which connect transmission systems to the GSWB

•

Clock and Synchronization Unit (CLS), which generates the clock signals For the BSC

•

Transcoder(TCSM2) which, though it is a separate network element Usually installed on the MSC site, is normally viewed as a functional unit Of the BSC

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The BSC3i with IP-Switch and Bit Group Switch

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5.1

Bit Group Switch

The Bit Group Switch conveys the traffic passing through the BSC and switches the tones to the subscribers of the exchange and to the trunk circuits. The Bit Group Switch also establishes the necessary connections to the signaling units and the internal data transmission channels, and is responsible for the sub multiplexing functions of the BSC. The Bit Group Switch switches on 8, 16, 32, and 64 kbit/s level. The operation of the Bit Group Switch is controlled and supervised by the Marker and Cellular Management Unit. The MCMU performs all necessary connecting And releasing functions. The Bit Group Switch Cartridge, SW1C-C, consists of power supply (PSC1-S) and four SW64B plug-in units, which all have 32 4 Mbit/s interfaces. The capacity of the GSWB is 256 2 Mbit/s PCMs. The Bit Group Switch of the BSC3i is a fully digital, one-staged, and nonblocking time switch with full availability. Its great advantage is its simplicity. After the Bit Group Switch has identified the correct time slots, it can always connect them in a uniform manner without using a special search path.

5.2

Call Control Computers

In the BSC3i, the call control functions are executed by microcomputers, called Call Control Computers. The Call Control Computers have an identical Central Processing Unit (CPU), which is based on the most suitable commercially available Intel microprocessors. The CPU board contains a microprocessor and a local Random Access Memory (RAM). Each Call Control Computer also contains the additional units that are required for performing specific tasks. A Compact PCI bus interconnects all the different plug-in units of each Call Control Computer.

5.3 Marker and Cellular Management Unit (MCMU) The Marker and Cellular Management Unit (MCMU) controls and supervises the Bit Group Switch and performs the hunting, connecting and releasing of the switching network circuits. The range of the tasks it handles makes up a combination of general marker functions and radio resource management functions. The MCMU is connected to the other computer units of the exchange, OMU and BCSU, through the message bus. It performs the control functions of a switching matrix and the BSC-specific management functions of the radio resources. The hardware of the MCMU consists of three modules: a microcomputer, a Switch Control Interface, and a Message Bus Interface

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Structure of the MCMU The marker functions of the MCMU control the Bit Group Switch. These control functions include the connection and the release of the circuits of the switching matrix. When the MCMU performs the marker functions, it exchanges messages with other Call Control Computers via the Message Bus (MB). The Switch Control Interface writes the required connections into the switch control memory and reads its contents. The switch control interface also performs various tests on the switching network, defined by the microcomputer, and generates the required timing signals. The cellular management functions of the MCMU are responsible for cells and radio channels that are controlled by the BSC. This responsibility is centralized in the MCMU. The MCMU reserves and keeps track of the radio resources requested by the MSC and the handover procedures of the BSC. The MCMU also manages the configuration of the cellular network. The cellular management functions of the MCMU do not require any specific hardware in addition to the standard DX 200 microcomputer and a Message Bus Interface Unit (MBIF). One BSC3i always includes two MCMUs that are permanently connected to the duplicated pair of the Bit Group Switches, the active MCMU to the active GSWB and the passive MCMU to the passive GSWB. Redundant Ethernet interfaces connect the MCMU to the duplicated LAN switch units.

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5.4

LAN switch units The integrated LAN switch provides access to the operator's IP network as a first level LAN switch. It provides uplink interfaces to the IP network (router) or to an additional LAN switch via the BSC3i connector panel. The LAN switch collects data from the computer and packet control units and sends it further to external routers and the IP network via 100 Mb/s uplink connections. Redundant LAN switch units provide ports of 10/100BaseT/Tx interfaces with RJ45 connectors via the BSC3i connector panel. There are in total 4 + 4 10/100BaseT/Tx uplink ports to the operator's IP network. The hardware implementation of a LAN switch contains 2 pcs of LAN switch (ESB20) plug-in units situated in both MCMU cartridges. An active MCMU unit is backed up by a redundant MCMU containing a similar configuration. The active MCMU and its LAN switch interconnect the active LAN connections and the redundant MCMU connects the redundant LAN connections. One LAN switch plug-in unit is dedicated for collecting user-plane traffic from packet Control units and another for collecting data from computer units. The LAN Switch Units of the MCMU clarifies the LAN connection principle in the BSC3i

The LAN Switch Units of the MCMU

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5.5

BSC Signaling Unit (BCSU)

The BSC Signaling Unit (BCSU) performs those BSC functions that are highly dependent on the volume of traffic. One BCSU can handle traffic in a maximum of 110 TRXs. The BCSU is housed in a cartridge of its own. It consists of two parts, which correspond to the A and Abis interfaces. The optional Packet Control Units (PCUs) are included in the BCSU. The A interface part of the BCSU is responsible for the following tasks: •

Performing the distributed functions of the Message Transfer Part (MTP) And the Signaling Connection Control Part (SCCP) of SS7

•

Controlling the mobile and base station signaling

•

Performing all message handling and processing functions of the signaling channels connected to it The Abis interface part of the BCSU controls the air interface channels associated with transceivers (TRXs) and Abis signaling channels. Every speech circuit on the Abis interface is mapped one-to-one to a GSM-specific speech/data channel on the air interface. The handover and power control algorithms reside in this functional unit.

The hardware of the BCSU consists of the following modules •

A microcomputer

•

A LAPD (Link Access Protocol on the D-Channel) interface

•

A combined SS7/LAPD interface for control of the ET

•

The Message Bus interface

•

The Packet Control Unit(s)

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Structure of the BCSU The interface units, that is, the SS7 and the LAPD protocols, are connected to the switching network via PCM connections. Similarly redundant Ethernet interfaces connect the BCSUs to the duplicated LAN switch units. The SS7 interface module is for the A interface. It contains a preprocessor, which is capable of handling a maximum of four signaling channels (64, 128 and 256 kbit/s). The signaling terminal is semi permanently connected to the time slots used for signaling. The BCSU uses the LAPD Interface to supervise the 2 Mbit/s circuits (time slot 0 handling) connected to the Bit Group Switch. The BCSU is equipped with LAPD interface terminals. The standard equipment of the BCSU includes two dedicated terminals for LAPD interfaces for signaling towards BTS and towards ET2, and one shared terminal for both Abis and Ater interface directions. Both LAPD dedicated signaling terminals can handle a maximum of 64 LAPD links. The bit rate of a single link can be 16 kbit/s, 32 kbit/s, or 64 kbit/s. The signaling terminal performs the layer 2 LAPD functions, but application-specific messages are handled by the microcomputer of the BCSU. According to the N+1 redundancy principle, there is one extra BCSU compared to the number set by the dimensioning rules. The additional unit is used only if one of the active units fails.

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The BCSU is part of the decentralized call control section in the DX 200 system. The BCSU is connected to the microcomputer network of the exchange via the message bus interface of its microcomputer and to the CCS and LAPD channels by semipermanent connections via the Group Switch (GSW). The control connection between the trunk circuit interfaces (ET) and the BCSU is also implemented by semipermanent connections. The BCSU is connected to the Clock Equipment (CLS) of the exchange by the 8 MHz and 8 kHz timing signals. The BCSU is connected to the Operation and Maintenance Unit (OMU) and the Marker and Cellular Management Unit (MCMU) by the timing and GSW switchover control signal.

5.6 Packet Control Unit (PCU) In the BSC3i there can be four logical Packet Control Units (PCUs), composed of two physical PCU-B plug-in units, per BCSU. A logical PCU is an entity handling the same functionality in the BSC as a physical PCU plug-in unit in older Nokia BSC models. In the Gb interface, one logical PCU handles one NSE. The PCU unit performs all the data processing tasks that are related to the (E)GPRS traffic. It implements both packet switched traffic-oriented Gb and Abis interfaces in the BSC. A PCU includes microprocessors and digital signal processors integrated to the same plug-in-unit to handle the tasks. The main functions are GPRS traffic radio resource management, for example connection establishment and management, resource allocation, scheduling, data transfer, MS uplink power control, Gb load sharing (uplink) and flow control (downlink).PCUs must be configured to every BCSU installed, but only the activated ones are to be used. This requirement comes from the general N+1 redundancy Principle of the fault tolerant DX 200 Computing Platform.

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5.7 Operation and Maintenance Unit (OMU) The Operation and Maintenance Unit (OMU) is an interface between the BSC and a higher-level network management system and the user. The OMU can also be used for local operations and maintenance. The OMU receives fault indications from the BSC. It can produce local alarm printouts to the user or send the fault indications to Nokia Net Act. In the event of a fault, the OMU automatically activates appropriate recovery and diagnostics procedures within the BSC. Recovery can also be activated by the MCMU if the OMU is lost. The tasks of the Operation and Maintenance Unit (OMU) can be divided into four groups: •

Traffic control functions

•

Maintenance functions

•

System configuration administration functions

•

System management functions

The traffic control functions include: • • •

routing administration routing state administration traffic administration

The maintenance includes the maintenance of the exchange, subscriber network and trunk circuits. These include the following functions: • supervision • alarm output • recovery • diagnostics The system configuration administration takes place by means of MML programs used to manage: • •

exchange expansions introduction of new features in the exchange and network

The system management includes the functions closely related to the operating system of the microcomputer in the OMU which serve the other operation and maintenance functions.

