Multi-Class QoS in 802.11 Networks Using Gentle Decrease of Multiple Contention Windows

IEEE 802.11 distributed coordination function (DCF) uses a binary exponential backoff algorithm to arbitrate simultaneous channel access by multiple stations. When a station senses that either the shared wireless channel is busy or a collision has occurred, that station must delay the transmission by backoff time. This backoff time, in legacy DCF, is uniformly selected from a common contention window range that, in turn, ensures equal chance of grabbing the wireless medium by each station. Therefore, legacy DCF does not support service differentiation or priority access to the channel. To support service differentiation and increased quality of service (QoS), we propose two enhancements to the procedures used for managing contention window in original protocol. These two enhancements can be easily incorporated within the legacy DCF mechanism. First, instead of a common window for all classes, each traffic class is assigned a contention window with minimum and maximum values which are specific to that class. Second, the backoff algorithm is adjusted to follow a multiple increase multiple decrease (MIMD) procedure. First enhancement provides service differentiation and the second improves performance for all traffic classes. Unlike previous schemes, our proposed scheme does not require protocol changes in the legacy DCF. We evaluate our scheme using extensive simulations and demonstrate that, in comparison with legacy DCF, our newly proposed scheme provides a throughput increase of about 30% for high priority flows and of about 20% for medium priority flows.