GSM – Global System for Mobile communications -Seminar Report

Abstract:

The Europeans realized in 1982 the Conference of European Posts and Telegraphs (CEPT) formed a study group called the Groupe Spécial Mobile (GSM) to study and develop a pan-European public land mobile system. GSM uses radio frequencies efficiently, and due to the digital radio path, the system tolerates more intercell  disturbances. Speech is encrypted and subscriber information security is guaranteed.It is grong very fastly and moe than 120 million users are currently using GSM serevices.

Introduction

At the beginning of the 1980s it was realised 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 “Groupe Spéciale 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.

IInterfaces 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 actions/procedures/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”. These two network elements will be discussed in greater detail in later chapters. 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 centralised 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 decentralised 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 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.

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 of one mobile equipment and inserted into another. In the GSM network, the SIM card identifies the user just like a traveller 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.

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. We will describe more about the transcoder functions later.

                            The Base Station Subsystem (BSS)

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. For this purpose, the GSM Technical Specifications define about 120 different parameters for each BTS. These parameters define exactly what kind of BTS is in question and how MSs may "see" the network when moving in this BTS area. The BTS parameters handle the following major items: what kind of handovers (when and why), paging organisation, radio power level control and BTS identification.

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.

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.

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:

·                                          CGI=MCC + MNC + LAC + CI

·         MCC            Mobile Country Code

·         MNC            Mobile Network Code

·         LAC Location Area Code

·         CI     Cell Identity

Let’s take an example of two adjacent BTSs. One serves an industrial area and the other a nightlife area.

Network Subsystem

The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN, and additionally provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. These services are provided in conjuction with several functional entities, which together form the Network Subsystem. The MSC provides the connection to the fixed networks (such as the PSTN or ISDN). Signalling between functional entities in the Network Subsystem uses Signalling System Number 7 (SS7), used for trunk signalling in ISDN and widely used in current public networks.

The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call-routing and roaming capabilities of GSM. The HLR contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile. The location of the mobile is typically in the form of the signalling address of the VLR associated with the mobile station. The actual routing procedure will be described later. There is logically one HLR per GSM network, although it may be implemented as a distributed database.

The Visitor Location Register (VLR) contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. Although each functional entity can be implemented as an independent unit, all manufacturers of switching equipment to date implement the VLR together with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR, thus simplifying the signalling required. Note that the MSC contains no information about particular mobile stations --- this information is stored in the location registers.

The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and encryption over the radio channel.

  

 Network Management Subsystem

The functions of the NMS can be divided into three categories:

·         Fault Management

·         Configuration Management

·         Performance Management

These functions cover the whole of the GSM network elements from the level of individual BTSs, up to MSCs and HLRs.

Fault Management

The purpose of Fault Management is to ensure the smooth operation of the network and rapid correction of any kind of problems that are detected. Fault management provides the network operator with information about the current status of alarm events and maintains a history database of alarms.

The alarms are stored in the NMS database and this database can be searched according to criteria specified by the network operator.

                     

Configuration Management

The purpose of Configuration Management is to maintain up to date information about the operation and configuration status of network elements. Specific configuration functions include the management of the radio network, software and hardware management of the network elements, time synchronisation and security operations.

                  

Performance Management

In performance management, the NMS collects measurement data from individual network elements and stores it in a database. On the basis of these data, the network operator is able to compare the actual performance of the network with the planned performance and detect both good and bad performance areas within the network.

 RADIO 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. This will be analysed later in this chapter.

                   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:

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. 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.

The figure below illustrates the TDMA principle.

                  

GSM uses digital techniques where the speech and control information are represented by 0s and 1s. How is it possible to transmit digital information over an analogue radio interface?

The digital values 0 and 1 are used to change one of the characteristics of an analogue radio signal in a predetermined way. By altering the characteristic of a radio signal for every bit in the digital signal, we can "translate" an analogue signal into a bit stream in the frequency domain. This technique is called modulation. Analogue signals have three basic properties: Amplitude, Frequency, Phase. Therefore, there are basically three types of modulation process in common use:-

·                  Amplitude Modulation

·                  Frequency Modulation

·                  Phase Modulation

                  

GSM uses a phase modulation technique over the air interface known as Gaussian Minimum Shift Keying (GMSK). In order to understand how it works, let’s take a simple example.

At the GSM air interface, the bit rate is approximately 270Kbits/s. (This will be explained later) At this bit rate, the duration of one bit is 3.69 ms, i.e. the value of the bit requires 3.69 ms of transmission time. GMSK changes the phase of the analogue radio signal depending on whether the bit to be transmitted is a 0 or a 1.

                  

The radio air interface has to cope with many problems such as variable signal strength due to the presence of obstacles along the way, radio frequencies reflecting from buildings, mountains etc. with different relative time delays and interference from other radio sources.

With such levels of interference, complex equalisation techniques are required with GMSK. 

    Transmission Through the Air Interface

 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 neighbouring 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 (describer later). 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.

               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.

                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 bi-directional 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.

               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.

Time Slots And Bursts

We have already seen that the technique used in air interface is Time Division Multiple Access (TDMA) where one frequency is shared by, at the most, eight users. 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 then falls silent for the next seven timeslots and then again sends the next burst and so on.

It can be seen that the mobile station is sending information periodically. All the mobile stations send their information like this. If we go back to the analogy of the army, the road is the radio carrier frequency, the vehicle is the TDMA frame and the seats in each  vehicle are the TDMA timeslots.

                     

In the air interface a TDMA timeslot is a time interval of approximately 576.9 s 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 :

·         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 theoretical distance of about 35 km is allowed between the base transceiver station and mobile station.

            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. In this section we will briefly discuss some of the problems that occur in air interface and some solutions. 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 travelled, 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:

·         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 analysing 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. (Ciphering is also carried out for security reasons).

