Simulation of message-based priority functions in carrier sense multiaccess/broadcast systems

The need for priority functions in Multiaccess Computer Communication Networks arises from applications with restrictions on packet delay. One alternative is to have a dedicated channel supporting very low loads to insure small delays; another alternative is to provide some priority mechanism by which packets that are time constrained can have priority over those which are not. One such mechanism, called Prioritized Carrier Sense Multiple Access (P-CSMA), recently proposed and analyzed by Tobagi (10-12), is studied here using simulation. The objective of this work is to extend the results obtained by the stochastic analysis and evaluate more completely the performance of P-CSMA. Three variations of the operation of the protocol are investigated, namely: nonpreemptive, semipreemptive and preemptive disciplines. In particular, we study the effect on average packet delay, packet loss and the variance of delay of several system parameters that prove to be interesting, such as: the number of stations, the number of buffers, the preemption discipline, etc. It is shown that priority functions indeed reduce packet delay, delay variance and packet loss for the high priority class.