GSM TECHNOLOGY
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INTRODUCTION
The development of GSM started in 1982 when a study group ‘Group Special Mobile’ was formed during Conference of European Posts and Telegraphs (CEPT) this group was to develop a Pan-European public cellular system in the 900 MHz range. Some of the basic criteria for their proposed system were:
• Good subjective speech quality
• ISDN compatibility
• Spectral efficiency
• Support for international roaming
• Support for range of new services and facilities
In 1989, GSM responsibility was transferred to European Telecommunication Standards Institute (ETSI) and commercial service was started in mid 1991. Although GSM was standardized in Europe, now it is operational in other continents also. The acronym GSM now aptly stands for Global System for Mobile Communication.
CELLULAR CONCEPT
Cellular radio was devised in order to make better use of limited resource of Radio Spectrum. Each Megahertz of spectrum will only support a comparatively a small number of simultaneous conversations and the same frequency must be reused many times in order to meet the capacity needed for national or regional service. Cellular radio achieves this by creating a honeycomb of cells over the region and assigning frequencies and
power levels in such away that the same frequency can be reused for different conversation two or three cells away.
Although most of the countries in Europe had their own analogue cellular system, they were not compatible with each other. So with the development in Political, Commercial and Industrial areas there arose a necessity for uniformity in cellular communication.
SYSTEM ARCHITECTURE
The functional architecture of a GSM system can be broadly classified into
• Mobile Station (MS)
• Base Station Subsystem (BSS)
• Network and Switching Subsystem (NSS)
• Operation Subsystem (OSS)
The MS and the BSS communicate via the Um interface or radio link. The BSS communicates with Mobile Service Switching Center across the A interface.
MOBILE STATION
This may be a standalone piece of equipment for certain services or support the connection of external terminals. The MS consists of the Mobile Equipment (ME) and a Subscriber Identity Module (SIM).
The ME is uniquely identified by the International Mobile Equipment Identity (IMEI), but it need not be personally assigned to one subscriber, The SIM which is a smart card provides personal mobility and the user can access the subscriber services. The subscriber can operate on any terminal just by inserting the SIM card in that GSM terminal. SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication and other security information’s. SIM card may be protected against unauthorized use by a password.
BASE STATIONSUBSYSTEM
The BSS is composed of three parts, Base Transceiver Station (BTS) and U3ase Station Controller (BSC). These two communicate across the standardized Abis interface. The third part is Transcoder and Rate Adaptation Unit (TRAU).
Base Transceiver Station:
This provides the GSM radio coverage within a cell. It comprises of radio transmitting and receiving equipment and associated signal processing units. This complements the radio features of ME.
Base Station Controller
This manages the radio resources for one or more BTS’s. It handles radio channel set-up, Handovers and frequency hopping. Handovers between BTS’s belonging to different BSC’s however can involve MSC’s but are still managed by the original serving BSC. It controls the transmission of information about Local Area Codes (LAC), signaling channel configuration and information’s about neighboring cells.