Prioritized-virtual-time CSMA: head-of-the-line priority classeswithout added overhead

The prioritized-virtual-time carrier sense multiple access (PVT-CSMA) protocol is a media access algorithm for cable- and radio-based local area networks that incorporates message-based priority classes. PVT-CSMA implements the head-of-the-line (HOL) priority queuing discipline, in which higher priority messages are always transmitted in preference to lower priority messages, and messages are transmitted first-come-first-served within each priority class. Expressions are obtained for both throughput as a function of offered load and mean delay as a function of throughput for asynchronous (unslotted) PVT-CSMA, which are easily solvable for any number of priority classes and for class-dependent message lengths, arrival rates, retransmission rates, etc. Specialization of the delay analysis to the single class case yields a substantial improvement over previous results for asynchronous virtual-time CSMA. Comparisons are made between this analysis and simulations of PVT-CSMA, and with simulations of Tobagi's P-CSMA (1982) protocol