fected by whether an end of use flag is included or not in the header of the last packet sent. Such a flag allows users to contend for the slot at the then the vulnerable period is reduced to t since transmission must next frame rather than having to wait a frame to discover whether it start at the start point of each slot. t is small for low propagation is empty. Equation 13.20 gives the throughput when no flag is delays on the medium. included and 13.21 when it is included. Both equations compare the CSMA can operate in three modes, 1-persistent, non-persistent throughput of the reservation ALOHA with the throughput (Ss) of a and p-persistent. In addition there is a variant called CSMA/CD, as slotted ALOHA system. described in the following sections. (13.20) For no flag 13.6.2.1 1-persistent CSMA In the 1-persistent CSMA all users listen to the line prior to trans- mitting. If they sense traffic they wait. When the line becomes free (13.21) the users who have been waiting transmit immediately, i.e. transmit For flag with probability one. Collisions will occur if two stations start transmission simultaneously, or if the delay in the transmission channel is such that after one channel has started transmission the second ready channel has not heard this and also starts transmission. In these equations k is the average number of packets transmitted After a collision the stations wait a random amount of time before before the user gives up his reserved slot. It varies between 1 and listening to the channel again. infinity. If all transmitted packets are assumed to be of constant length and The maximum throughput of a reservation ALOHA(S, is given by Equations 13.22 and 13.23, where the value for slotted ALOHA the load (G) has a Poisson distribution, then the throughput is given by Equation 13.24. It is also assumed in this calculation that all users is given by can sense the transmission of all other users. Contention minimisation systems 13/11 a = 0.2 a=0.4 a=1.0 Offered load (G) Figure 13.14 Effect of channel prop