The Efficacy of Source Rate Control in Achieving Fairness in Wireless Mesh Networks
The use of 802.11-based wireless mesh networks (WMNs) as an alternative network backbone technology is growing rapidly. The primary advantages of this approach are ease of deployment and lower cost. However, such networks typically exhibit poor fairness properties, often starving nodes if they are too many hops distant from the gateway. Researchers have shown a growing interest in this problem in recent years. Many solutions proposed amount to some level of source rate control, either by policing directly at the source, or via TCP congestion control reacting to a gateway-enforced rate limit. However, there has been limited study on the effectiveness of source rate control. In this thesis we first demonstrate that source rate control can only partially solve the fairness issue in 802.11-based WMNs, with some routers experiencing an undesirable degree of unfairness, which we call structural unfairness. We then identify the four necessary factors that cause structural unfairness. If we can eliminate or reduce any one of these conditions, we can eliminate or ameliorate the unfairness problem. We first investigate two techniques to improve 802.11 MAC scheduling: fixing the contention window and packet spacing at every router node, both means achievable with commodity 802.11 hardware. We show that the combination of these mechanisms provides a significant gain in fairness. We also perform case studies using another three techniques, channel re-assignment, routing changes, and careful router placement, to remove or reduce other necessary conditions. We demonstrate that these techniques, whenever applicable, can eliminate the unfairness problem entirely at times, or at least improve the situation.