Analysis of the saturation throughput and node contention levels in the 802.11 MAC protocol

Several analytical models were proposed in the literature to provide throughput performance evaluation of the 802.11 MAC protocol. The performance parameter “Saturation Throughput” is defined as the limit reached by the system throughput as the offered system load increases. This parameter is evaluated under overloaded conditions, i.e., the transmission queue of each station is assumed to be always nonempty. Our extensive simulation results obtained using NS2 for the p-persistent 802.11 MAC protocol show different behavior. The p-persistent throughput linearly increases with the load to a certain point and reaches a constant saturation throughput. Unlike the standard 802.11, there is no peak value followed by a drop in the throughput of the p-persistent 802.11. Our simulation results clearly show that the saturation throughput is often attained without 100% contention among all the nodes. This is an important observation that contradicts the assumption used in the literature for the analytical modeling of the 802.11 MAC protocol. We are currently using this observation in the design of the load estimation component of the admission control mechanism for WLANS.