he address codeword. There are a number of different types of control channel messages that can be sent by the base station to the mobiles: Aloha messages Sent by the base station to invite and mobile stations to access the system. Requests Sent by radio units to request a call to be set 'Ahoy' messages Sent by the base station to demand a response from a particular radio unit. This may be sent to request the radio unit to send his unique identifier to ensure it should be taking traffic through the base station. Acknowledgements These are sent by both the base stations and the mobile radio units to acknowledge the data sent. Go to channel messages These messages instruct a particular mobile radio unit to move to the allocated traffic channel. Single address messages These are sent only by the mobile radio units. Private mobile radio 243 Short data messages These may be sent by either the base station or the mobile radio unit. Miscellaneous messages Sent by the base station for control applications. One of the problems encountered by mobile signalling systems is that of clashes when two or more mobile radio units try to transmit at the same time on the control channel. This factor is recognized by the system and is overcome by a random access protocol that is employed. This operates by the base station transmitting a synchronization message inviting the mobile radio units to send their random access message. The message from the base station contains a parameter that indicates the number of timeslots that are available for access. The mobile radio unit will randomly select a slot in which to transmit its request but if a message is already in progress then it will send its access message in the next available slot. If this is not successful then it will wait until the process is initiated again. Although the data is transmitted as digital information, the audio or voice channels for the system are analogue, employing FM. However, some work has been carried out to develop completely digital systems. The main system