is assumed that there is a finite time t which both senders take to detect that a collision has occurred. Available? They then wait random periods of time (T, 1 and T2) before retrans- mitting their data, which in this case does not collide with any other data on the line. The data which is transmitted from the source and eventually Random reaches its destination is the throughput S. The total traffic on the Transmit back-off line, which includes all retransmissions, is the offered traffic G, both delay usually being normalised to the channel capacity, as stated earlier. Also, as seen from Figure 13.6, collisions and retransmissions add to the delay between the transmission of a packet and it being Transmission correctly received at the source. successful? The performance of an ALOHA system have been analysed by many researchers (e.g. Abramson, 1977; Binder et al., 1975a). It will be assumed that there are a very large number of users (infinite) each sending small amounts of data, in the form of packets, with a Figure 13.5 Flow diagram of a pure ALOHA access Poisson distribution and having a mean of S packets per unit time. system For stability 0 < S < 1. The offered load (G) is also assumed to have a Poisson distribution and G S, with G 1 S when the load is low a data source has information to send, usually a packet of data, it and there are few collisions. It is also assumed that all users are does SO without regard for any other data source which may also be equal, i.e. they have equal probability of transmission. using the transmission medium. It then waits the appropriate The performance of an ALOHA channel is given by Equation 13.9 amount of time for an acknowledgement. If this is not obtained the and is plotted in Figure 13.7. source assumes that a collision has occurred. It then waits for a random amount of time before retransmitting the same information. Ge-20 (13.9) A random wait or back-off time is required to ensure that two sources, which have just collided, do not again collide when they From this i