PRIORITIZED-VIRTUAL-TIME CSMA - HEAD-OF-THE-LINE PRIORITY CLASSES WITHOUT 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 well-known 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. This priority discipline works by altering the way in which stations manipulate the "virtual clocks" that scan the time axis for message arrivals, to ensure that higher priority messages are encountered by the virtual clocks before lower priority messages. Thus, unlike other approaches for incorporating message-based priority classes in CSMA protocols, ours works implicitly, without adding any channel overhead compared to the nonpriority case. 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 our delay analysis to the single class case yields a substantial improvement over our previous results for asynchronous virtual-time CSMA. Comparisons are made between our analysis and simulations of PVT-CSMA, and with simulations of Tobagi's P-CSMA protocol.