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Access time control for TCP fairness in WLANs

Wireless local area networks (WLANs) based on IEEE 802.11 standard have been widely used in infrastructure and ad hoc networks. Although the MAC layer of 802.11 WLAN provides fairness in accessing the wireless channel, the channel capacity is allocated unfairly among stations and flows due to effect of the upper layer protocols such as TCP and asymmetric network conditions. In this dissertation, this unfairness problem of TCP traffic in the IEEE 802.11 based infrastructure and ad hoc networks is studied. In the first part of the dissertation, the per-station fairness in infrastructure WLANs is studied. Under the 802.11 distributed coordination function (DCF), there exists unfairness between uplink and downlink TCP flows in infrastructure WLANs. In addition, when each station has a different number of flows, the channel capacity is allocated unfairly among stations. In order to resolve these problems, the Access Time Control (ATC) scheme and the Distributed Access Time Control (DATC) scheme are proposed. In the ATC scheme, an access point (AP) controls the rate of TCP flows based on the channel access time of each station so that it can provide per-station fairness regardless of the number and direction of flows in each station without sacrificing throughput. On the other hand, in the DATC scheme, each station controls the rate of its TCP flows based on the channel access time without coordination of the AP. By applying these schemes, per-station fairness among stations can be achieved, and the QoS of traffic with high priority can be protected from the best-effort traffic in the 802.11e networks. Also, the network throughput decreases little even when there exist stations with low transmission rate. Since the proposed schemes do not require any modification of the MAC and TCP layers, they can be easily implemented in current devices. The properties of the proposed schemes are investigated and their effectiveness is verified through ns2 simulations. In the second part of the dissertation, the fairness in ad hoc networks is studied. In ad hoc networks, each station may forward some flows for other stations as well as sends its own flows. Therefore, the number of flows to deliver may be different among stations. However, since the 802.11 DCF provides an equal chance for sender stations to access the channel regardless of their number of flows, it can induce unfairness among flows. In addition, the hidden node problem degrades throughput and fairness of TCP flows in multihop ad hoc networks. In order to resolve these problems, the ATC scheme for ad hoc networks is proposed. In the proposed scheme, by fairly allocating the channel access time of a flow to each link and by accounting the interference level from hidden nodes, fairness and throughput are enhanced. Through ns2 simulations for various topologies, the effectiveness of the proposed scheme is shown.