Short-term fairness for TCP flows in 802.11b WLANs

Wireless local area networks (WLANs) based on the IEEE 802.11 technology are becoming increasingly popular and widely deployed. However, the growing need for quality of service (QoS) guarantees is difficult to implement in distributed systems like WLANs, where the random access protocol and the unpredictability of the wireless channel hamper their interaction with well-established architectures like DiffServ. Even without trying to provide deterministic QoS guarantees, simpler requirements are hard to get by. For example, the basic requirement of providing fair access to all users is conflicting with the nature of higher-layer protocols: TCP is fair only under certain conditions, hardly met by 802.11b WLANs. Another basic requirement is the protection for short-lived TCP flows, that are sensitive to losses during the early stages of the TCP window growth. The main contribution of this paper is the proposal of an LLC-layer algorithm that can be implemented at both access point (AP) and wireless stations (WSs). The algorithm aims at guaranteeing fair access to the medium to every user, by awarding longer transmission opportunities to WSs that experienced short channel failures. At the same time, such award mechanism can protect short-lived flows while they strive to get past the critical "small window regime". We outline the proposed solution and present a simulation study that shows the effectiveness of the new algorithm in comparison to the standard 802.11b implementation.