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The system management functions are divided into the following categories: • input and output system • input and output function management • MMI system • Data communications • file management • system support functions

The OMU consists of microcomputers similar to the Call Control Computers. In addition, the OMU contains I/O interfaces for local operation. The Operation and Maintenance Unit (OMU) consists of the following modules: • Microcomputer •

Alarm interface

•

Message Bus Interface

•

Peripheral device interface

•

Ethernet interface

Structure of the OMU

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The alarm interface module connects internal wired alarms to the OMU from the BSC cartridges, power supply, air conditioning equipment, etc. This module provides both input and output interfaces for external alarms to Nokia NetAct. The OMU communicates with the Call Control Computers of the BSC via the Message Bus. The CPU controls the peripheral device interface module, which is used to connect disk units, visual display unit, and printer to the OMU. A mirrored pair of Winchester disk units, and one magneto-optical disk unit can be controlled by the OMU. The disk units are installed in the same OMU cartridge using dedicated plug-in unit adapters. The digital X.25 PCM-based O & M interface module is used for the network management interfaces implemented in time slots. This module provides an X.25 connection via the A interface time slot. It also provides an O & M interface for the transcoders and the transmission equipment. The LAN interface module provides an Ethernet interface according to 802.3. Redundant LAN interfaces from the CPU are interconnected to the duplicated LAN switch units.

5.8

Message Bus (MB)

A duplicated high-speed Message Bus (MB) is used for data transfer between the OMU and the Call Control Computers of the BSC3i The length of each message is determined individually by a message length parameter at the beginning of the message. The sender and the receiver of the message are indicated in the address field of the message. The receiver can be a single microcomputer, or it can be a group of microcomputers specified by the broadcast address. The hardware of the Message Bus consists of several parallel twisted pairs, which carry the actual data and also control the information required for the message transfer. In the event of a failure, the standby Message Bus takes over the functions of the active bus without interfering with the ongoing calls.

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5.9

Exchange Terminal (ET)

The ET performs the electrical synchronization and adaptation of external PCM lines. It performs the HDB3 (ET2E), or B8ZS or AMI (ET2A) coding and decoding, inserts the alarm bits in the outgoing direction and produces PCM frame structure. All ET2 plug-in units contain two separate ETs. All 2,048 Mbit/s (in the ETSI environment) or 1,544 Mbit/s (in the ANSI Environment) interfaces for the MSC, the SGSN and the BTSs are connected to the Exchange Terminals. The Exchange Terminals adapt the external PCM circuits to the GSWB and synchronies to the system clock. Synchronization is included in the bit frame. The ETs of the BSC3i are housed in ET4C-B cartridges. One ET4C-B cartridge can contain up to 32 ET2E-S, ET2E-SC or ET2A plug-in units; however in the second ET4C-B cartridge only 30 plug-in units can be activated. Thus there can be a maximum of 62 ET2-units in active use. Each ET is connected to the switching network and the Clock Unit of the BSC3i via permanent, wired connections. The ETs are also connected to the LAPD interface via a LAPD link. In the incoming direction, the ET2 decodes the 2,048 Mbit/s (ETSI) or 1,544 Mbit/s (ANSI) signal of a circuit into data signals. The decoder decodes the line code (HDB3 in the ETSI environment, B8ZS or AMI in the ANSI environment) into binary form. At the same time, the ET2 is synchronized to the bit rate of the incoming signal. In the outgoing direction, the ET2 receives a binary PCM signal from the Switching network and generates the PCM frame structure. The resulting signal is converted into a line code and transmitted further onto the 2,048 Mbit/s (ETSI) or 1,544 Mbit/s (ANSI) circuit.

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5.10 Clock and Synchronization Unit (CLS) The Clock and Synchronization Unit (CLS) distributes timing reference signals to the functional units of the BSC3i. It can operate synchronously with the timing references it receives from the digital PCM trunks. Four PCM reference inputs with priority order are provided for the timing reference signals. The oscillator of the CLS is normally synchronized to an external source, usually an MSC, through a PCM line. Up to three additional PCM inputs are provided for redundancy. The processor chooses the highestpriority interface which is in order, for the phase lock from the six synchronization inputs. The options are the following: •

A maximum of four 8 kHz synchronization signals are received from the Exchange Terminals. The interface with the Exchange Terminal is symmetrical or current-controlled.

•

A signal n x 4 kHz (n = 2, 3,..., 4 096) divisible by 4 kHz is received from up to two frequency standards (FS). The interface with external frequency standard is separated by a transformer and both a balanced and unbalanced synchronization signal can be connected to it.

5.11 Peripheral devices The peripheral devices of the BSC3i are: • Disk units • Magneto-optical (MO) disk unit • Printer • Visual display unit • External alarm unit • Side Cable Conduit for raised floor installations

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Disk Units The BSC3i has a duplicated system disk unit (Winchester). The software can be loaded locally to the BSC by using a magneto-optical disk. The system disk units contain the operative software as well as fallback software of the BSC and BTS software including the BTS HW database. Traffic measurements are also stored on the system disks. All the disk drives are connected to the OMU, which controls the system disk units. Magneto-optical disk The BSC has a magneto-optical disk drive to facilitate temporary service operations, for example data pdates/downloads. In the BSC, a visual display unit is used as a user interface. With a VDU terminal, the user can perform normal operational functions with MML commands and diagnostic tests on various units. External alarm unit (optional) The optional external alarm unit (EXAU) provides a visual alarm of the fault indications of the BSC. The EXAU panel is located in the telecommunications site rooms, outside the network element. It is easy to install to all wall materials. Side Cable Conduit for raised floor installations (optional) If there is a raised floor in the equipment room, the incoming and outgoing cables can be placed under the floor and connected to the equipment racks through a vertical cable conduit, which can be placed at either side of the BSC3i cabinet.

5.11 Configurations, capacity and Performance of the BSC3i In the BSS, the BSC is a stand-alone network element that is connected to the surrounding network elements by means of standard PCM interfaces. The compact BSC3i is only available in one cabinet configuration. It can be equipped flexibly with a number of TRXs and trunk circuits. The Nokia BSC3i, High Capacity BSC can be equipped with: • A maximum of 660 TRXs • A maximum of 248 BTSs • a maximum of 248 BCFs .

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Capacity of the BSC3i With reference to the model below, the maximum processing capacity of a BSC3i is 3920 Erl/117000 BHCA, giving full support to 660 FR TRXs.

Circuit switched processing capacity of the BSC3i The above-mentioned traffic processing capacity of the BSC can be achieved with all S10.5-level features activated and with the new BSC3i hardware environment. The maximum traffic processing capacity of the BSC3i supports 660 full rate TRXs or 330 half rate TRXs. However, some future features might have an influence on the capacity of the BSC3i. Circuit switched data calls are taken into account in the reference model of call traffic in the following way: • one data call with one radio time slot is seen as one mobile originating or terminating call • one full rate data call is seen as one mobile originating or terminating call • one high speed circuit switch data (HSCSD) call with two or more radio time slots is seen as several calls, based on how many time slots are reserved for the call. For example, if two time slots are reserved (2*14,4 kbit/s) the call is seen as two mobile originating or terminating calls. Different types of connections are provided as follows:

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• a maximum of 256 PCM connections connected to the non-blocking switching matrix • 4 + 4 10/100-BaseT/X Ethernet connections are provided for the IP interfaces The GSM network has been originally specified and designed to use 64 kbit/s CCS7 links in the Ainterface. Due to various reasons, the CCS7 links can be congested in the A-interface. The situation can be improved with higher capacity CCS7 links on the A-interface between the BSC and MSC: • a maximum of 16 SS7 signalling links. Optionally wider 128 or 256 kbit/s signalling links can be configured instead of standard 64 kbit/s links. LAPD signalling links can be configured to 16, 32 or 64 kbit/s speeds. Introduction of HR will enable TRX configurations of more than 18 radio channels which leads to increased load in measurement reporting. Because of this, the capacity of a 16 kbit/s signalling link is not sufficient in all cases. Therefore it is recommended that also with HR the TRX configurations should be restricted to the maximum of 18 radio channels if 16 kbit/s signalling links are used. With TRX configurations of more than 18 radio channels a 32 kbit/s LAPD link is highly recommended for supporting the telecom signalling which half rate requires. The overload of the signalling link can be monitored by the Telecom LAPD link supervision with a possibility to set an alarm in an overload situation. In the BSC3i the packet handling capacity is provided by Packet Control Units (PCUs): • packet handling capacity and connectivity per each logical PCU is 64 BTSs, 128 TRXs, and 256 traffic channels (16 kbit/s) for GPRS/EDGE use • maximum capacity with full configuration (12 physical PCU units, 24 logical PCU units) is 256 x 24 = 6144 traffic channels (16kbit/s) per BSC3i for GPRS/EDGE use The packet handling capacity mentioned above can be achieved with the newHW environment. However, some future features might have an influence on the packet handling capacity of the BSC.

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5.12 BSC INTEGRATION In this phase, the MSC, transcoder and the BSC are configured to enable the MSC and the BSC to communicate properly with each other. The configuration can be made in many different ways depending on the equipment, or capacity. For example it is possible to implement only one PCM line between the MSC and the BSC or to use several separate PCM lines for redundancy or capacity purposes. When the TCSM2 is used, the A interface is always multiplexed. Provided that the BSC is equipped with an 8 Kbit group switch (GSWB), four separate A interface lines can be put into one highway PCM, making it possible to have up to maximum 120 full-rate speech channels in one highway PCM cable. In half-rate configuration the maximum number of speech channels is 210. The combination of half-rate and full-rate can be used as well . The A interface is defined in accordance with CCITT#7 Signalling System. Three protocol layers are used: the Message Transfer Part (MTP), the Signalling Connection Control Part (SCCP), and the BSS Application Part (BSSAP). The MTP's task is to provide a reliable means of data transmission. It consists of a signalling link, a signalling link set, and a signalling route set. The SCCP complements the services of the MTP by providing connectionless and connection-oriented network services. It consists of SCCP subsystems. The BSSAP uses the services of the MTP and the SCCP. It takes care of actual GSM/DCS-related interaction between the MSC and the BSS. Typical tasks of the BSSAP are call control, location updates, handover management, paging etc. It has no counterparts in terms of BSC MMI but is created along with the SCCP. Whith TCSM2, the time slots can be allocated more freely. However, the maximum efficiency is achieved if Nokia's recommendation is followed. Signalling channels, and possibly NMS connections, are always allocated beginning from the end of the frame. This optimises the number of the traffic channels available for the fourth tributary. This is due to the fact that only the time slots preceding the first signaling time slot can be used as speech channels in the fourth tributary. For example, if signaling links are allocated to time slots 31 and 27, only the time slots 25 and 26 are available for the fourth tributary - even if time slots 30, 29, and 28 are not used at all.

