Virtual Time CSMA: Why Two Clocks Are Better than One

A new carrier sense multiple access (CShiA) algorithm, called virtual time CSMA, is described and analyzed. This algorithm uses a novel approach to granting access to the shared broadcast channel based on variable-rate clocks. Unlike other CSMA algorithms, the operation of virtual time CSMA reduces to the ideal case in the zero propagation time limit: a work-conserving, first-come first-served M/G/1 queueing system. The algorithm does not appear to be difficult to implement, but offers better throughput-delay performance than existing CSMA algorithms. A simple closed form technique for estimating the mean message delay is presented. This technique is of independent interest because of its applicability to certain " sliding window " tree conflict resolution algorithms. Extensive numerical results for the algorithm are presented, including comparisons with simulation and with other CSMA algorithms.