ously very important that interference should not be caused by the transmitted signal to the received one. Particularly the receiver must not be overloaded by the nearby strong transmitted signal. Although the GSM system has many similarities with the basic concept for a cellular telecommunications system, there are naturally many improvements that have been made. To enable different organizations to be able to develop compatible pieces of equipment, the GSM specifica- tions were set down. The original set of documentation extended to over 8000 pages, which although lengthy enabled the development to proceed relatively smoothly and to be adopted by many countries and organizations. The system supports several features that were not available on the old analogue services. The one that has gained the most popularity is the 264 Newnes Guide to Radio and Communications Technology short message service (SMS) that is very widely used, especially by the young for whom it is a cheap way of communicating. The mobile station (MS) consists of the mobile radio transceiver with display and the digital processing circuitry. It also contains a Subscriber Identity Module (SIM). This is a small card contained in the phone that enables the personal information set up on the phone to be transferred to another phone. It is particularly useful when upgrading phones. By transferring the card from one phone to another it enables items such as phone number, phone book and the like to be transferred. Two identities are associated with the phone. One is the International Mobile Equipment Identity (IMEI) and this is contained in the phone itself. The other is the International Mobile Subscriber Identity (IMSI) and this is contained in the SIM card. The IMEI and IMSI are independent and this allows for both the phone and the user to be identified separately. The base station subsystem (BSS) comprises two sections. One is the base station transceiver sub-system (BTS) and the other is the base station controller (BSC). Often one controller is