t is seen that the maximum throughput of an ALOHA retransmit their data. Many algorithms are in use for calculating this channel occurs when G = 0.5 and is equal to 0.184, i.e. the channel random time. Also the acknowledgement process varies depending utilisation is limited to 18.4%. This is the maximum possible utili- on the transmission system being used. For broadcast systems, such sation since, as seen later, the delay in packet transmission reaches as satellites, the sender listens for its message to be broadcast from unacceptable values as this theoretical utilisation figure is ap- the satellite, and if it does not occur within a given time (allowing proached. Retransmission Retransmission Total Collision Figure 13.6 Illustration of packet collision and retransmission in ALOHA Pure contention systems 13/7 Slotted ALOHA ALOHA G (channel traffic) Figure 13.7 Throughput in pure ALOHA and slotted ALOHA channels If the offered load exceeds 0.5 the number of collisions increases Figure 13.8 shows the plot of delay for a systems operating over at a rapid rate, so that although the offered load is increasing this is channels with significant and variable delay. This also indicates the made up of retransmitted traffic and the useful traffic, or throughput, very large delays which are experienced as the theoretical maximum actually decreases, until it eventually falls to zero. throughput of the ALOHA channel is approached. Although ALOHA has low utilisation it is frequently used for In order to increase throughput the retransmission should be systems which have many users who are transmitting low volume randomised over longer intervals, to reduce the probability of colli- bursty traffic. sions on retransmission. This means that the value of x, in Equation If the average length of a message is equal to T seconds, the 13.13, should be large. However, as seen from Equations 13.14, this average propagation delay in the medium is T seconds, and the would also result in increased delay, SO that design of multiple average n