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5.12.1 GENARAL CGR: Circuit Group Name. The identifier used will be EFRDR Total Number of A I/f E1 PCMs: Depend on the design Number of SS7 links (64Kb/s): Depend on the design. Physical BSC PCM Port#: BSC-side physical PCM port number used by each Transcoder. The Nokia rule is to assign the ET ports dedicated to the connection to the MSC starting from the number 32 and carrying on with steps of 8 Ets. If it is necessary to assign other A interface Ets, it will be recommended to check the BSC configuration and to reserve ET which are not associated to the A-bis interface, starting from the last available. Logical PCM Nb: Starting at ZERO for each BSC. This index is used for the CIC numbering. The four number used to identify the PCM generated by an ET port are consecutive within the same transcoder card. Timeslot: Timeslots that filled with traffic channels. CIC: Circuit Identification Code. Combines the logical PCM number and the Timeslot SLC: Signalling Link code. Starting from zero. SS7 TS#: Timeslot in the signalling link where the SS7 info is carried OMC-R link TS#: Timeslot where the O&M information is carried

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The following table shows the configuration values of a standard implementation.

Phys ical BSC PCM Port # 32

40

Logi cal PCM Nb 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Voice/Data - SS7 - OMC-R Trunks detailed breakdown Voice/Data trunks SS7 link OMC-R link Timeslot # CIC # Timeslot CIC # # From To From To From To From To SLC TS # TS # 1 1 1 1 1 1 1 1

15 15 15 15 15 15 15 15

0-1 1-1 2-1 3-1 4-1 5-1 6-1 7-1 -

0-15 1-15 2-15 3-15 4-15 5-15 6-15 7-15 -

17 16 16 16 17 16 16 16

30 31 31 19 30 31 31 19

0-17 1-16 2-16 3-16 4-17 5-16 6-16 7-16 -

0-30 1-31 2-31 3-19 4-30 5-31 6-31 7-19 -

0

16

31

1

16

31

To explain the limitation of the fourth A interface, it is attached a scheme that contains the Nokia interface between BSC and Transcoder (A_ter). Here it is the configuration of the A_ter interface used for BSC integration carrying 110 traffic channels and one signaling links.

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It is obvious that there is no more space in the A_ter interface to allocate traffic channels. During the commissioning procedure of the transcoder, these channels will be mapped into the A interface. Network Indicator (NI): The type of network used is always NA1. BSC Point Code: Numeric identifier of the associated to the SP. MSC Point Code: Numeric identifier of the associated to the SP.

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6 Frequency Hopping 6.1 Internal Description of Frequency Hopping The radio interface of the GSM uses frequency hopping. Frequency hopping consists of changing the frequency used by a channel at regular intervals.

6.1.1 Properties of frequency hopping Frequency Hopping (FH), is a feature designed to the GSM to increase quality and capacity in an urban propagation environment. This is achieved by means of frequency diversity and interference diversity. Frequency Hopping in the GSM means that the frequency of a radio time slot (RTSL) is changing burst by burst. The frequency remains the same during a burst (0,577 ms). All dedicated channel types and their associated channel types (TCH /SACCH /FACCH , SDCCH/SACCH ) can hop. The Packet Broadcast Control Channel (PBCCH ) can also hop. A call hopping over 4 frequencies.

The exact frequency on a certain moment of time is defined as a function of frequency hopping parameters related to a cell and a logical Radio interface channel and the Radio interface absolute frame number. The frame number synchronises MS and BTS operation on a time basis. There are two main options for the implementation of frequency hopping; Baseband Hopping and Radio Frequency Hopping. Both of these are supported by Nokia's BSS. If the transceivers of the BTS are of the hopping synthesiser type you can choose the frequency hopping mode between non-hopping, RF hopping, and BB hopping.

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Baseband hopping management There are two different hopping groups used with baseband hopping in each BTS. When the Intelligent Frequency Hopping (IFH) feature is used, an additional hopping group 3 is in use. Group 1 All radio time slots (RTSL-0), except the BCCH time slot, on the BTS belong to Group 1. It is managed through FHS-1 in the BSC's database. HSN1 is related to this group. Group 2 All radio time slots (RTSL-1 to 7) on the BTS belong to Group 2. It is managed through FHS-2 in the BSC's database. HSN2 is related to this group. If baseband hopping is used on the BTS, all radio time slots belonging to the BTS are defined as hopping. The only exception to this is the BCCH time slot, which is always defined as non-hopping. In case there are dedicated signalling channels (SDCCH, CBCH ) on the BCCH time slot, they do not hop either. These hopping groups are maintained by adding and removing the Absolute Radio Frequency Channel Numbers (ARFN) of the TRXs used in frequency hopping in the BTS. Based on the available ARFNs in the BTS, the following hopping parameters, stored in the BSS Radio Network Configuration Database (BSDATA), are updated: " Mobile Allocation (MA) " Mobile Allocation Index Offset (MAIO) The ARFN of a TRX is always removed from the hopping systems when the user sets the TRX to the administrative state LOCKED. The ARFN is also removed from hopping if the system takes the TRX out of operational use due to a Carrier Unit (CU) or Transceiver Unit fault in the BTS. These operations cause clearing of calls in a BB hopping cell. The frequency hopping systems are configured and the parameters are implicitly defined by radio network configuration management in the BSC, when a BTS is created. Through the BSC MMI, you can define whether the BTS uses frequency hopping or not. If baseband hopping is used in the BTS, you have to give a hopping sequence number (HSN) for all the hopping groups in the BTS. If baseband hopping is used in the BTS, the modification of the administrative state or the ARFN of a TRX, and the creation or deletion of a TRX is possible only when a BTS has first been set in the administrative state LOCKED.

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Figure 4 TRX BTS using baseband hopping shows an example of a 4-TRX BTS using baseband hopping:

RF hopping management RF hopping can be used when the transceivers of the BTS are of the hopping synthesiser type. Then you can choose the frequency hopping mode between nonhopping, RF hopping, and BB hopping. When the transceivers of the BTS are of the conventional type (Nokia 2nd generation base stations), only non-hopping or BB hopping are possible. The Intelligent Frequency Hopping (IFH) feature introduces an additional hopping group The frequencies for a hopping cell are defined by attaching the cell to one of the mobile allocation frequency lists (MA-lists) defined by the operator. The system calculates the MAIOs, and the operator gives the HSN for the cell. The BCCH transceiver cannot hop, but it transmits a continuous BCCH frequency. Note that the MA used must contain at least as many frequencies as there are unlocked hopping transceivers in the BTS. The MA or the HSN can be changed only when the hopping cell is locked. You can create up to 255 mobile allocation frequency lists (MA-lists) and use them freely with different cells. One list can contain up to 63 frequencies. Changes in the transceiver configuration of an RF hopping cell affect only the transceiver that is being handled, other transceivers are not disturbed. The hopping mode can be changed when the BTS is locked. Nokia Talk family of base stations require that, in addition to the BTS being locked, the changes between hopping modes are done via changing the hopping mode first to non- hopping mode. If an alarm is received from the BCF indicating a fault that has an effect on frequency hopping, the BSC blocks the faulty object.

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Figure 3-TRX BTS using RF hopping shows an example of a 3-TRX BTS using RF hopping.

Freeform RF Hopping Adjacent frequencies cannot be used in the mobile allocation frequency list since it would cause adjacent channel interference within the cell and also between the cells in the same BTS site. However, in order to fully benefit from the very tight reuse factor and fractional loading in the hopping network offered by the use of RF hopping, the operator needs means to share the same frequency sets between adjacent sectors of a BTS site. At the same time channel collisions and adjacent channel interference have to be avoided. Thus the operator needs to be able to define all the frequency hopping parameters, including MAIOs. The operator is provided with two parameters for MAIO manipulation to use RF hopping more flexibly and more efficiently: " User defined parameter to set the starting point for allocation of the MAIOs per cell, i.e., the lowest MAIO in a cell can be bigger than zero. " User defined parameter to allow discontinuous MAIO numbering to be used in a cell, for example, MAIOs 0,2,4,6. Hopping management with common BCCH Hopping groups in BB and RF hopping segments

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The multiband network will support frequency hopping within each band of operation. Frequency hopping between the bands of operation is not supported. In the segment, the resources of different types are grouped as separate BTSs. All the resource types have their own hopping parameters and hopping groups. Figure Hopping groups in a segment gives an example of the different hopping groups of a segment in the Common BCCH Control feature. The segment architecture enables the network to have BTSs without a BCCH RX. This reduces the amount of hopping groups in the regular area of a BTS due to no need for a separate group for the BCCH TRX in RF hopping. In the case of BB hopping, TSL0 and other TSLs are separated and these are seen as two different hopping groups.

6.2 Parameters of frequency hopping The BCF type (TYPE) , the hopping mode of the cell (HOP/UHOP) , the Mobile Allocation Frequency Lists (MA) band and frequencies, the identification number of the MA list attached to the cell (MAL/UMAL) , the initial frequencies of the transceivers (FREQ) , the cell based Hopping Sequence Numbers (HSN1, HSN2 and UHSN) , MAIO Offset (MO/UMO) , and MAIO Step (MS/UMS) are defined with the MML commands of the BSC or by Nokia NetAct through the Q1 interface. On the Abis O&M interface, the hopping mode and the MA, MAIOs, and HSNs are sent to the BCF when it is configured. If the BCF does not accept the configuration and sends a negative acknowledgement, an alarm is set. Even so, the data is stored in the BSC database until the user modifies it again. With the MAIO Offset parameter you can set the lowest MAIO value per sector to other than 0. This makes it is possible to use the same MA list for two or more sectors in the site without collisions. However, because the sectors controlled by one BCF operate in frame synchronisation, the HSN values must be equal between the sectors in order to each frequency to be used only once during one frame period. If the HSN values differ between the sectors, collisions will occur regularly. Using the MAIO Offset parameter requires that the sectors operate in frame synchronisation. With the MAIO Step parameter you can use successive channel numbers within a single cell.

