TCP performance optimization in multi-cell wireless local area networks

In this paper, we consider the throughput allocation problem in an extended (or multi-cells) IEEE 802.11 wireless network. We first demonstrate the severe throughput imbalance that can take place between downlink TCP flows even in simple multi-cell WLANs. Then to solve this unfairness problem, we derive an analytical model that describes the interaction between different TCP flows at the MAC layer, and formulate the throughput allocation problem as a nonlinear optimization problem subject to certain fairness requirements. Real world complexity such as hidden terminals, packet transmission retry limit, and the unique characteristics of TCP traffic are considered. Unlike alternative approaches that rely on modifying the TCP sender or implementing active queue management at the network layer, solving our optimization problem yields the optimal MAC layer contention windows settings that can lead each TCP flow to its target end-to-end throughput. Among the practically appealing characteristics of this approach are: i) its locality and the possibility of implementing it at the AP and within the ESS, whereas alternative approaches rely on changing the TCP traffic source; and ii) its simplicity, i.e., it is achieved by controlling a single parameter per-node, whereas alternative approaches based on AQM require the tuning of many parameters and achieve mitigated results. Simulation results show that our approach can achieve a fair throughput allocation and attest to the accuracy of our proposed method. Copyright 2009 ACM