 has to be standardized SO that suppliers can manufacture in higher volumes and thereby reduce costs to acceptable levels. Most trunked radio systems follow the same format. This is most widely known as MPT 1327. To implement trunked PMR a network of stations is set up. These stations are linked generally using land lines, although optical fibres and point to point radio are also used. In this way the different base stations are able to communicate with each other. Private mobile radio 241 In order to be able to carry the audio information and also run the variety of organizational tasks that are needed the system requires different types of channel to be available. These are the control channels of which there is one in each direction for each base station or trunking system controller (TSC). A number of different control channels are used SO that adjacent base stations do not interfere with one another, and the mobile stations scan the different channels to locate the strongest control channel signal. In addition to this there are the traffic channels. The specification supports up to 1024 different traffic channels to be used. In this way a base station can support a large number of different mobile stations that are communicating at the same time. However, for small systems with only a few channels, the control channel may also act as a non-dedicated traffic channel. The control channels use signalling at 1200 bits per second with fast frequency shift keying (FFSK) sub-carrier modulation. It is designed for use by two-frequency half duplex mobile radio units and a full duplex For successful operation it is essential that the system knows where the mobiles are located SO that calls can be routed through to them. This is achieved by base stations polling the mobile stations using the control channel. To make an outgoing call the mobile transmits a request to the base station as requested in the control channel data stream from the base station. The mobile transmits its own code along with that of the destinatio