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When both MAIO Offset and MAIO Step parameters are used, even re-use 1/ 1 is possible in a 3-sector BTS site, without any co-channel collisions or detrimental interference from adjacent frequency channels. Note that you have to make sure that the MA-list is long enough. For example, if you have a 3-sector site, 4 TRXs in each, and you want to share a single MA-list between all of those with 400 kHz minimum channel separation, you have to reserve at least 18 frequencies for the MA-list plus 3 BCCH frequencies, that is 21 frequencies altogether.

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6.3 Restrictions to frequency hopping There are some restrictions concerning the cell allocation frequencies on the Abis and Radio interface messages of the GSM1800/GSM1900 system. The following allocation coding methods can be used: "variable bit map", "256 range" and "512 range". Only one hopping mode (BB or RF) can be active on a BTS at a time, or if the site type is Nokia Talk-family, on a BCF at a time. Cyclic and random hopping are not allowed simultaneously on one Nokia 2nd generation base station site. A floating TRX cannot be used in BB hopping. The values of the Training Seqency Codes (TSCs) of all the TRXs of a BB hopping layer must be equal. If the BCCH/PBCCH TRX's frequency is used in a BB hopping layer, then the TSCs of all TRXs of that BB hopping layer must be the same as the BTS colour code (BCC).

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6.4 User interface of frequency hopping 6.4.1 Creation of BB hopping BTS A Baseband Frequency Hopping BTS is created in a similar way to any BTS; 1. First the BCF is created. In this phase the BTS generation is given to the system, some checks later on are based on this information (for example RF hopping is not possible with Nokia 2nd generation base stations). 2. When the BTS is created, hopping related parameters of the BTS can be set. 3. The rest of the parameters and procedures are similar to the parameterization of a non-hopping BTS. 4. The hopping configuration parameters are transferred from the BSC to the BTS site when the user unlocks the BTS. The BSC sends a configuration message via the Abis O&M link to the BTS including CA, MA, MAIO , HSN, ARFN , and hopping mode. The BCF (BTS-OMU) of the site then re-formats the hopping related parameters and forwards them to the hopping control units of the BTS 5. When the BTS is up and running, the hopping system is in place and working even though no signal (except BCCH) is transferred. The call- related hopping parameters (MA, MAIO etc.) over the Abis telecom interface are not configuring the BTS any more, they go to the MSs.

6.4.2 Creation of RF hopping BTS The creation of an RF hopping BTS has MA-handling as an extra step; 1. First the BCF is created. In this phase the BTS generation is given to the system, some checks later on are based on this information (for example, RF hopping is not possible with 2nd generation BTS). 2. Before RF hopping of the BTS can be fully defined, the MA-lists must be defined at the BSC-level. 3. When the BTS is created, the hopping related parameters of the BTS can be set. The BTS must also be attached to an MA-list. 4. steps 3) , 4) and 5 ) in Creation of BB hopping BTS.

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Mobile Allocation Frequency List With the commands of the BCCH and Mobile Allocation Frequency List and RA Handling command group you can create, modify, remove, and display mobile allocation frequency list objects in the BSS Radio Network Configuration Database. With the commands of the Base Transceiver Station Handling command group you attach any of these lists to a BTS Creation of mobile allocation frequency list With the CREATE MOBILE ALLOCATION FREQUENCY LIST command you can create a mobile allocation frequency list in the BSS Radio Network Configuration database. The mobile allocation frequency list is identified by a number. The frequency band type of the mobile allocation frequency list is an obligatory parameter. Deletion of mobile allocation frequency list With the DELETE MOBILE ALLOCATION FREQUENCY LIST command you can remove a mobile allocation frequency list from the BSS Radio Network Configuration Database. The list identification has to be given as a parameter. You cannot remove the MA-list if it is still used by a BTS. Modifying mobile allocation frequency list With the MODIFY MOBILE ALLOCATION FREQUENCY LIST command you can add or remove frequencies to/from mobile allocation frequency lists. Interrogation of mobile allocation frequency list data With the OUTPUT MOBILE ALLOCATION FREQUENCY LIST command you can display the mobile allocation frequency list data stored in the BSS Radio Network Configuration database. The list identification can be given as a parameter. If the lists are used by some BTSs, the numbers of the related BTSs are printed out. If you do not identify the list, all the mobile allocation frequency lists are displayed.

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6.5 Frequency Hopping Implementation Description When frequency hopping is used, the Basic Call procedure differs from the non- hopping case only in the channel assignment phase. The radio resource management program block (RRMPRB) in the BSC selects a free logical dedicated channel in a hopping BTS in the same way as in the non- hopping BTS. The channel selection procedure is influenced by the idle channel interference levels measured by the base station. In a hopping BTS the idle channel interference measurements are done in all frequencies included in the Mobile Allocation list. If hopping is not used, the radio interface channel can be defined with ARFN , Radio Time slot number and subchannel number. If hopping is used the ARFN is replaced with MA, MAIO and HSN. The MA determines which frequencies from the Cell Allocation are used in the hopping sequence. After the selection of a dedicated channel the BSC determines the required MAIO, HSN and MA for the selected channel and sends them to the mobile station in the IMMEDIATE_ASSIGNMENT or ASSIGNMENT message. The MS reads the Cell Allocation from the System Information Messages which are broadcast on the BCCH and, in the case of GPRS , from the Packet System Information Meassages which are broadcast on the PBCCH . The base station has received the used MA, MAIO and HSN in the BTS configuration phase via O&M signalling link. Example: Baseband Hopping Four consecutive calls are made in a cell with four TRXs. Baseband hopping is used in the cell. For all calls, the BSC allocates the RTSL-2 in different TRXs. Figure Example of baseband hopping illustrates how the bursts are organised in the air interface. In this case only RTSL-2 is observed.

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Example: RF Hopping RF Hopping on TRX-2 in a two TRX cell. The first call is made on RTSL-2 of the non-hopping BCCH TRX and the second call is made on RTSL 2 ot the RF Hopping TRX. Figure Example of RF hopping illustrates how the bursts are organised in the air interface. In this case only RTSL-2 is observed.

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6.5.1 Fault Management of Frequency Hopping/recovery examples The following three examples show the recovery of a hopping BTS.

FU fault is the trigger in this example (Frame Unit (FU) and Carrier Unit (CU) are functional parts of Nokia 2nd generation base stations). The fault can be internal (for example a FU hardware) or external (for example losing a LAPD -link of the TRX). This fault type and recovery mechanism is not applicable for later BTS generations due to more integrated hardware. 1) The BTS alarms the BSC, or the BSC detects a non-functional LAPD. TRX-2 is suspected to be faulty. 2) The BSC clears all the calls which are allocated to those Abis circuits corresponding to TRX-2. Calls on TRXs 1, 3 and 4 remain untouched. 3) The BSC blocks TRX-2 in order not to allow new traffic for Abis circuits related to it. In this fault type the BSC can assume that all the Carrier Units (CU) are functioning properly, so the hopping parameters can be left untouched. MSs handled by TRXs 1, 3, and 4 in this example can still hop over all 4 frequencies.

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Initial configuration: as in Figure Initial configuration of baseband hopping BTS The fault in this example is on the BCCH Carrier Unit. The fault is such that the CU of TRX-1 or the Transceiver Unit of TRX-1 does not work anymore. 1. The BTS alarms the BSC. TRX-1 is suspected to be faulty. 2. The BSC blocks all the TRXs of the cell for a while. This causes clearing of all ongoing calls of the cell. Frequency redefinition procedure is not used because: " in fault cases operation should be as straightforward as possible, no complex procedures should be triggered " it is not certain, especially in case of smaller configurations, that any messages can be carried over the Radio interface reliably once one frequency (or more) is missing from a hopping group " it is not certain whether the MSs would still use the BTS concerned at the time of frequency redefinition. 3. The BSC calculates new hopping parameters. 4. The BSC deblocks TRXs 2, 3 and 4. New hopping and TRX parameters are transferred to the BTS. TRX-2 is restarted with BCCH on RTSL-0 , and hopping is reconfigured. 5. The BSC allows new traffic for TRXs 2, 3 and 4.

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TRX fault, RF hopping BTS

Initial configuration of RF hopping BTS.

1. The BTS alarms the BSC, or the BSC detects a non-functional LAPD. TRX-2 is suspected to be faulty. 2. The BSC clears all the calls which are allocated to those Abis circuits corresponding to TRX-2. Calls on TRX-1 remain untouched. 3. The BSC blocks TRX-2 in order not to allow new traffic for Abis circuits related to it.

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6.6 BTS Implementation of Frequency Hopping The Nokia 2nd generation base stations' implementation is baseband hopping only. RF hopping is not supported now nor in the future. All carrier units have fixed frequencies. The hopping function is realised by multiplexing baseband digital bit streams between Frame Unit and Carrier Units. Physically, these bit streams are carried over a 10 Mbit/s serial two-way link (FHI-link, Frequency Hopping Interface link). All links are fed through a special Frequency Hopping Unit (FQHU) that takes care of the actual multiplexing and hopping law calculation. FigureFunctional units for frequency hopping in 2nd gen. BTS clarifies the set-up.

Functional units for frequency hopping in 2nd gen. BTS. The FQHU itself is basically a multiplexing device capable of multiplexing 12 simultaneous serial links. FQHU has two processors, one general purpose processor for Q1-bus interface and housekeeping functions and one DSP dedicated for frequency hopping algorithm calculation. The switching matrix is a passive device into which the DSP loads (with the assistance of FIFO & associated clocking circuitry) required switch settings twice during a time slot. The first load is used to control data transfer from FU to CU, second load is for the CU->FU transfer. Figure Loading switch settings illustrates this further.

