Carrier Sense Multiple Access with Message-Based Priority Functions

We consider packet communication systems of the multiaccess/broadcast type, exemplified by ETHERNET [1] and single-hop ground radio networks [2], in which all communicating devices share a common channel which is multiaccessed in some random fashion. Among the various random access schemes known, carrier sense multiple access (CSMA) has been shown to be highly efficient for environments where the propagation delay is short compared to the transmission time of a packet on the channel [3]-[5]. In this paper, we describe a new version of CSMA which incorporates message-based priority functions, referred to as prioritized CSMA (P-CSMA). The scheme is based on the principle that access right to the channel is exclusively granted to ready messages of the current highest priority level. It can be made preemptive or nonpreemptive, and is suitable to fully connected broadcast networks with or without the collision detection feature. We analyze the p-persistent protocol of P-CSMA with two priority levels and derive the throughput-delay characteristics for each priority class. Finally, we discuss numerical results obtained from the analysis and from simulation, and thus evaluate the effect of priority functions and preemption on the throughput-delay characteristics for each class.