Subject: Mobile Computing

Difficulty: Medium

Marks: 8 Marks

GSM system consists ofthree major components:

(i) Base Station System (BSS).

(ii) Operation and Maintenance Center (OMC).

(iii) Network and Switching Subsystem (NSS).

(i) Base Station System (BSS): This system consists of Mobile Station (MS), Base Station Controller (BSC),Base Transreceiver Station (BTS). As shown in Fig. the BSS and NSS connected to each other via A interface(solid lines) and the connection to OMC via O interface (dashed lines).

1. Base Station Subsystem (BSS): GSM system consists of many BSS, each one is controlled by Base Station Controller (BSC). BSS performs all the functions which are required to maintain connection to MS, coding/decoding of voice etc. BSS also contains Base Trans receiver Stations (BTS).

2. Base Station Controller (BSC): BSC provides all the control functions and physical link between MSC and BTS. BSC is connected to BTS and MSC (Mobile Switching Center). Base Trans receiver Station (BTS): BTS is responsible for handling radio interface to the mobile station. It is connected to MS via Uminterface and it is also connected to BSC via the Abis interface. The Um interface contains all mechanism for wireless interface (TDMA, FDMA etc.). The BTS is a radio equipment (trans receiver or antenna) needed to service each cell in the network.

(ii)Operation and Maintenance Center (OMC): OMC is connected to all equipments in switching system and to the BSC. It maintains operation of the GSM network by observing the handovers, system load, blocking rates etc. OMC provides network overview and allow network engineers to monitor, diagnose and troubleshoot every aspect of GSM network.

(iii) Network and Switching Subsystem (NSS): NSS is responsible for performing call processing and subscriber related functions. It also includes Mobile Switching Center (MSC), Home Location Register (HLR), Visitor Location Register (VLR), Authentication Center (AUC), Equipment Identity Register (EIR) etc.

Mobile Switching Center (MSC): It is used to handle communication between different MS connected to different BSCs. The function of MSC is to locate different MS and associated BTS, call switching and authentication etc. Home Location Register (HLR): It is a database for managing the mobile subscriber. HLR stores permanent data of subscriber which include subscribers service profile, location information and its activity. A home subscriber charges are less thanthe roaming subscriber.

Visitor Location Register (VLR): It is a database which consists of temporary information about subscribers which is used by MSC in order to provide services to visiting subscriber. MSC updates the VLR by determining which users are in roaming. Once, the roaming mobile information is updated, then MSC sends necessary information to roaming mobile subscribers so that roaming mobile call can be properly routed.

Authentication Center (AUC): This authentication center is used to provide authentication and encryption method that is used to verify the user identity and ensure the confidentiality and secrecy of each call.

Equipment Identity Register (EIR): It contains a list of all valid MS equipment within the network, where each MS is known by it's IMEI.

This IMEI is divided into three groups.

1. White IMEI: All known IMEI.

2. Black IMEI: All stolen mobile handset.

3. Gray IMEI: Handset that is uncertain.