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Loading switch settings. The hopping unit is common for a BTS site, the sectors of a BTS use the same hopping unit. The data transmitted between FU and CU is digital, in downlink direction the modulating bits are transmitted to CU for actual modulation, upconversion and power amplification. In uplink direction the data consists of digital samples of the baseband Iand Q-signals. The FHI-link can be duplicated for reliability or because of diversity. If diversity is not used, the other link will act as a hot redundancy, which means that it is immediately and automatically taken into use if the primary link fails. When diversity reception is used, the other link is used for carrying the signal from the diversity receiver. When two links are employed, a second FQHU is also required.

Figure Data flow in the diversity case shows the logical flow of data and does not represent a physical link structure. On downlink the redundancy continues. The same data is transmitted through both links. On uplink one link carries data from the primary receiver and another carries data from the diversity receiver.

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When redundancy is used the same data is transmitted through both links. FQHU is capable of supporting a maximum of 15 hopping groups at a time. This is sufficient as in a three sector IFH configuration the number of hopping groups used is 12. Both random and cyclic hopping modes are supported but not simultaneously. This means that the whole site (= all sectors) must be using either cyclic hopping or random hopping. With random hopping the HSN can be selected freely. Then all hopping groups can have a different HSN at the same time. The number of CUs must equal the number of FUs, the maximum number of TRXs supported is 12. The specification requires that transmission and modulation on the BCCH is continuous and no power control takes place for dedicated channels located on the BCCH carrier. This is ensured in the following way. In the serial data from FU to CU there is control information for transmit power level, the transmitter on/off bit and the BCCH-bit. The DSP processing a BCCH time slot sets the BCCH-info bit and also includes the BCCH power level. The carrier unit that receives a data burst where the BCCH-bit is set forces the transmitter to operate on the power level indicated in the burst for 8 consecutive time slots. Power level information and the power on/off-bit are ignored during those 8 time slots thus ensuring that the power level set for the BCCH is maintained constantly. All FUs generate constantly valid transmit bursts towards CU. If a channel is active, these bursts contain traffic data, such as speech or signalling. When a channel is idle the FUs keep on generating dummy bursts but the transmitter on/ off bit is set to off-position. Due to baseband hopping, these bursts are directed to all CUs in turn. When this burst is directed to a CU transmitting the BCCH, this bit (and the power level) is ignored and the data bits are used to modulate the carrier. In the following example there are two TRXs using baseband hopping and the cyclic algorithm. The BCCH time slot is not hopping, thus TSL-0 of the TCH- TRX cannot hop either. The remaining time slots 1-7 are hopping. The resulting data stream is transmitted next to the CUs. The BCCH time slot ensures that power remains constant in every time slot. When no calls are active, a dummy burst is transmitted in idle time slots, here denoted with `I' and `i'. On the BCCH, the transmitter is active all the time since the BCCH time slot is repeated in every eighth time slot. On the TCH carrier, transmission takes place only when data of an active call is routed through it. In the following example the transmitter is active in time slots 6 and 4 on even and odd frames, respectively.

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For FU failure cases the FQHU also has a dummy burst generation logic, which makes it possible to replace the signal from a FU with an FQHU-generated dummy burst. This is necessary after a FU failure when the FU itself cannot generate these bursts. The OMU controls this feature, whenever it detects a situation where a FU is incapable of producing a dummy burst for idle channels, it commands the FQHU to fill in for this FU. When the FU is back in service, the fill-in is discontinued. The dummy burst generation logic is common to all FUs, and one or all FUs can be replaced simultaneously.

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7 Maintenance

7.1 Changing plug in units of BSC: The first thing that we have to do in this kind of operations is making Fallback of running package before the changes affecting any unit. After the creation of the fallback package, the state of the concerned unit must be checked with ZUSI command. For example, to change AS7 plug in unit in BCSU n° 7. The state of BSCUs could be checked by ZUSI command. ZUSI:BCSU; EXECUTION STARTED DX 200

BSC26RJT

2006-08-03 01:26:56

WORKING STATE OF UNITS UNIT MB STATE LOCATION BCSU-0 38 SP-EX BCSU-1 31 WO-EX BCSU-2 32 WO-EX BCSU-3 33 WO-EX BCSU-4 34 WO-EX BCSU-5 35 WO-EX BCSU-6 36 WO-EX TOTAL OF 07

INFO IDLE -

UNITS

COMMAND EXECUTED WORKING STATE AND RESTART HANDLING COMMAND In the normal working state of Nokia BSC, 06 BCSUs must be in WO-EX (workingexecuted) state and the 7th must be in SP-EX state to guarantee the redundancy. There are 5 permanents states for computer units and the changing of unit state should be done as shown in the figure using ZUSC command :

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WO-EX

Working Executed

SP-EX

Spare Executed

TE-EX

Test Executed

SE-OU

Separated Out of Use

SE-NH

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There are 2 temporary states (Non permanents) : WO-RE and SP-RE. which show the restart process of the units. In this case, we can’t make any change. We have to wait until the stabilisation of the unit. The state of BCSU 7 is WO-EX, we have to change it to SE-NH with ZUSC command : (WO-EX SP-EX TE-EX SE-OU SE-NH). ZUSC : BCSU, 6 :SP ; EXECUTION STARTED STATE TRANSITION EXECUTED UNIT = BCSU-7 NEW STATE = SP-RE NEW ACTIVE UNIT = BCSU-0 COMMAND EXECUTED WORKING STATE AND RESTART HANDLING COMMAND After few seconds, we can check the BCSU states with ZUSI command : ZUSI:BCSU; EXECUTION STARTED DX 200

BSC26RJT

2006-08-03 01:28:58

WORKING STATE OF UNITS UNIT MB STATE LOCATION BCSU-0 30 WO-EX BCSU-1 31 WO-EX BCSU-2 32 WO-EX BCSU-3 33 WO-EX BCSU-4 34 WO-EX BCSU-5 35 WO-EX BCSU-6 37 SP-EX

INFO IDLE

The same operation must be done to change BCSU state to TE, SE-OU and SE-NH :

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ZUSC:BCSU,6 /* IDENTIFY POSSIBLE NEW WORKING STATE: ACTUAL WORKING STATE AND INFO IS: UNIT MB STATE LOCATION INFO BCSU-6 37 SP-EX IDLE FOLLOWING NEW STATES ARE DEFINED: WO WORKING TE TEST */ ZUSC:BCSU,6:TE; EXECUTION STARTED STATE TRANSITION EXECUTED UNIT = BCSU-6 NEW STATE = TE-EX COMMAND EXECUTED WORKING STATE AND RESTART HANDLING COMMAND ZUSC:BCSU,6 /* IDENTIFY POSSIBLE NEW WORKING STATE: ACTUAL WORKING STATE AND INFO IS: UNIT STATE LOCATION INFO BCSU-6 TE-EX FOLLOWING NEW STATES ARE DEFINED: WO WORKING SP SPARE SE SEPARATED */ ZUSC:BCSU,6:SE; EXECUTION STARTED STATE TRANSITION EXECUTED DDU

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UNIT = BCSU-6

NEW STATE = SE-OU

COMMAND EXECUTED WORKING STATE AND RESTART HANDLING COMMAND < ZUSC:BCSU,6:SE; LOADING PROGRAM VERSION 18.4-0 EXECUTION STARTED STATE TRANSITION EXECUTED UNIT = BCSU-6 NEW STATE = SE-NH COMMAND EXECUTED WORKING STATE AND RESTART HANDLING COMMAND After few seconds, we can check the BCSU states with ZUSI command : ZUSI:BCSU; EXECUTION STARTED DX 200

BSC26RJT

2006-08-03 02:4:08

WORKING STATE OF UNITS UNIT MB STATE LOCATION BCSU-0 30 WO-EX BCSU-1 31 WO-EX BCSU-2 32 WO-EX BCSU-3 33 WO-EX BCSU-4 34 WO-EX BCSU-5 35 WO-EX BCSU-6 37 SE-NH TOTAL OF 7 UNITS INCORRECT STATES

INFO

1

COMMAND EXECUTED WORKING STATE AND RESTART HANDLING COMMAND

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Now, the state of BCSU 6 is SE-NH. To change the plug in unit, we have to switch off the power of the BCSU 6 (PSC), After the change of the PIU, we have to hand the BCSU to its normal state (SE-NH SE-OU TE-EX SP-EX WO-EX) with ZUSC command.

ZUSI:BCSU; EXECUTION STARTED DX 200

BSC26RJT

2006-08-03 02:9:50

WORKING STATE OF UNITS UNIT MB STATE LOCATION BCSU-0 38 SP-EX BCSU-1 31 WO-EX BCSU-2 32 WO-EX BCSU-3 33 WO-EX BCSU-4 34 WO-EX BCSU-5 35 WO-EX BCSU-6 36 WO-EX TOTAL OF 7

INFO IDLE -

UNITS

COMMAND EXECUTED WORKING STATE AND RESTART HANDLING COMMAND >

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7.2 TRX TEST LOOP The main Purpose of the TRX test loop is to verify that the TRX is working correctly and that it’s not damaged. The principle of TRX test loop is to test the functionality of TRX unit (TX, RX and Diversity). Test signal simulates the real uplink downlink communication between BTS and MS during a phone call. The TRX test loop can not be done if the Base Band (BB) hopping mode is activated. We are going to make TRX test loop for TRX 7 of BTS n° 26. The first thing to do is to deactivate the base band hopping mode after locking the concerned BTS : * TO LOCK the BTS with forcing handovers, we have to use ZEQS command : ZEQS:BTS=26:L:FHO,25; in this, we gived 25s for forcing Handovers to the adjacent cells. * To CHANGE HOPPING MODE state, we have to use ZEQE command : ZEQE:BTS=26:HOP=N; After the deactivation of the hopping mode, we have to UNLOCK the BTS with ZEQS command : ZEQS:BTS=26:U; After the restart of the BTS, the TRX test loop can be started with ZUBS command : ZUBS:BTS=26,TRX=7; After few seconds, the TRX test loop report can be monitored with ZUBP command : ZUBP:TR:BTS=26,TRX=7; The result of the TRX test loop looks like : DX 200 BSC26RJT 2006-09-03 8:35:12 RADIO NETWORK TEST REPORT TRX TEST BCF-025 BTS-026 ZMDA001B TX_RTSL-000 RX_RTSL-005

