by Equation 13.18. 13.5.4 Controlled ALOHA (13.18) An ALOHA channel can rapidly move into congestion if the traffic approaches the theoretical maximum, due to collisions and retrans- missions. Several methods have been devised (Lam and Kleinrock, Figure 13.9 compares the delay curves between a typical pure 1975) for controlling the traffic SO as to reduce congestion. The ALOHA and slotted ALOHA system, where it is seen that for low prime methods are to reduce the probability of retransmission of throughput (few users and low traffic) pure ALOHA has a lower collided packets in a time slot, SO reducing G, and to deny certain delay. users the right to transmit over a period of time, which in effect reduces the number of users. 13.5.3 ALOHA with capture ALOHA with capture is a modification to the basic slotted ALOHA 13.6 Contention minimisation systems system (Roberts, 1975) which works on the principle that if two signals have different strengths, then on collision the stronger one In pure contention systems a user takes no account of other users, will be 'captured' by the receiver and will get through, SO that only and therefore transmits when it has any data available. There are the weaker signal needs to be retransmitted. This, in effect, reduces however several multiple access protocols which, although basi- the retransmissions by half, and gives almost a 50% increase in cally of the contention type, attempt to minimise the amount of throughput over slotted ALOHA, as seen by Equation 13.19. It is contention between users. Some of these contention minimisation assumed that if three stations collide none of them get through, i.e. systems are described in this section. no one station is strong enough to dominate any two others. 13.6.1 Reservation ALOHA S = 1 (13.19) This multiple access method is very similar to the ALOHA (pure and slotted) systems described in the previous section, but uses Contention minimisation systems 13/9 Slotted ALOHA with capture Slotted ALOHA Unslotted ALOHA Offered load (G) Figure 13.10 C