Prioritized-Virtual-Time CSMA:Head-of-the-Line Priority Classes without Channel Overhead

摘要

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.Access to the channel is controlled by a novel algorithm that makes use of multiple,variable-rato 'clocks'.Unlike other approaches for incorporating priority classes in CSMA protocols,ours adds no channel overhead compared to the non-priority case.PVT-CSMA implements the well-known Head-of-the-Line (HOL) queueing 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.Using a novel analytical method based on the M/G/1 HOL queue and an additional Poisson approximation,we find simple expressions for both throughput as a function of offered load and mean delay as a function of throughput.The model is easily solveable for any number of priority classes and for ciassdependent messageengths,arrival rates,retransmissian rates,etc.Comparisons are made between our analysis and simulations of PVT-CSMA,and with simulations of other priaritized CSMA protocois.