TRX-007

TEST RESULT: PASSED TEST EXECUTED: 2006-09-08 8:35:12 TRANSMITTED POWER......................... 47 DBM DDU

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MAIN RX: BIT ERROR RATIO .....................(BER).. 0.0615 % RX SENSITIVITY..................................... -106 DBM DIVERSITY RX: BIT ERROR RATIO .....................(BER).. 0.0000 % RX SENSITIVITY..................................... -103 DBM END OF REPORT COMMAND EXECUTED SINGLE RADIO NETWORK TEST HANDLING COMMAND

* In Ultrasite product, if the value of the Bit Error Ratio (BER) exceeds 2%, the TRX is damaged and must be changed. * The value of RX sensitivity could not be less than -115 dbm for main RX and Diversity. * Test fails if the Transmitted Power value is outside + 4 dB of the nominal power value (for Ultrasite TRX, it must be 46,5 + 4 dbm). After the test loop, we have to turn up to the initial configuration. The hopping mode must be activated using ZEQE command : 1/ First, we have to LOCK the BTS with ZEQS command : ZEQS:BTS=26:L:FHO,25; 2/ We have to activate the hopping mode with ZEQE command : ZEQE:BTS=26:HOP=RF; 3/ Then, the BTS must be unlocked with ZEQS command : ZEQS:BTS=26:U;

7.2.1 Lock/Unlock of BCF, BTS or TRX. If a problem occurs on a network element (BCF, BTS or TRX), the first thing to do is to restart this network element by doing LOCK and UNLOCK. We have to remember to unlock every network element that we locked before (BCF, BTS or TRX).

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To lock or unlock a TRX, the used command is ZERS : * ZERS:BTS=26, TRX=7:L; Lock TRX 7 under BTS 26. * ZERS:NAME=ZHA014B,TRX=7:U; Unlock the TRX 7 under the cell ZMDA001B. * ZERS:BTS=26, TRX=7&8:L; Lock the TRX 7 and 8 under BTS 26. * ZERS:NAME=ZHA014B, TRX=7&&9:L:FHO,25; Lock the TRX 7,8 and 9 under the cell ZMDA001B with forced Handovers in 25s. Nokia recommends always to make a lock of BTS with forced handovers (FHO) to avoid the drop of calls which are in the concerned BTS. To lock or unlock a BTS (Cell or Sector), the used command is ZEQS : * ZEQS:BTS=26:L; Lock the BTS n°26. * ZEQS:NAME=ZMDA001B:U; Unlock the cell ZMDA001B. * ZEQS;BTS=26:L:FHO,25; Lock the BTS 26 with forced handovers in 25s. * ZEQS:NAME=ZMDA001B:L:FHO, 25; Lock the BTS ZMDA001B with forced handover in 25 s To lock or unlock a BCF, the used command is ZEFS : * ZEFS:25:L; Lock the BCF n°25 or site ZMDA001. * ZEFS:25:U; Unlock the BCF n°25. We can never give the SITE NAME as parameter in ZEFS command. The execution printout of ZEFS command is: BSC BSC26RJT

2006-09-03 08:54:32

BASE CONTROL FUNCTION BCF-25 STATE LOCKED COMMAND EXECUTED

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7.3 UNIT DIAGNOSTIC : The tasks of the diagnostic is, firstly, to locate hardware failures in the BSC units, with an accuracy equivalent to the maintenance requirements, and secondly, to verify that the hardware is functioning properly. If errors occur, the system generally informs you of the necessary actions by producing a text diagnosis with fixed headers. The software automatically detects failures and produces a diagnostic report or an alarm printout on them. To repair the fault, you must replace the plug-in unit that the report shows to be faulty or you must determine the location of the failure by making diagnostics. The diagnostic work done by the diagnostic system is directed to the functional units of the system (total diagnoses) or to functional entities of the system as seen from the point of view of diagnostics (partial diagnoses). A total diagnosis is divided into partial diagnoses. If a fault is located, the faulty flag (FLTY) of the unit occurs (if the system has set it on earlier) and the unit state changes to the TE-EX state. Before starting unit diagnostic, we have to make sure that the concerned unit is in TE-EX state. To start unit diagnostic, the command used is ZUDU. For example, we are going to make diagnostic to BCSU 6 : ZUDU:BCSU,6 /* IDENTIFY TEST FOR UNIT: TEST TOTAL POWER YES PROC NO CPU YES RAM YES ETHER NO SYSB YES PCUC NO PCUD NO PCUL NO PCUT YES AS7 YES

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BCSU PLUG-IN UNITS MBIF_UA CP6LX CP6LX CP6LX CP6LX MBIF_UA CP6LX PCU PCU PCU PCU AS7_V

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OMITTED PARAMETER REFERS TO TOTAL TEST OF UNIT

*/

ZUDU:BCSU,6; *************************************** DIAGNOSIS JOB ACTIVATED UNIT = BCSU-6 TEST=TOTAL COMMAND EXECUTED DIAGNOSTICS HANDLING COMMAND To Interrogate about the currently active diagnostics, the command used is ZUDQ : < ZUDQ; LOADING PROGRAM VERSION 4.3-0 CURRENT DIAGNOSTIC TESTS: UNIT BCSU-6

TEST CPU

INITIAL TIME 9:00:28

COMMAND EXECUTED DIAGNOSTICS HANDLING COMMAND After the execution of Diagnostic, the Diagnostic Report can be displayed using ZUDH command : ZUDH:BCSU,6; DX 200

BSC26RJT

2006-09-03 9:05:32

DIAGNOSTIC REPORT HISTORY UNIT = BCSU-6 REPORT-CLASS = ALL 09:05:32 DX 200

BSC26RJT

PARTIAL DIAGNOSIS EXECUTED DX 200

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BSC26RJT

DATE = 2006-09-03 TIME =

2006-09-03 BCSU-6

9:05:32 POWER

2006-09-03 9:05:32

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PARTIAL DIAGNOSIS EXECUTED DX 200

BSC26RJT

BSC26RJT

PARTIAL DIAGNOSIS EXECUTED BSC26RJT

RAM

BCSU-6 SYSB 2006-09-03 9:05:32 BCSU-6

PCUT

2006-09-03 9:05:32

PARTIAL DIAGNOSIS EXECUTED DX 200

BCSU-6

2006-09-03 9:05:32

PARTIAL DIAGNOSIS EXECUTED DX 200 BSC26RJT

DX 200

CPU

2006-09-03 9:05:32

PARTIAL DIAGNOSIS EXECUTED DX 200

BCSU-6

BSC26RJT

BCSU-6

AS7

2006-09-03 9:05:32

DIAGNOSTIC REPORT BCSU-6 PARTIAL DIAGNOSIS DIAGNOSTIC PROGRAM DIAGNOSIS

TOTAL 0000 3999

3999 TOTAL DIAGNOSIS EXECUTED - UNIT OK END OF REPORT END OF DIAGNOSTIC REPORT HISTORY The unit is functioning properly when the total diagnostic is executed and “UNIT OK” field occurs in the end of the report. If the system detects a fault during diagnostic, the diagnostic execution is stopped and the affected fault is shown in the Diagnostic Report. The position of the faulty plug in unit is shown in the Diagnostic Report. This plug in unit must be changed.

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8 Introduction Of Transcoder In the case of the air interface, the media carrying the traffic is a radio frequency, but, in the PSTN originated call, the traffic signal is also carried through fixed networks. To enable the efficient transmission of the digital speech information over the radio Air Interface the digital speech signal is compressed. For transmission over the air interface, the speech signal is compressed by the mobile station to 13Kbits/s (Full Rate) or 5.6Kbits/s (Half Rate). This compression algorithm is known as "Regular Pulse Excitation with Long Term Prediction" (RPE-LTP). However, the standard bit rate for speech in the PSTN is 64Kbits/s Therefore, a converter has to be provided in the network to change the bit rate from one to another. This is called the Transcoder (TC). If the TC is located as close as possible to the MSC with standard PCM lines connecting the network elements, we can, in theory, multiplex four traffic channels in one PCM channel. This increases the efficiency of the PCM lines. But when connecting to the MSC, the multiplexed lines have to be de - multiplexed. In this case the unit is called Transcoder and Submultiplexer (TCSM). In the Nokia solution, the submultiplexing and the transcoding function is combined in one equipment which is called a TCSM2E.

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8.1 Purpose of TCSM2 The second generation Transcoder and Submultiplexer (TCSM2) is used in the GSM900/GSM1800/GSM1900 digital cellular network to provide transcoding function for traffic channels. This function is located in the Base Station Subsystem (BSS). The TCSM2 is used together with the Nokia DX 200 BSC and Nokia BTS (Base Transceiver Station). TCSM2 units are functional units of the BSC, they can be physically located either on the BSC or MSC site. The major telecommunications interfaces of the TCSM2 are the A-interface towards the MSC and the Ater-interface towards the BSC. The capacity of a single TCSM2 is up to seven digital (MSC-side) trunks. The number of TCSM2 units serving a particular BSC is selected on the basis of the capacity of the BSC. One individual block of the TCSM2 is called a Transcoding and Rate Adaptation Unit (TRAU). The TRAU function essentially includes converting the 64 kbit/s traffic channels arriving from the MSC into channels of 8 kbit/s, 16 kbit/s, 32 kbit/s or 64 kbit/s rate and further multiplexing these channels to fit in the timeslots of the trunk towards the BSC. The other direction (BSC to MSC) works according to the same principle (in reverse). Telecommunications functions of the TCSM2 are listed below: • • • • •

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transcoding and rate adaptation of traffic channels carried between the BTS and the TRAU submultiplexing of 8, 16, 32 or 64 kbit/s capacity TRAU frame channels onto 64 kbit/s timeslots through-connection of selected entire timeslots providing the interface functions for the trunks receiving the clock synchronisation from the MSC direction trunks and being part of the synchronisation chain extending down to the BTSs

