GLOBAL SYSTEM FOR MOBILE COMMUNICATION

GSM is a second generation of cellular telephone system system developed to solve fragmentation problems. It is the first digital cellular telephone system to use services of wide range of network services.

GSM SERVICES

GSM telephone services can divide into three categories. They are

1) Bearer services. 2) Tele services 3) Supplementary services.

The most basic bearer services provided by GSM are telephony. With GSM Analog signal are encoded into digital and then transmitted through the network, as data stream. There is also an emergency service where the closest emergency provider is notified by dialing three digits simultaneously to 91 services.

GSM offer services where users can send and receive data at rates up to 9600bps to subscribe in POTS, ISDN network, and circuit switched public data network using a wide variety of access methods and protocols.

Since GSM is a digital network a modem is not required between user and gsm network.

GSM also provides short message services (SMS), which is bi-directional service for sending alphanumeric message up to 160 bytes in length. SMS can be send simultaneously to multiple receivers.

In supplementary services call forwarding and call barring is included.

GSM SYSTEM ARCHITECTURE:

GSM consist of three major interconnected subsystems that interact among one another and with subscriber through network interfaces.

The three primary subsystems are:

THE BSS is also known as radio subsystem because it provides and manages radio transmission path between mobile units and mobile switching center (MSC). The BSS also manages the radio interface between between mobile units and GSM subsystem. Each BSS consist of BSCs, which are used to connect the MSC to the NSS. The NSS manages switching function for the system and allows the mobile switching network to communicate with other telephone networks. Such as the ISDN and PSTN .the OSS supports operation and maintenance of system.

The entire subscriber related functions and call processing is done through switching system. It is divided into following units:

• HLR (Home location register)
  
• VLR (Visitor location register)
  
• EIR (Equipment identity register)
  
• MSC (Mobile switching center)
  
• AUC (Authentication center)

HLR as name suggests is a database used to store permanent data about subscribers such as Location information. It is location specific i.e. in which location subscriber is subscribed. Beyond this area he/she will be in roaming.

VLR is also a database but contains temporary information, which is required by the MSC for the visiting subscriber. Through this subscriber is found in a particular area and is connected to the network and hence outside world. VLR collects the data about subscriber from the HLR.

EIR is also a database, which contains information about the identity of mobile equipment of the subscriber. It prevents their unauthorized use.

AUC as the name suggests is used for verification of user identity. It is a very important part of network.

MSC - Switching functions of the network are done through MSC. It controls calls to and from other telephone and data systems.