e a shorter walk time. Generally they central station then allocates the data slots to users according to their have better performance than polling (Kleinrock and Scholl, 1977) priority and demand. There are usually three classes of traffic, but are more complex since users need to be intelligent to make and priority, normal and bulk, and these are served in that order. recognise reservations, and be synchronised to the reservation slots. Fixed assignment can be used to access the reservation slots, when the protocol is called Fixed assignment PODA (FPODA). Alterna- tively, when there are a large number of users, with light traffic, 13.7.3.4 contention can be used for access to the reservation slots, the system being called Contention PODA (CPODA). However a central sta- The Multi-level Multi-Access protocol (Rothauser and Wild, 1977) tion still manages the allocations based on reservations. overcomes the problem of high delay on light loads, experienced by MSAP/BRAP when there are many users. For example for a 1000 user system the worst case delay could be 1000 reservation time 13.8 References slots with a mean of 500 slots. In MLMA a user broadcasts its reservation in a given format, for Abramson, N. (1977) The Throughput of Packet Broadcasting example radix 10, as illustrated in Figure 13.38 for 100 users. Channels, IEEE Trans. Commun., COM-25, January. Suppose that users numbered 45, 49, 66 and 61 wish to transmit Betsekas, D. and Gallager, R. (1987) Data Networks, Prentice-Hall. data. They will first broadcast their most significant station number Binder, R. et al. (1975a) ALOHA Packet Broadcasting - A Retro- in Frame A. The system now knows that any number of twenty users spect, Proc. NCC. in the 40s and 60s group wish to transmit. Binder (1975b) A Dynamic Packet Switching System for Satellite In Frame C the 40s users are invited to place their reservations, Broadcast Channels, Proc. ICC, pp. 41-1 to 41-5a. indicating their second digit. Stations 45 and 49 do SO and are Black, U. (1989) Data Networks: Conc