by February 7, 2003 0 comments

During the 1980s, various European countries like UK, France and Germany were developing and implementing mobile technologies in their homelands. With all these independent efforts, a need was felt to consolidate and develop a standard so that the R&D could be combined, and economies of scale realized. Hence in 1982 a study group called (Groupe Special (with accented e) Mobile or GSM was formed. The developers of GSM chose an unproven (at the time) digital system, and provided 8000 pages of recommendations that had enough flexibility to allow competitive innovation, but at the same time ensured proper internetworking between the components of the system. 

At present, there are hundreds of functional GSM networks in an equally large number of countries, and the acronym now aptly stands for Global System for Mobile telecommunications.

A GSM based PLMN (Public Land Mobile Network) can broadly be broken up into 3 parts:

  • The Mobile Station
  • The Base Station Subsystem, and
  • The Network Subsystem
The GS of telecommunication
1G systems used analogue modulation and provide only for voice transmission.
2G (second generation) GSM provides voice and limited data services and uses digital modulation with improved audio quality.
2.5G systems recently introduced enhance the data capacity of GSM and
mitigate some of its limitations. 
The new 3 G (third generation) cellular services known as UMTS (Universal Mobile Telecommunications System) or IMT-2000 will sustain higher data rates and opens the door to many Internet style applications.

The Mobile Station
The MS (Mobile Station) is the subscriber end of the network and consists of the GSM device (cellphone) and the SIM (Subscriber Entity Module). Each mobile device has a globally unique IMEI (International Mobile Equipment Identity) and each SIM has a IMSI (International Mobile Subscriber Identity), which is used to identify and authorize the subscriber for various services. The SIM card may be protected against unauthorized number using a PIN (Personal Identity Number). 

Base Station Subsystem (BSS)
Mobile networks consist of many BTS (Base Transceiver Stations), each of which form a “cell” covering a certain geographical area. The BTSs are the ones responsible for directly communicating with the mobile devices. Several BTSs are controlled together by a BSC (Base Station Controller). The BSC provides connectivity to the network subsystem.

Network subsystem
The MSC (Mobile services Switching Center) is the central component of the network subsystem. It communicates with all the BSCs and is responsible for providing all the functionalities to mobile devices like, registration, authentication, call routing etc. A network also, usually, has GMSC (Gateway MSCs) to communicate with external (landline) networks like ISDN, PSTN etc.

Many registers are also maintained which contain information necessary for smooth functioning of the network. The HLR (Home Location Register) stores information about the current location of all subscribers of the network. This information is necessary for routing all calls/ messages to their intended destinations. A VLR (Visitor Location Register) covers one or more cells and stores information about the subscribers currently under its area of influence. The EIR (Equipment Identity Register is used to authenticate and store equipment, rather than subscriber data, while the AUC or AC (Authentication Center) stores and validates the users’ PIN. The MSC communicates with these registers whenever it needs some information.

Making a call
When a mobile unit is switched on, it tries to contact a BTS that is in range and logs onto the network. The BTS also transmits the frequencies of neighboring BTSs to the mobile unit so it can determine if it’s getting better reception from any of them. It transmits these measurements back to the BTS, which in turn passes this information to the BSC. The authority to let the phone switch BTSs lies with the BSC, since it has a much wider view of what is going on in the network. For an incoming call, the HLR finds out which VLR knows where the phone is, and the VLR then sends a signal on the unit’s paging channel. The unit responds, and the call is set up.

Other services 
In addition to voice calls, GSM networks also provide many other services, and we are familiar with most of them. There’s the ubiquitous SMS–the capability to send short text messages from one phone to another. There are the data services, which exploit GSM’s digital nature. This allows one to access the Internet from a GSM phone, and many service providers are offering this service. You can send email from the handset and lots more. One limitation of GSM is that currently it has a maximum data transfer rate of only 9 kbps, but new developments like HSCSD (high-speed circuit switched data) and GPRS (general packet-switched radio) are pushing this up quite a bit. 

Kunal Dua with Anuj Jain

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