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8.2 Operating environment The TCSM2 is connected with 2048 kbit/s trunks to the MSC (A-interface) as well as the BSC (Ater-interface). The figure below shows the operating environment of the TCSM2

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Interface to the MSC The interface to the MSC consists of one or more digital 2048 kbit/s trunks. The synchronisation for the TCSM2 is extracted from a trunk signal from the MSC. The gross bit rate of the traffic channel is 64 kbit/s. Signalling related to call-handling is provided by Common Channel Signalling (CCS) at a bit rate of 64 kbit/s. Interface to the BSC The interface to the BSC is a digital 2048 kbit/s trunk. The bit rate of a traffic channel is either 8, 16, 32 or 64 kbit/s. Timeslot Allocations The routing of signals between the BSC-side and MSC-side trunk interfaces is controlled by the chosen timeslot allocation. Different allocations can be programmed and loaded into the TCSM2. The selected allocation type must also be supported by the BSC. Different TCSM2s of the same BSS may use different allocations. Each Ater trunk may carry different channels. Each of the A-interface PCMs can be individually programmed to the appropriate circuit type. The maximum number of Ainterface PCMs supported by a TCSM2 is dependent on the circuit types. For Full Rate (FR) traffic use, the TCSM2 uses an allocation with 16 kbit/s traffic channels. Half Rate (HR) traffic uses either an allocation with 16 kbit/s or an allocation with 8 kbit/s traffic channels. High Speed Circuit Switched Data (HSCSD) connections consist of one to four 16 kbit/s streams over the Ater interface. The TCSM2 is capable of using mixed allocations, with the BSC-side trunk capacity divided into separate 16 kbit/s and 8 kbit/s portions of channels. A number of 64 kbit/s timeslots can be configured to be through-connected transparently (Common Channel Signalling, O&M connection, etc.) between one of the MSC-side lines and the BSC-side line. TRAU interface A single TCSM2 can operate in different transcoding function modes, depending on the DSP program variant. The basic program performs FR transcoding (13 kbit/s speech channel / 16 kbit/s TRAU frame) and HR transcoding (5.6 kbit/s speech channel / 8 kbit/s TRAU frame). Different transcoding programs can coexist in the DSP memory, and depending on the transcoding type they are invoked either by command or automatically based on the received TRAU frame type.

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8.3 Functional structure 8.3.1 Block structure Each TCSM2 unit of a particular BSC is independent of the others, even if located in the same rack. The block diagram of the TCSM2 unit is presented in the figure. The unit consists of four main blocks: • • • •

the Transcoder Controller plug-in unit TRCO a number (maximum 14) of Transcoder plug-in units TR16-S the Power Supply Converter plug-in unit PSC1 that supplies the +5V and -5V operating voltages to the TRCO and TR16-Ss the Exchange Terminal plug-in units ET2E

The TRCO incorporates a microcomputer that controls and supervises the operation of the TCSM2. The TRAU functions are performed by 16 DSPs in the TR16-S unit. The TCSM2 has up to seven trunk interfaces towards the MSC and one trunk interface towards the BSC. The functions related to the line interfaces are handled in the ET2E plug-in unit (two trunk interfaces per unit).

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8.4 Operation of TCSM2 8.4.1 Plug-in units Transcoder Controller plug-in unit (TRCO) The TRCO plug-in unit controls the overall TCSM2 functions. It provides a path for the trunk signals to the ET2E and TR16-S plug-in units. These internal trunk signals are called Transcoder Interface (TRI) and Line Interface (LI) signals. They have a bit rate of 4096 kbit/s and fill the time slots of two PCM trunks. The BSC controls the TCSM2 unit over the LAPD Operation and Maintenance (O&M) link. A local session is possible by a terminal attached to the front panel of the TRCO. The TRCO includes five LAPD functions, four of which control the ET2E plug- in units and one of which communicates with the BSC. The TRCO incorporates an 80186 processor, with the proprietary Pectus operating system and FLASH, PROM and RAM. Exchange terminal plug-in unit, ET2E The ET2E plug-in unit provides the interface functions for the two trunks. The applicable standard for the physical characteristics is G.703. The interface is a 120 & balanced (ET2E, ET2E-S) or a 75 & coaxial (ET2E-C, ET2E- SC) interface. The interfaces of the first of the ET2Es are shared, so that the first trunk interface takes care of the BSC direction trunk and the second takes care of the first MSC direction trunk. The ET2E incorporates an 80188 processor with the proprietary Pectus operating system. This processor is the common control unit for both the trunk interfaces. Transcoder plug-in unit, TR16-S Traffic channels and through-connected timeslots are handled by the TR16-S plug-in units. Each TRAU block is realised with one Digital Signal Processor (DSP), which performs the transcoding and rate adaptation functions in both transmission directions. A DSP can alternatively perform the switching-through of a 64 kbit/s timeslot between the BSC-side and MSC-side trunk interfaces. The DSP is a fixed point processor with 40 MIPS capacity. It has 5 kwords of internal RAM and an address space of 64 kwords for program and data memory. Each DSP of the TR16-S is permanently allocated to its particular timeslot on a particular MSC-side line. This allocation is based on the slot of the TR16-S in the cartridge and the logical number of the DSP in the TR16-S. The allocation onto the BSC-side bit or bits is controlled by the Submultiplexing Address bus (SUBA).

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The DSP has two bi-directional time-divided serial interfaces; one for the BSC- side and the other for the MSC-side. The BSC-side interface of the DSP is the first subsignal of the first Transcoder Interface (TRI). The MSC- side interface of the DSP is a serial interface where the signals of the Transcoder Control Interface bus (TRCI) and one TRI are combined. The TRCI bus bits use even bits of the signal and the TRI bits use the odd bits.

8.4.2 Synchronisation The TCSM2 unit supports hierarchical synchronisation of the BSS. The unit is synchronised to one of the MSC-side PCM line signal inputs or to the 2048 kHz clock (via the External synchronisation Input ESI). The unit supervises the incoming PCM signals. If there is an error, the signal is rejected and synchronisation is taken from another signal. The TRCO provides a 2048 kHz clock output (External synchronisation Output, ESO) for measurement purposes. The PCM line signal outputs and TRCO - TR16-S internal 4096 kbit/s data signals are bit and frame synchronised to the master clock. Due to elastic buffers, the received PCM signal frame phase can be arbitrary (for a slip-free operation, however, the average bit rate must be the same as the master clock in the TRCO). The incoming data is clockbuffered in the ET2E plug-in unit. The BSC synchronises to one of its TCSM2s. The returning PCM signals from the BSC thus have the same average frequency. Slip-free operation is also possible in the BSC to MSC direction. The synchronisation source automatically changes under the following conditions: • incoming signal is missing • frequency of incoming signal is outside the limits set for it • incoming signal is an Alarm Indication Signal (AIS) • frame synchronisation is lost • Cyclic Redundancy Check (CRC) multiframe alignment is lost.

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8.5 TRCO - TR communication links The connections between the TRCO and TR16-S. Transcoder Interfaces (TRI) Four TRI serial interface buses transfer the signals of the traffic channels and throughconnected time slots between the TRCO and the TR16-Ss at a bit rate of 4096 kbit/s. The TRI signals thus each carry two 2048 kbit/s signals. The interfaces are synchronised to the master 8192 kHz and 8 kHz clock signals of the TRCO. The TRI signals thus have equal phasing with respect to one another. Transcoder Control Interface (TRCI) The TRCI is a bi-directional internal bus. The bus transfers the control (for example, operating mode settings) and supervision messages (for example, alarms) between the TRCO and the TR16-S plug-in units as well as the program code of the TR16-S units. The TRCI consists of three parts: an address, an 8-bit data part and a control part. The TRCO is the master while the TR16-Ss are slaves on the bus. Via this bus, the TRCO verifies that the TR16-S obeys the SUBA bus data switching commands, thereby securing the SUBA and TRCI bus functions. Submultiplexing Address bus (SUBA) The SUBA bus is a uni-directional 8 bit parallel bus. Via this bus the TRCO controls the traffic channel routing function of each DSP in each TR16-S unit in real time. The DSPs transmit their data signals to the different bit positions of the BSC-side internal trunk, based on the control information received over the SUBA bus. The interface is synchronized to the master 8192 kHz and 8 kHz clock signals of the TRCO, so that the data is written to the bus at instants when each time slot position starts.

8.6 TRCO - ET communication links Traffic signals between the TRCO and ET2E are carried over the LI. The ET2E O&M link uses time slot 0 of the 2048 kbit/s signal (the first 2048 kbit/s signal of the ET2E). The program code is also loaded over this time slot. In addition, the ET2E transmits the two clock signals to the TRCO (extracted from the signals received by the PCM).

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8.7 Circuit Types It is possible to configure the TCSM2 equipment with O&M commands to be one of six circuit types (A to F). Following circuit types are supported: • • • • • •

A_ circuit type means that the Ater subchannel handled by the TRAU is a 16 kbit/s channel, containing FR or EFR speech or FR data traffic. B_ circuit type means that the Ater subchannel handled by the TRAU is an 8 kbit/s channel, containing HR speech or HR data traffic. C_ circuit type means that the Ater subchannel handled by the TRAU is an 8-16 kbit/s channel, containing FR or EFR speech, FR data, HR speech or HR data traffic. D_ circuit type means that the Ater subchannel handled by the TRAU is an 8-32 kbit/s channel, containing FR or EFR speech, FR data, HR speech or HR data traffic or HSCSD max 2 × FR data traffic. E_ circuit type means that the Ater subchannel handled by the TRAU is an 8-64 kbit/s channel, containing FR or EFR speech, FR data, HR speech or HR data traffic or HSCSD max 4 × FR data traffic. F_ circuit type means that the Ater subchannel handled by the TRAU is a 8-16 kbit/s channel, containing AMR speech.

