Architecture of GSM system:

Architecture of GSM system:

There are two main parts of GSM system. One is known as BSS ( Base Station Subsystem) or the Access Network and the other one is NSS (Network Subsystem) or Core Network.

Functioning of BSS in GSM system

Using BSS the mobile station Accesses the network. When the user or the mobile station want to make a call or use any of the network services, then it needs to use SIM in the mobile system. There are two identities, one is of the mobile station, which is IMEI No. ( International mobile equipment identity no.). The second is the identity of the SIM, which is characterised by IMSI no (International Mobile subscriber identity). Whenever user want to use the network then he needs a valid SIM and valid IMSI no. When ever user or mobile station wants to use the network, he will use his/ mobile station to connect with BTS ( base Transceiver Station) over the radio interface or air interface. The BTS is connected to BSC (Base Station Controller). There can be many BTS connected to one BSC. In any Mobile Network there are more than one BSC. BSC is connected to TRAU (Transcoding and Rate adoption). 

Function performed by BSC. 

First is channel allocation, ie. Uplink and down link frequencies between Mobile station and BTS and is known as Abis interface and this interface can be coaxial cable, or optical fiber, or a microwave link. The other functionality of BSC is handing over the signal from one BTS to other BTS, when user is moving. One more function of BSC is power control of Mobile station and that of BTS. All the traffic from all the BTSs is agreegated by BSC and sent to TRAU.             

Functionality of TROU is to compress/decompression the signals between Mobile station and MSC to use channel frequencies efficiently.

Core Part/NSS (Network Sub System) of GSM system

The central part of the NSS is the MSC( Mobile switching centre). There can one or more than one MSCs in GSM Network. MSC is very similar to the Digital switching system for fixed Telephone Network. It establishes calls between the different users and process those calls. MSC has some functionality of switching system and has some additional functionality which are specific to Mobile Network. To accomplish these additional functionality MSC needs to have certain data bases like HLR, VLR, authentication centre, OMC, Equipment identity Register. These data basis helps MSC to accomplish the tasks which have been allocated to it. 

When the mobile station is switched on, it registers itself with the Network. It is the MSC which registers the Mobile station with itself. Similarly the call establishing and routing is done by MSC. To connect the call, MSC needs to allocate resources like a channel, which needs to be allocated between mobile station & BTS and between BSC & MSC. When ever the mobile station/user is moving then MSC takes care that, to which BTS it should be connected. It keeps on changing the BTS depending upon the location of the mobile station. So handing over the call from one BTS to other BTS. If there is change of BSC because of movement of switching station the MSC also hand over the call from one BSC to the other BSC.

The inter-working function (IWF) is a method for interfacing a wireless telecommunication network with the public switched telephone network (PSTN). The IWF converts the data transmitted over the air interface into a format suitable for the PSTN. Also IWF allows the mobile user to use internet @ 9.6 kbits per sec. But now a days this is no more being used, because of slow data rate. The function of IWF is protocol conversation and data rate adoption.

Data Bases of MSC

We have one data base which is called as HLR ( Home Location Register). HLR contains the data of all the users in the Home Network. This is the central location in any Mobile Network. Normally there is only one HLR in the entire Network. But some times there is more than one HLR, to take care of break downs etc. The additional HLR works in Backup mode. Also to reduce signalling load on one HLR.            

Another data base in MSC is VLR ( Visitor Location Register). When mobile station from different MSC is roaming in this MSC, then this MSC has data of this mobile station in VLR Register. This data will be maintained in the VLR till the time the mobile station remains in the network of this MSC. So the VLR Register shows the active users present in the network of this MSC.

Another data base in the MSC is authentication Centre. The authentication center (AuC) is a key component of a global system for mobile communications (GSM) home locator register (HLR). The AuC validates any security information management (SIM) card attempting network connection when a phone has a live network signal. The AuC provides security to ensure that third parties are unable to use network subscriber services.

There is one more data base known as EIR ( Equipments Identity Register). The Equipment Identity Register (EIR) is a database that contains a record of the all the mobile stations (MS) that are allowed in a network as well as an database of all equipment that is banned, e.g. because it is lost or stolen. The identity of the mobile station is given by the International Mobile Equipment Identity (IMEI). Each time a call is made, the MSC requests the IMEI of the mobile station, which is then send to the EIR for authorisation. EIR has three lists, one is white list, which has data of all IMEI nos. of all valid subscribers. Then there is black list, which contains IMEI nos. of blocked subscribers, which are either stolen or have some other problem. The last list is Grey list, which contains IMEI nos of subscribers which are being watched or are under observation.

Now come OMC ( Operation and Maintenance Centre), which has data of all Equipment in the network. It takes of any failures or faults of any equipment in the network or degradation of parameters of the network. It also has alarms, which help in detecting faults or failures in the network system. OMC has connection with MSC and all BSCs. It helps in trouble shooting the faults and optimising the performance of the network. Operation Maintenance Center (OMC) is used to monitor and maintain the performance of each Mobile Station (MS), Base Station (BS), Base Station Controller (BSC) and Mobile Switching Center (MSC) within a GSM system. The OMC has three main functions which are:

1.To maintain all telecommunications hardware and network operations with a particular market.

2. Manage all charging and billing procedures.

3. Manage all mobile equipment in the system.

The OMC is dedicated to each of these tasks and has provisions for adjusting all base station parameters and billing procedures, as well as for providing system operators with the ability to determine the performance and integrity of each piece of subscriber equipment in the system.

The Serving GPRS Support Node (SGSN) is a main component of the GPRS network, which handles all packet switched data within the network, e.g. the mobility management and authentication of the users. The SGSN performs the same functions as the MSC for voice traffic. The SGSN and the MSC are often co-located. The SGSN is connected to the BSC. The SGSN is the service access point to the GPRS network for the mobile user. On the other side the SGSN relays the data between the SGSN and relevant GGSN (and vice versa). The SGSN handles the protocol conversion from the IP used in the backbone network to the sub-network-dependent convergence protocol (SNDCP) and logical link control (LLC) protocols used between the SGSN and the mobile users. These protocols handle compression and ciphering. The SGSN is also responsible for the authentication of GPRS mobiles. When the authentication is successful, the SGSN handles the registration of the mobile to the GPRS network and takes care of its mobility management.

SMSE is the part of MSC which takes care of SMS servies. SMS or Short Message Service is a service provided by an operator of GSM / CDMA besides Voice service. SMS and Voice is primary service in common mobile operator. The SMS is non-circuit related, while the circuit voice service is so important to understand in terms of the protocol used. It is better to know the basic protocol used in SMS that is SS7.

This is the architecture of GSM system and how it operates.

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