ed, the reservation slots begin again. If the load is low then reservation slots will repeat continuously until a user is ready to transmit and makes a reservation. If a user is ready to transmit data after its reservation slot has passed it must wait until the next cycle for it to reappear before it S-ALOHA can make a reservation and transmit. The worst case delay occurs for a user located towards the end (user F in Figure 13.36). On (1-Y) average it must wait N/2 reservation slots before it has access to its Throughput reservation slot and then, if all users have made reservations, it must wait a further (N-1) data slots before it can begin transmission. To this must be added the transmission propagation time and access Figure 13.34 Delay-throughput characteristic for slotted times, to obtain the total delay. Users who are allocated reservation ALOHA and reservation systems slots at the beginning of the reservation cycle (user A in Figure 13.36) only have an average delay of N/2 plus propagation and access delays. Reservation Data slots : = 3 slots 13.7.3.3 The BRAP and MSAP ABCDEFABCDEFABCDER The Broadcast Recognition with Alternating Priorities (Chlamtac, 1976) and the Mini Slotted Alternating Priorities (Scholl, 1976) protocols are very similar in operation, and overcome the two major limitations of the bit-map protocol. These are: Figure 13.35 Frame structure for R-TDMA 1. The delay under light load, caused by ready users who have made a reservation but have to wait until the end of the reser- vation cycle before they can transmit. tion slot and now there is no contention since no other user can 2. The asymmetry in the protocol, where some users get preferen- transmit in it. tial treatment over others, depending on their position within If the owner of a data slot does not wish to use it then it is assigned the reservation frame. to other ready users. This is usually done in a round robin fashion. Figure 13.37 illustrates the operation of the MSAP and BRAP protocols. As for the bit-map protocol reservation 