An accurate closed-form formula for the throughput of long-lived TCP connections in IEEE 802.11 WLANs

Abstract

There is a vast literature on the performance modeling of the 802.11 MAC protocol, but the interplay between the TCP dynamics and the self-regulating behavior of the 802.11 CSMA/CA access method has not been sufficiently investigated. In addition, it has been observed that TCP stations in a WLAN are sporadically active due to the TCP flow control mechanisms. This makes traditional saturated models of the network capacity of 802.11 WLANs not very accurate. We presented a rigorous analytical model to calculate the distribution of the number of active TCP stations in a WLAN with multiple long-lived TCP flows [R. Bruno, M. Conti, E. Gregori, Performance modelling and measurements of TCP transfer throughput in 802.11-based WLAN, in: Proceedings of the IEEE MsWiM 2006, Torremolinos, Malaga, Spain, 2006, pp. 4-11, [1]]. Starting from this analysis, in this paper we derive a simple but accurate closed-form expression of the per-connection TCP throughout as a function of the average duration of collisions, the average backoff period and the TCP packet size. We validate this formula through performance tests carried out on a real WLAN. Then, we use our model to mathematically study the optimal minimum contention window size to maximize the TCP throughput. Our analytical results indicate that the initial window size specified in the 802.11b standard is about two times larger than the optimal one. However, we also show that the TCP throughput performance is not very sensitive to changes of the minimum contention window size, and that the TCP flows significantly underutilize the channel bandwidth also in the optimal MAC operating state.