2.4.2 GSM Architecture :

2.4.2 GSM Architecture :

Mobile Station [MS] :

The Mobile Station MS consist of:

·        ME: Mobile Equipment .

·        SIM : Subscriber Identification Module .

SIM card

The SIM card is allocated and activated by the provider upon completion of the contract . It is realized by means of chip which contains a variety of permanent and temporary information for the subscriber (e.g. personal telephone register ) and about him / her.a long with the personal ( secret ) ID numbers (IMSI – I nternational Mobile Subcriber Identity,TMSI-Temporary Mobile Subcriber Identity )these stored information are for example algorithms and keys for ciphering the transmission .

Base Station Subsystem [ BSS] :

The Base Station Subsystem is responsible for managing the radio network , and it is controlled by an MSC . Typically ,one MSC contains several BSSs. A BSS itself may cover a large geographical area consisting of many cells ( a cell refers to an area covered by one  or more frequency resources ).The BSS consists of the following elements : 

·         BSC Base Station Controller

·        TC Transcended

·        BTS Base Transceiver Station

Base Station Controller [BSC] :

The BSC is the central network element of the BSS and it controls the radio network.

It has several important tasks, some of which are presented in the following :

·        Connection establishment between the MS and the NSS

             All calls to and from the MS are connected through the group switch of the BSC (GSWB ).

·        Mobility management

The BSC is responsible for initiating the vast majority of all handovers based on measurements reports sent by the MS during a call.

·        Statistical raw data collection

Information from the Base Transceiver Station, Transcoders, and BSC are collectd in the BSC and forwarded to the NMS (Network Manegment Subsystem) from which the network quality and status is obtained.

·        BTS and TC control

All the BTSs and TCs are connected to the BSCs .The BSC maintains the BTSs.Transcoders are also maintained by the BSC.

Base Transceiver Station [BTS] :

   The BTS is the network element responsible for maintaining the air interface and minimizing the transmission problems.

The BTS has several very important tasks, some of which are presented in the following :

·        Ciphering

Both the BTS and MS must be able to cipher and decipher information in order to protect the transmitted speech and data in the air interface.

·        Speech Processing

Speech Processing refers to all the functions of the BTS performs in order to guarantee an error-free connection between the MS and the BTS. This includes tasks like speech coding, channel coding, interleaving and burst formatting.

·        Timing Advanced[ TA]

Calculation of the distance of the MSs from the BTS; the MSs are informed of necessary transmission advance.

·        Frequency Hopping

A feature which enhances the reliability of information transfer.

    TC-Transcoder :

Transcoder converts from one speech compression format to another.For transmission over the air interface , the speech signal is compressed by the mobile station to 13 k bits (full rate and enhanced full rate) or 5.6 k bits (half rate ).

Network Switching Subsystem [ NSS ] :

The Network Switching Subsystem [ NSS ] contains MSC , VLR , HLR , AC and EIR .

Mobile Services Switching Centre [MSC] :

The MSC is responsible for controlling calls in the mobile network.It identifies the origin and destination of a call ( mobile station or fixed telephone ),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 .

The MSC is responsible for several important tasks , such as the following :

·        Call Control

MSC identifies the type of call, the destination , and the origin of a call . It also sets up ,supervises , and clears connections .

·        Initiation of Paging

Paging is the process of locating a particular mobile station.

·        Charging data collection

Visitor Location Register [VLR] :

VLR is a database which contains information about subscribers currently being in the service area of the MSC/VLR, such as :

·        Identification numbers of the subscribers.

·        Security information for authentication of the SIM card and for ciphering.

·        Services that the subscriber can use .

The VLR carries out location registrations and updates.It means that when a mobile station comes to a new MSC/VLR serving area , it must register itself in the VLR , in other word perform a location update. A mobile subscriber must always be registered in a VLR in order to use the services of the network .Also the mobile stations located in the own network are always registered in a VLR .

The VLR database is temporary , in the sense that the data is held as long as the subscriber is within its service area .It also contains the address to every subscriber‘s Home Location Register.

Home Location Register [ HLR ] :

HLR maintains a permanent register of the subscribers , for instance subscriber identity numbers and the subscribed services .In addition to the fixed data , the HLR also keeps track of the current location of its customers . The MSC asks for routing information from the HLR if a call is to be set up to a mobile station ( mobile terminated call ).

Authentication Center [AC] :

The Authentication Center provides security information to the network , so that we can verify the SIM cards (authentication between the mobile station and the VLR, and cipher the information transmitted in the air interface ( between the MS and the Base Transceiver Station). The Authentication Center supports the VLR ‘s work by issuing so –called authentication triplets upon request.

Equipment Identity Register [EIR] :

As for AC , the Equipment Identity Register is used for security reasons . But while the AC provides information for verifying the SIM cards , the EIR is responsible for the International Mobile Equipment Identity ( IMEI )checking (checking the validity of the mobile equipment ).

When performed , the mobile station is requested to provide (IMEI) number .This number consists of type approval code , final assembly code and serial number of mabile station .

    The EIR contains three lists :

·        Mobile equipment in the white list is allowed to operate normally .

·        If we suspect that a mobile equipment is faulty , we can monitor the use of it .It is then placed in the grey list .

·        If the mobile equipment is reported stolen , or it is otherwise not allowed to operate in the network , it is placed in the black list .

Network Management Subsystem [NMS] :

The Network Management Subsystem (NMS) is the third subsystem of the GSM network in addition to the Network Switching Subsystem (NSS) and Base Station Subsystem (BSS),which we have already discussed . The purpose of the NMS is to monitor various functions and elements of the network.

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

·        Fault management

·        Configuration management

·        Performance management

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