ires a high degree of control. To enable all the switching and control functions to take place the base station is linked to control and switching centres. To avoid all this being routed back to very large centralized centres, some of this is done locally. Typically there are centres sometimes called mobile telephone exchanges (MTX) or mobile switching centres (MSC) that control a number of base stations and smaller localized controllers often called base station controllers (BSC) that might control a small group of cells, possibly a complete cluster. Monitoring and routing of the mobile phone is first accomplished by the local base station controller. Once the mobile is about to move out of the set of base stations controlled by the localized controller it hands control to the larger switching centre. However, it should be remembered that different mobile phone systems may operate in slightly different ways although the underlying principles are the same. Control In order that a cellular phone system is able to work, the base station has to be able to control many aspects of the operation of the phone. Which frequencies are to be used, knowledge of where the mobile handset is SO that calls can be routed to it, and many other functions all need to be accomplished for it to be able to operate correctly. In order to achieve these control functions most systems have dedicated control channels that are used purely for this purpose and they do not carry any voice or other user data. The first function that must be completed when the mobile handset is turned on is that of registration. After the phone has performed its own internal power on self-check it scans the channels for the strongest control channel. Once it finds this it identifies itself on the uplink to the base station. Data including the individual phone ID and the number is sent regardless of whether a call is to be made. This information is then recorded back at the main system. In this way the system can register that the phone is within a particular cel