Contention-Based Airtime Usage Control in Multirate IEEE 802.11 Wireless LANs

In a multirate wireless LAN, wireless/mobile stations usually adapt their transmission rates to the channel condition. It is difficult to control each station´s usage of network resources since the shared channel can be overused by low transmission-rate stations. To solve this problem, we propose a distributed control of stations´ airtime usage which 1) always guarantees each station to receive a specified share of airtime, and 2) keeps service for individual stations unaffected by other stations´ transmission rates. Such airtime control enables service differentiation or quality of service (QoS) support. Moreover, it can achieve a higher overall system throughput. The proposed airtime usage control exploits the Enhanced Distributed Channel Access (EDCA) of the IEEE 802.11e standard . Two control mechanisms are proposed: one based on controlling the station´s arbitration inter-frame space (AIFS) and the other based on the contention window size. We show how the stations´ airtime usage is related to the AIFS and contention window size parameters. Using this relation, two analytical models are developed to determine the optimal control parameters. Unlike the other heuristic controls or analytical models, our model provides handles or parameters for quantitative control of stations´ airtime usage. Our evaluation results show that a precise airtime usage control can be achieved in a multirate wireless LAN