A cross-layer approach for per-station fairness in TCP over WLANs

Abstract

In this paper, we investigate the issue of per-station fairness in TCP over IEEE 802.11-compliant wireless local area networks (WLANs), especially in Wi-Fi hot spots. It is asserted that the hot spot suffers from the unfairness among wireless stations in exploiting the wireless medium. The source of this unfairness is analyzed from two aspects, TCP-induced asymmetry and MAC-induced asymmetry; the former causes TCP congestion control with a cumulative acknowledgment mechanism to prefer the wireless sending stations to the wireless receiving stations, whereas the later exacerbates the unfairness problem in the hot spots. By investigating the interaction between TCP congestion control and MAC contention control, we reveal that, even when a wireless station has a sufficiently large amount of traffic to send, it cannot always participate in the competition to access the wireless medium since the attempts to access the medium in the IEEE 802.11 MAC layer are controlled by the TCP congestion control. Therefore, we propose a cross-layer feedback approach to assure per-station fairness and to ensure high channel utilization. In this approach, we introduce the notion of channel access cost to quantify the system-wide traffic load and per-station channel usage. The access cost is estimated at the MAC in an access point and conveyed to the TCP sender. Then, the TCP sender adjusts its sending rate based on the access cost, so as to assure per-station fairness. The novel aspect of the approach is that it does not entail any modification of either the TCP or the IEEE 802.11 MAC. Furthermore, it neither resorts to any complex fair scheduling, nor compromises the modularity of the layered architecture. The simulation results indicate that the proposed approach can provide both per-station fairness and high channel utilization, irrespective of network configurations.