Global System for Mobile Communications.

 In the early 1980, analog cellular telephone systems were experiencing a period of rapid growth in western Europe, particularly in Scandinavia and the United Kingdom and to a lesser extent in France and Germany . Each country subsequently developed its own cellular telephone system.Which was incompatible with everyone else 's system operated at different frequencies , and all were analog .

 In 1982, the Conference of European posts and Telegraphs formed a study group called Groupe Special Mobile(GSM) to study the development of a pan- European public land mobile telephone system using ISDN. In 1989, the responsibility of GSM was transferred to the European Telecommunications Standards Institute (ETSI) , and phase I of the GSM  specifications was published in 1990.

GSM has the advantage of being designed from scratch with little or no concern for being backward compatible with any existing analog cellular telephone system. GSM provides its subscribers with good quality , privacy , and security. GSM is sometimes refferred to as the Pan -European celluar system.

The first GSM system developed was GSM -900 , which operates in the 900-MHZ band for voice only. Phase 2 was introduced in 1995 , which included facsimile, video, and data communications services. After implementing PCS frequencies in 1997 GSM -1800 and GSM-1900 were create GSM is a second-generation cellular telephone system initially developed to solve the fragmentation problems inherent in first- generation cellular telephone systems in Europe.

GSM telephone services can be broadly classified into three categories: bearer services , teleservices, and supplementary services.Probably the most basic bearer service provided by GSM is telephony.

With GSM analog speech signals are digitally encoded and then transmitted through the network as a digital data stream.There is also an emergency service where the closest emergency service provider is notified by dialing three digits similar to 911 services in the United States.

A wide variety of data services is offered through GSM, where users can send and receive data at rates up to 9600 bps tp subscribers in POTS, ISDN networks , packet switched public data networks , and circuit switched public data networks using a wider variety of access methods and protocals such as X.25. In addition, since GSM is a digital networks , a modem is not required between the user and the GSM network.