GSM (Global System FOR Mobile Communication)

In 1982, the CEPT(Conference of European Post and Telecommunications.It is an association of telephone and telegraph operators in Europe,) established a working group called Groupe Special Mobile (GSM) to develop a new land mobile system. Their working language was French.

The proposed criteria was following by the GSM Group for the system

• good speech quality
• low cost for mobiles and service
• roaming(International)
• handheld terminals
• spectral efficiency
• support for introduction new services
• compatibility with ISDN.

In 1989, the responsibility for GSM development was transferred to the ETSI(European Telecommunications Standards Institute), and phase 1 of the GSM specification was published in 1990. The first commercial service was launched in 1991.

When the official language of the GSM group changed from French to English, GSM was changed from Groupe Speciale Mobile to Global System for Mobile Communications.

In 1994, phase 2 data/fax services were launched, and in 1995, the GSM phase 2 standard was completed. The first GSM services in the United States were launched.

GSM uses a combination of both the time division multiple access (TDMA) and frequency division multiple access (FDMA) technologies. With this combination, more channels of communications are available, and all channels are digital.

The GSM service is available in four frequency bands: 450-MHz

The higher frequency bands provide additional capacity and higher subscriber densities.

One of the unique benefits of GSM service is its capability for international roaming because of the roaming agreements established between the various GSM operators worldwide.

Basically the GSM system is consists of Below ;
MS - Mobile Station(Mobile Phone)
BTS - Base Transceiver Station
BSC - Base Station Controller
MSC - Mobile Switching Center
GMSC - Gateway Mobile Switching Center

Usually the MS is server by the nearest BTS.When the call is originating by the MS ;
It could be the both call originating phone and the call terminating phone serves by a same BTS as below

And also could be the both call originating phone and the call terminating phone serves by different BTS's with a same BSC (Several BTS together are controlled by one Base Station Controller (BSC)as below

And could be the both call originating phone and the call terminating phone serves by different BTS's with different BSC's but the MSC is same as below

All details of Mobile Phones are recorded as a data base on MSC as below;

HLR(Home Location Register) - Database is used to store information about each subscriber
VLR(Visitor Location Register) - Database is determine the position of the subscriber that currently coverd by the switching center
AuC(Authentication Center) - Database is used for authentication activityes of the system
EIR(Equipment Identity Register) - Databes keeps track of the type of equipment that exist at the mobile station