Modeling Fairness And Clock Drifts Under Synchronized CSMA Contention

A broad class of CSMA protocols use synchronized contention in which nodes periodically contend at intervals of fixed duration. Both analysis and operation of these protocols assume perfect clock synchronization. However, in practice, clocks are not perfectly synchronized and drifts can vary widely, depending on the synchronization mechanism used in the network. We introduce an analytical model that quantifies the interplay of clock drifts with key performance factors such as contention window size, carrier sense, and usage of guard time. Utilizing this model, we investigate the isolated and combined impact of these factors to per-flow throughput and fairness properties in both single-hop and multi-hop networks. Our model reveals conditions on protocol parameters under which the throughput of certain flows can exponentially decrease with clock drift; while at the same time, it enables solutions that can offset such problems in a predictable manner.