er Total channel bandwidth Input Maximum throughput = 1 where y< (13.35) Register Output to line Figure 13.34 compares the delay-throughput characteristics of slotted ALOHA and a general reservation systems. Slotted ALOHA reaches a maximum throughput of 1/e (0.368), as in Equation 13.16, Input whilst typical values of Y for reservation systems is 0.1 to 0.3, giving Buffer maximum throughputs from 0.7 to 0.9. However the delay within the reservation systems is higher than in ALOHA, especially when there are a relatively small number of users, and the data is 'bursty'. Figure 13.33 A simplified representation of the interface to a register insertion network 13.7.3.1 Reservation TDMA position is located by a pointer. As the register fills up the pointer moves to the left. Initially the pointer is at the extreme right, Reservation TDMA (Weissler et al., 1978) is similar to the reserva- indicating that the register is empty. Data is clocked into the register tion ALOHA system except that TDMA is used to access the one bit at a time and clocked out again. If the line is very busy the reservation slots. register will be full, but if it is lightly loaded then empty periods will The channel is divided into N reservation slots and kN data slots build up and there will be spare room in the register. per frame, as in Figure 13.35, where N is the number of users. Each When the user wishes to transmit data this is moved into the user is allocated one data slot within each of the k subgroups, for his buffer. It then checks to see there is sufficient room in the register, use, and one reservation slot. Reservations are made in the reserva- 13/20 Multiple access techniques Priority is now given to low valued users. The first station who puts a '1' in its slot transmits in the first data slot following the end of the reservation slots. In Figure 13.36 this is user A. The next slot is allocated to the next user in sequence who marked its reservation Reservation slot, i.e. user D. When all the users who reserved slots have transmitt