 with slotted channels. When a user has CSMA/CD algorithms require that each user which detects a colli- data to transmit it senses the channel and if it is free it transmits with sion transmit a short jamming signal to immediately inform all other probability p i.e. delays transmission to the next slot with prob- users that a collision has occurred on the line. ability 1-p. If the next slot is idle then the user will again transmit with 13.6.3 Asymmetrical protocols probability p, and SO on. If the slot is not free at any time then the user waits a random amount of time and starts again. If initially the We have so far been considering the case where all users have an user had found that the slot was busy then it would have waited until equal probability (p) for transmission. This is know as a symmetri- the next slot and applied the algorithm. cal protocol. If somehow it were possible to divide the users into a The throughput of the system is given by Equation 13.25, where smaller number of groups, even if this meant splitting the overall K is given by Equation 13.26. channel capacity, then it can be shown that there is a greater overall chance of users being able to transmit without contention, resulting S -GE"(1+PGK) (13.25) in enhanced throughput. The ideal system would divide all users in such a way that in any one group there is only one active user, so that the chance of a successful transmission is assured. The trick is in determining a 13/12 Multiple access techniques 0.1 persistent CSMA Nonpersistent CSMA Slotted ALOHA 1 persistent CSMA Offered load (G) Figure 13.15 Comparison of channel throughput for ALOHA and CSMA, with channel delay factor a = 0.01. 1 persistent CSMA Slotted Nonpersistent CSMA ALOHA Level 3 Level 2 Optimum p-persistent CSMA Level 1 Figure 13.17 Operation of an adaptive tree walk protocol at node 2 then the protocol moves further up the tree to Level 3 and Throughput node 4, limiting the users to A and B. By moving up the tree each time a collision occurs, the protocol eventually fin