A Minimum Contention Window Control Method for Lowest Priority Based on Collision History of Wireless LAN

Abstract

Because of the widespread adoption of mobile devices, many applications have provided support for wireless LAN (WLAN). Under these circumstances, one of the important issues is to provide good quality of service (QoS) in WLAN. For this purpose, Dhurandher et al. improved the distributed coordination function (DCF). In this method, the contention window (CW) is divided into multiple ranges. Each range is independent of all other ranges and is assigned to a different priority. Although the highest-priority throughput increased using this method, throughput for the other priorities decreased significantly. To overcome this problem, this paper proposes a minimum contention window control method for two (high and low) priorities. In the method, all nodes are assumed to use real-time applications or data transmission. The former real-time frames are high priority and are sent by UDP. The latter data frames are low priority and are sent by TCP. The purpose of the proposed method is not only to provide good QoS for the highest priority but also to prevent deterioration in the QoS for other priorities in WLAN. For this purpose, the proposed method keeps the CW for the high priority at a low value and controls the CW for the low priority based on the collision history. Finally, the network simulations demonstrated that the proposed method reduces the decrease in the average total throughput of the low priority frames as well as reducing the packet drop rate of both priorities, compared with those for the DCF and Dhurandher's method. From a simulation scenario where there are only low priority flows in wider bandwidths, all methods give almost same average total throughput and packet drop rate, but the results also suggest that the CW range in the proposed method should be reduced to improve the average total throughput, when no congestion occurs.