Virtual Time CSMA: Why Two Clocks Are Better than One

A new carrier sense multiple access (CSMA) 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.

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