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 Introduction:
 Background and Requirements:
At the beginning of the 1980s it was realized that the European countries were using many different, incompatible mobile phone systems. At the same time, the needs for telecommunication services were remarkably increased. Due to this, CEPT (Conférence European Posts Telecommunications) founded a group to specify a common mobile system for Western Europe. This group was named “Groupe Spéciale Mobile” and the system name GSM arose. This abbreviation has since been interpreted in other ways, but the most
Common expression nowadays is Global System for Mobile communications.
At the beginning of the 1990s, the lack of a common mobile system was seen to be a general, world -wide problem. For this reason the GSM system has now spread also to the Eastern European countries, Africa, Asia and Australia. The USA, South America in general and Japan had made a decision to adopt other types of mobile systems which are not compatible with GSM. However, in the USA the Personal Communication System (PCS) has been adopted which uses GSM technology with a few variations.
 Advantages of GSM:
Due to the requirements set for the GSM system, many advantages will be achieved. These advantages can be summarized as follows:
 GSM uses radio frequencies efficiently, and due to the digital radio path, the system tolerates more intercellular disturbances.
 The average quality of speech achieved is better than in analogue cellular systems.
 Data transmission is supported throughout the GSM system.
 Speech is encrypted and subscriber information security is guaranteed.
 Due to the ISDN compatibility, new services are offered compared to the analogue systems.
 International roaming is technically possible within all countries using the GSM system.
 The large market increases competition and lowers the prices both for investments and usage.
 GSM has two frequency bands:
1) 900MHz band, the frequencies are,
890-915MHz (uplink)
935-960MHz (downlink)
2) 1800MHz band, the frequencies are,
1710-1785MHz (uplink)
1805-1880MHz (downlink)
 Evolution of GSM:
Year Milestone
1982 GSM formed
1986 field test
1987 TDMA chosen as access method
1988 memorandum of understanding signed
1989 validation of GSM system
1990 Pre operation system
1991 commercial system start-up
1992 coverage of larger cities/airports
1993 coverage of main roads
1995 coverage of rural areas
 ARCHITECTURE OF GSM NETWORK:
In a GSM Network, this decentralized intelligence is implemented by dividing the whole network into three separate subsystems:
•Network Switching Subsystem (NSS)
•Base Station Subsystem (BSS)
•Network Management Subsystem (NMS)
Nowadays, GSM specifications define two truly open interfaces. The first one is between the Mobile Station and the Base Station. This open air interface is appropriately named the “Air interface”. The second one is between the Mobile Services Switching Centre – MSC (which is the switching exchange in GSM) and the Base Station Controller (BSC). This interface is called the “A interface”.