Public Access Mobile Radio systems (PAMR) using trunking technology allow more efficient use of the frequency spectrum, as all the users of the system jointly use a common pool of channels. This counteracts one of the limiting factors on the growth of conventional Private Mobile Radio systems (PMR), frequency congestion [1]. Moreover the public trunked network offers the user many advantages over the private network. The connection to the public is simple and it is easier for small organizations to start up a radio operation with this network than with a private network.

European PAMR operators are encouraged to use signalling standards such as the current MPT 1327 or the upcoming TETRA [2], capable of supporting a number of advanced network features which include automatic registration and radio unit location, call-queueing when radio channels are busy, individual, group and fleet calls, and data transmission. As it is public, the operator limits the call duration to favour an equal share of the resources (radio channels) available. Many users coming from the conventional mobile radio systems, where no time limitation exists, need a training period to get used to this characteristic, as it obliges them to change their talking habits. As trunked PAMR systems can operate on two different levels of management [2], on the basis of messages (message trunking) and transmissions (transmission trunking), an examination of message and transmission holding time has been made. When using message trunking, the system allocates the radio channel from the beginning to the end of the conversation. On the other hand, with transmission trunking, the system allocates a radio channel when one partner presses his or her push-to-talk button (PTT) and deallocates the radio channel when he or she releases the PTT button to change talking partners. Thus, during the silence produced by this change, no channel is occupied. The advantage of transmission trunking is that there is no waste of channel occupancy during the pauses in the conversation. The drawback is that a more complex control system with more signalling is needed, where the distribution of the transmission occupancy is more crucial in the performance than in the distribution of the message holding time when trunking by message. In message trunking, the lack of radio channel only causes a delay and the user has to wait to begin the conversation, while in transmission trunking, it can produce an unpleasant effect of clipped words.