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 freqencies in the cell.

 Propagation Delay

As you remember, 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.

                  

Transmission between BSC and BTS

There are three alternative methods to provide the connections between a BSC and several BTSs. The method used will depend on a number of factors such as the distance between the Base Station Controller (BSC) and Base Transceiver Station (BTS), the number of TRXs used at a particular BTS site, the signalling channel rate between Base Station Controller (BSC) and Base Transceiver Station (BTS).

There are three options available: point-to-point connection, multidrop chain and multidrop loop

Point-to-point connection indicates that the Base Station Controller (BSC) is connected directly to every BTS with a 2Mbit/s PCM line. This is a simple and effective method particularly in cases when the distance between BSC and BTS is short. However,  if the BSC -BTS distance is a few kilometres whereas the distance between a group of BTS’s is much shorter, it does not make sense to draw a point-to-point connection to every BTS. One PCM line has ample capacity to transfer data to several BTSs simultaneously. Therefore, it is possible to draw just one BSC - BTS connection and link the BTSs as a chain. This technique is called "multidrop chain". The BSC sends all the data in one 2Mbit/s PCM line and each BTS in turn analyses the signal, collects the data from the correct timeslots assigned for itself and passes the signal to the next BTS.

But there is one problem with a multidrop chain. Consider what would happen if there is a malfunction somewhere along the line and the chain breaks. More BTSs are isolated and, if the BSC is not informed, it will continue to send data. The solution to this problem is called "multidrop loop" and instead of a chain we connect the BTSs in the form of a loop. Previously a dynamic node was needed to split the signal into the two directions around the loop, but later versions of BTS are capable of carrying out this function. The flow of the signal is similar to the signal flow in multidrop chain, except that a BTS will change the “listening” direction if the signal from one side fails. This ensures that the BTSs always receive information from the BSC even if the connection is cut off at some point in the loop.

The Concept of Multiplexing

According to GSM 900 and GSM 1800 specification, the bit rate in the air interface is 13 Kbits/s and the bit rate at the Mobile Services Switching Centre (MSC) and PSTN interface is 64 Kbits/s. This means that the bit rate has to be converted at some point after the signal has been received by the BTS and before it is sent to other networks. But the specifications do not put a constraint as to where exactly the conversion should take place. This brings up some interesting scenarios.

The actual hardware which does the conversion from 13 Kbits/s to 64 Kbits/s and vice versa is called a transcoder. In theory this piece of equipment belongs to the Base Transceiver Station. However, by putting the transcoder at a different place we can take some advantages in reducing the transmission costs.

If the transcoder is placed at the BTS site (in the BSC interface), then the user data rate from BTS to Base Station Controller (BSC) would be 64 Kbits/s. The transmission for this would be similar to standard PCM line transmission with 30 channels per PCM cable. The same would also apply between BSC and MSC.

If we put the transcoder somewhere else, say just after MSC, then also we can not get significant advantage. This is because although after transcoding the bit rate reduces to 13 kbit/s we still have to use the PCM structure to send the traffic channels, with 8 bits per time slot. However since after transcoding we have a bit rate of 13 Kbits/s and an additional 3 Kbits/s (making 16 Kbits/s) only two bits per time slot will be used. The other 6 bits are effectively wasted. The next figure shows these two types of connections.

Independent from its actual position, the transcoder belongs to the BSS even if it is placed next to the MSC. (When the TC is placed away from the BTS it is called a Remote TC according to the GSM recommendations).                 

But the real advantage comes if we use the second configuration shown in the figure with another piece of hardware called submultiplexer. We saw that from the MSC data comes out at 64Kbits/s rate and from the Transcoder it comes out at 16Kbits/s. Each PCM channel (time slot) has 2 bits of information. It appears that we are able to put in data from other 3 PCM lines also here by multiplexing. However there are other issues as well such as Common Channel Signalling information, OMC data and some other network information which can not be transcoded. Thus we are able to multiplex 3 PCM lines and send 90 channels in one PCM line from MSC (transcoder) towards the BSC. The BSC is able to switch 2 bits per time slot (or 1 bit) to the correct direction. The next figure shows the configuration.

Location updating

A powered-on mobile is informed of an incoming call by a paging message sent over the PAGCH channel of a cell. One extreme would be to page every cell in the network for each call, which is obviously a waste of radio bandwidth. The other extreme would be for the mobile to notify the system, via location updating messages, of its current location at the individual cell level. This would require paging messages to be sent to exactly one cell, but would be very wasteful due to the large number of location updating messages. A compromise solution used in GSM is to group cells into location areas. Updating messages are required when moving between location areas, and mobile stations are paged in the cells of their current location area.

The location updating procedures, and subsequent call routing, use the MSC and two location registers: the Home Location Register (HLR) and the Visitor Location Register (VLR). When a mobile station is switched on in a new location area, or it moves to a new location area or different operator's PLMN, it must register with the network to indicate its current location. In the normal case, a location update message is sent to the new MSC/VLR, which records the location area information, and then sends the location information to the subscriber's HLR. The information sent to the HLR is normally the SS7 address of the new VLR, although it may be a routing number. The reason a routing number is not normally assigned, even though it would reduce signalling, is that there is only a limited number of routing numbers available in the new MSC/VLR and they are allocated on demand for incoming calls. If the subscriber is entitled to service, the HLR sends a subset of the subscriber information, needed for call control, to the new MSC/VLR, and sends a message to the old MSC/VLR to cancel the old registration.

 Advantages of GSM

·         GSM uses radio frequencies efficiently, and due to the digital radio path, the system tolerates more intercell disturbances.

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

·         Data transmission is supported throughout the GSM system.

·         Speech is encrypted and subscriber information security is guaranteed.

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

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

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