A Contention Window Control Method Using Priority Control Based on the Number of Freezes of Wireless LAN
Because of the widespread adoption of mobile devices, many applications now provide support for wireless LAN (WLAN). This makes it an important issue to provide good WLAN quality of service (QoS). For this purpose, Dhurandher et al. improved the distributed coordination function (DCF). Although the highest-priority throughput increased using this method, throughput for other priorities decreased significantly. The authors proposed a minimum contention window control method based on the collision history for two (high and low) priorities, to improve Dhurandher's method. That method can keep the same average throughput for high-priority traffic as Dhurandher's method yields, but decreases the average total throughput of not only the low-priority traffic but also the high-priority traffic when congestion occurs. To overcome this problem, the method introduced in this paper improves the contention window control method by using a priority control based on the number of freezes, which is defined as a count of the times that a mobile node lets its backoff timer be frozen by the other nodes. The number of freezes is introduced as a more suitable indication of congestion, and to control the contention window for the low priority. Finally, network simulations executed on two scenarios using continuous flows or on-off flows for the low-priority and demonstrated that the proposed method retains the average total throughput of both priority frames as well as reducing the number of freezes of both priorities and the packet delay and jitter for high-priority frames, compared with the controls in Dhurandher's method and the previous method. However, the fairness-index for low-priority flows showed a little unfair in the proposed method.
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