nsmit does so without Figure 13.3 Effect of a on the idealised curve of Figure regard for any other user of the common transmission medium. Very little sensing or synchronisation mechanism is required, the prime requirement is for an acknowledgement system which can detect when the data has not been correctly received, usually due to a collision with information transmitted by another source, so that it Total information flow or throughput (S) can be retransmitted. Total capacity of the medium (C) In contention protocols any overhead involved in assigning chan- nel access is independent of the number of users, but depends (13.8) instead on the channel traffic. This system is most efficient when there are a large number of users, each sending small amounts of 'bursty' traffic. Multiple access Contention Non-contention contention minimisation Reservation ALOHA ALOHA Slotted ALOHA ALOHA with capture CSMA/CD Controlled ALOHA Asymmetrical protocols Contention rings Fixed Demand Reservation assignment assignment Polling Reservation TDMA Token passing Bit-map protocol Slotted ring BRAP/MSAP Register insertion Figure 13.4 Classification of multiple access techniques 13/6 Multiple access techniques 13.5.1 Pure ALOHA for up-link, down-link and processing delays) then it assumes a collision has occurred. The pure ALOHA multiple access system (usually referred to sim- Figure 13.6 illustrates the process of packet collision within ply as ALOHA) was developed by the University of Hawaii for use ALOHA. If any part of two packets overlap (e.g. the first bit of one with satellite based transmission systems. Its operation can be and the last bit of another) then they are both destroyed. This means explained by the simplified flow diagram of Figure 13.5. Whenever that there is a vulnerable period equal to the the sum of the two packet lengths (or twice a packet length, if all packets have the same length) when collision can occur. In Figure 13.6 packets from users B and D are seen to overlap and collide. After this has occurred it 