Fig 1 shows GSM architecture. The network manly consist of User Equipment (UE), Base transceiver station (BTS), Mobile switching center (MSC). The GSM contains most of the necessary capabilities to support packet transmission over GSM. The critical part in the GPRS network is the mobile to GSN (MS-SGSN) link which includes the MS-BTS, BTS-BSC, BSC-SGSN, and the SGSN-GGSN link .fig 1 shows block diagram of GSM architecture.

User Equipment (UE)—These are the users .Number of users are controlled by one BTS
1. The mobile stations (MS) communicate with the base station subsystem over the radio the radio interface.
2. The BSS called as radio the subsystem, provides and manages the radio transmission path between the mobile stations and the Mobile Switching Centre(MSC).It also manages radio interface between the mobile stations and other subsystems of GSM.
3. Each BSS comprises many Base Station Controllers(BSC) that connect the mobile station to the network and switching subsystem (NSS) through the mobile switching center
4. The NSS controls the switching functions of the GSM system.It allows the mobile switching center to communicate with networks like PSTN, ISDN, CSPDN, PSPDN and other data networks.
5. The operation support system (OSS) allows the operation and mantanance of the GSM system. It allows the system engineers to diagnose,troubleshoot and observe the parameters of the GSM systems.The OSS subsystem interacts with the other subsystems and is provided for the GSM operating company staff that provides service facilities for the network.

Base station(BSS)-- The following stations subsystem comprises of two parts:
1. Base Transceiver Station (BTS).
2. Base Station Controller(BSC).
The BSS consists many BSC that connect to a single MSC. Each BSC controls upto several hundred BTS.

Base Transreciver Station(BTS)-BTS
It has radio transreciever that define a cell and are capable of handelling radio link protocols with MS.
Functions of BTS are
1. Handelling radio link protocols
2. Providing FD communication to MS.
3. Interliving and de- interliving.

Base station controller(BSC) IT manages radio resources for one or more BTS.It controls several hundred BTS al are connected to single MSC.
Functions of BTS are
• To control BTS.
• Radio resource management
• Handoff management and control
• Radio channel setup and frequency hoping

Network subsystem( NSS)
1.It handels the switching of GSM calls between external networks and indoor BSC
2.It includes three different data bases for mobility management as
A .HLR (Home Location Register)
B .VLR (Visitor Location Register)
C. AUC (Authentication center)

Mobile switching center (MSC)--
It connects fix networks like ISDN ,PSTN etc.
Following are the functions of MSC
1. Call setup, supervision and relies
2. COLLECTION OF BILLING INFORMATION
3. Call handelling / routing
4. Management of signelling protocol
5. Record of VLR and HLR

HLR (Home Location Register) - Call roamiming and call routing capabilities of GSM are handeled.It stores all the adminstartive information of sub scriber registered in the networks.IT maintance unique international mobile subscriber identity.(IMSI).

VLR (Visitor Location Register) - It is a temporary data base.It stores the IMSC number and costomer information for each roaming customer visiting specific MSC.

Authentication center - It is protected database .It maintance authentication keys and algorithms.It contain s aregister called as Equipment Identity Register.

Operation subsystem(OSS) - IT manages all mobile equipment in the system 1)management for charging and billing procedure 2)To maintain all hardware and network operations

Interfaces used for GSM network : (ref fig 2)
1)UM Interface –Used to communicate between BTS with MS
2)Abis Interface— Used to communicate BSC TO BTS
3)A Interface-- Used to communicate BSC and MSC
4) Singling protocol (SS 7)- Used to communicate MSC with other network .