There are always two HR traffic channels or one FR traffic channel in one 16 kbit/s subchannel between the BSC (Base Station Controller) and the BTS (Base Transceiver Station) i.e. in the Abis interface. In case of circuit types "C - E", however, the TRAU handles only one HR traffic channel or one FR traffic channel in the lowest 8 or 16 kbit/s subchannel respectively in the Ater-interface. Each Ater trunk may carry different channels. Each of the A-interface PCMs is individually programmed to the appropriate circuit type: A, B, C, D, E or F. The maximum number of A-interface PCMs supported by a TCSM2 is, however, dependent on the circuit types.

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8.8 Basic functions and features of a TRAU A TRAU has a 64 kbit/s PCM interface towards the MSC and an 8, 16, 32 or 64 kbit/s GSM interface towards the BTS. These interfaces are called the A interface and Abis interface, respectively. The downlink direction is the opposite of this (MSC to BTS). In addition to the PCM and GSM interfaces, the TRAU has a logically separate 64 kbit/s interface to the TRCO plug-in unit which is used for transmitting O&M messages. The information is transferred between the BTS and the TRAU in TRAU frames consisting of either 320 bits or 160 bits, and therefore taking 20 milliseconds to transfer one TRAU frame at the transfer speed of 16 kbit/s or 8 kbit/s, respectively. During HSCSD operation, up to four 16 kbit/s data traffic channels are transferred to a TRAU simultaneously. For the synchronization of the FR frames there is a pattern of 16 zeros at the beginning of each frame. The first bit of the 16-bit or 8-bit words which generates the frame, is a synchronization bit, depending on the type of the frame, whether speech or data, respectively. For the synchronization of the HR frames, there is a pattern of 8 zeros in the beginning of each frame. The first bit of the 8-bit words that generate the frame is a synchronization bit. there are also control bits in all the frames. The control bits contain data about the type of the frame and a varying amount of other type-specific information. In speech frames the last four bits in FR and the last two bits in HR are reserved for time alignment purposes. There are five types of frames defined for FR depending on the type of information they contain: speech frame, data frame, extended data frames, O&M frame and idle speech frame. There are three types of frames defined for HR depending on the type of information they contain: speech frame, data frame and O&M frame. When the TRAU is not performing any operation, it is in idle state. It announces this with an O&M message to the TRCO plug-in unit. It also transmits idle pattern, that is 01010101, 0101, 01, 0 or 1, depending on the Ater circuit type, in the downlink direction, and 01010100 in the uplink direction. When the call is set up, the BSC reserves one TRAU and transmits information about the set-up of the call to the BTS. After receiving the call set-up request, the BTS reserves the capacity required for the transmission of the call from the radio interface and starts to transmit TRAU frames that correspond to the type of the call to the TRAU (in the uplink direction). Upon receiving a frame, the TRAU starts to transmit similar frames in the downlink direction and the call has been set up. Even during a call, it is possible to change the type of the call by simply changing the type of TRAU frames coming in the uplink direction.

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When the call is terminated, the TRAU enters idle state after receiving idle pattern for a duration of three consecutive TRAU frames. In FR the idle pattern is a 01 combination in the 16 kbit/s subchannel. In HR the idle pattern is either all zeros or all ones, depending on the location of HR TRAU frame within the 16 kbit/s subchannel, whether in the first bit or the second. If the circuit type is "C" the idle pattern of the 16 kbit/s subchannel is either 01 or 11, because the BSC fills the unused subchannel with ones, and there may be either one or two subchannels in idle state. The idle pattern for circuit types "D" and "E" is 01 in each 16 kbit/s subchannel According to the GSM specifications, the TRAU should enter idle state after receiving the idle bit pattern for one second. However, a shorter time has been chosen because in principle it is possible that a new call will start during the one second period, therefore making it impossible to set speech and state variables and tables to the correct initial values.

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8.9 Discontinuous transmission Discontinuous transmission (DTX) is a mechanism which allows the radio transmitter to be switched off during speech pauses. The purpose is to reduce the power consumption of the transmitter, which is important for mobile phones, and to reduce the overall interference level on the radio channels, which affects the capacity of the network. Discontinuous transmission is implemented by means of three main structural elements. On the transmitting side a Voice Activity Detection (VAD) is required for detecting whether the signal in question contains speech or just background noise. The VAD function has been defined in the GSM specifications and it is mainly based on analyzing the energy of the signal and spectral changes. In addition, a function for counting the parameters of background noise is required on the transmitting side. On the receiving side comfort noise is generated on the basis of noise parameters received from the transmitting side, in order to avoid disturbing variations of complete silence and speech with background noise for the listener on the receiving side. All parts of the discontinuous transmission are mainly based on internal variables of speech codec.

8.9.1 Functions of the transmitting side (TRAU downlink DTX) The function taking care of discontinuous transmission on the transmitting side is called TX DTX Handler (Transmit DTX). It continuously delivers speech frames to the radio system. The frames are marked with a flag SP (Speech) which is located in the control bits, if the frame in question contains speech, or if the frame is a SID frame (Silence Descriptor), which contains information about the background noise for generating comfort noise on the receiving side. The SP flag is determined on the basis of the VAD flag (Voice Activity Detection), which is received from the VAD unit. When the value of this flag becomes zero, i.e. no speech is detected in the signal, the SP flag is also changed into a zero after the number of frames required for counting the parameters of background noise. Then the transmission unit of the radio system transmits the frame marked with a zero flag and containing noise parameters, after which the radio transmission is terminated. However, the TX DTX Handler continues to transmit frames containing noise information to the radio system which transmits one of these frames to the radio path at certain intervals, in order to update the noise parameters of the receiving side. When later speech is again detected in the signal, the value of the SP flag is changed to a one and continuous transmission restarts. On the transmitting side of discontinuous transmission, a function for counting the parameters of background noise is required. The parameters which describe background noise have been chosen from the normal speech parameters which give information about the level and spectrum of the background noise. These are further averaged over four speech frames. One common value is counted for the four block maximums during four speech frames. These parameters are sent to the radio path. All speech parameters which are normally transmitted are therefore not transmitted and some of the parameters are replaced with a SID code word which is generated from 95 zeros. All the other unnecessary parameters are coded into the value zero.

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8.9.2 Functions of the receiving side (TRAU uplink DTX) The discontinuous transmission of the receiving side is handled by RX DTX Handler (Receive DTX). From the radio system it receives frames which are handled on the basis of three flags received in the control bits. A BFI flag (Bad Frame Indicator) indicates if the frame in question contains usable information, that is, if a frame, for example, has changed so much on the radio path, that it cannot be reconstructed in the radio system of the base station, the frame is marked faulty with this flag. When this kind of a bad frame or a frame marked with value one of the BFI flag is received, the speech parameters of the frame in question will be replaced with the speech parameters of the previous frame, before decoding. If there are several consecutive frames marked with the BFI flag, the muting operations according to the GSM specifications are performed. The only exception to the handling of the BFI flag is uplink DTX, which means that a valid SID update frame has been received, but no normal speech frame with the BFI of the value zero has been received after that. In this kind of uplink DTX situation, the frame equipped with a BFI flag only means that the generation of comfort noise should be continued. An alternative implementation would be to transmit idle frames. The two-bit SID flag sent by the radio system is classified to a certain class on the basis of errors in the code word contained by the frame. On the basis of this classification, it is decided how the frame is going to be used. If the SID is of the value two and the BFI of the value zero, the frame is a valid SID frame, which can be used for updating noise parameters. The TAF (Time Alignment Flag) indicates if the frame in question has been used for signalling outside this subsystem, i.e. it usually indicates the time of the next SID updating. The operations related to the generation of comfort noise, such as muting etc. are performed from the combinations of the three flags mentioned before as defined in the GSM specifications. In general, the generation of comfort noise is started or the comfort noise is updated when a new valid SID frame has been received.

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8.10 Tandem Free Operation (TFO) Tandem Free Operation (TFO) is a procedure, which can be used to enhance speech quality in mobile-to-mobile (MS-MS) calls. In normal MS-MS call the speech signal is encoded in transmitting mobile station and then decoded in transcoding equipment for the fixed part of the network. On the other side of the network another transcoder encodes the speech samples for transmitting over air interface and the receiving mobile station finally decodes them for listening. From speech coding point of view there are two speech codecs (encoder-decoder pairs) forming a tandem connection in MS-MS call. Because the speech coding methods used in GSM network are lossy, the speech signal is degraded every time it is encoded and decoded. When MS-MS call is formed with TFO, the speech signal is not decoded and encoded between the two transcoders thus resulting a call in which the speech encoding and decoding is made in mobile stations only. There are some requirements, which have to be fulfilled for a successful TFO call: • • •

the call is an MS-MS call both transcoders support TFO the path between the transcoders is digitally transparent

A digitally transparent path between the two transcoders means that TFO signaling and coded speech samples are not corrupted by any in-path equipment normally used in transmission lines, for example echo canceller. The in-path equipment must also support TFO, so that they can be commanded to go to a transparent mode for TFO. In A interface the TFO data is exchanged in TFO frames which are as TRAU frames in Abis interface. Some of the control bits are used differently in TFO frames and some of the sync-one bits are changed by embedded TFO messages but the parameterized speech samples are in same format as in TRAU frames. The bits of the frames are sent in least significant bits of the PCM samples in A interface. The same speech signal is transmitted in A interface in the form of ordinary PCM samples and in the TFO frames. The availability of PCM samples guarantees fairly good speech continuity in those situations when TFO discontinues due to handover. Before the TFO protocol can go to operation state it has to ensure that the requirements of TFO are fulfilled. This is done by sending so called in-band signaling messages in A interface. Message bits are sent in the LSB of every 16th PCM sample.

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9 Conclusion The mobile communication is a very vast field and there are lots of things yet to explore. During this project I was able to relate our theoretical knowledge with the actual practical event happening in the industries. We thoroughly understand many concepts, which are use practically. This industrial training also helps us to be familiar to the actual work environment prevailing in the industries and helped as our valuable experience to adjust our self before entering into the actual industrial environment.

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10 References Systra.pdf BSC operation and Maintenance training document Nokia UltraSite EDGE BTS.pdf Nokia FlexiHopper Microwave Radio product Overview.pdf TRAU_1.pdf TRAU_2.pdf NED-Nokia electronic Document Server

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