In both FDMA and TDMA systems, channels should not be assigned to a mobile on a permanent basis. A fixed assignment strategy would either be extremely wasteful of precious bandwidth or highly susceptible to co-channel interference. Instead channels must be assigned on demand. Clearly, this implies the existence of a separate up-link channel on which mobiles can notify the base station of their need for a traffic channel. This up-link channel is referred to as the random-access channel because of the type of strategy used to regulate access to it.
Figure 3: Mobile-originating call establishment
The successful procedure for establishing a call that originates from the mobile station is outlined in Figure 3. The mobile initiates the procedure by transmitting a request on the random access channel. Since this channel is shared by all users in range of the base station a random access protocol, like the ALOHA protocol, has to be employed to resolve possible collisions. Once the base station has received the mobile's request it responds with an immediate assignment message which directs the mobile to tune to a dedicated control channel for the ensuing call setup. Upon completion of the call setup negotiation a traffic channel, i.e., a frequency in FDMA systems or a time slot in TDMA systems, is assigned by the base station and all future communication takes place on that channel. In the case of a mobile-terminating call request, the above sequence of events is preceded by a paging message alerting the mobile of the call request.
