is therefore given by Equation 3.95, where Y is the total The protocol has only two steps: transmission attempt rate. The channel is "sensed". If it is found to be idle the packet is Pr { Successful transmission transmitted immediately. = Pr { No transmission in vulnerable period (3.95) 2. If the channel is "sensed" busy the packet transmission is rescheduled using the retransmission distribution. At the end of the delay period the protocol moves to step 1. The total throughput is the rate of making successful trans- Figures 3.13(a) and 3.13(b) show successful and unsuccessful missions, as in Equation 3.96. transmissions respectively. The analysis for non-persistent CSMA is as follows. During the start of a transmission there is a short Throughput = Transmission attempt rate period during which other transmissions may start before all other Prohability of success mobiles become aware that the transmission has started. This inter- val, in units of packet transmission time is T as already mentioned. ye-2y (3.96) Making the usual assumption that transmission attempts are taking place at rate a as a Poisson process, the probability that the packet This is maximum for Y = 1/2 SO that the throughput cannot exceed is transmitted successfully is that there are no further transmission 1/2e. Note that the total rate of transmissions and retransmissions is attempts during this vulnerable period, which is given by expression being modelled as a Poisson process of rate Y. Even if the process of 3.97. fresh transmissions is Poisson. the assumption that the retrans- missions are independent Poisson can only be justified if the retrans- (3.97) missions take place at random over long intervals following the collisions which give rise to them. The throughput can be increased by adopting a slotted version of Contending packets may arrive at any time during the initial this protocol SO that packets can only be transmitted at the beginning vulnerable period and it is the last of these arrivals which determines of each slot which was 