Feedback based distributed admission control in 802.11 WLANs

A distributed connection admission control (CAC) scheme where stations independently admit or reject flows based on channel utilization threshold is easy to implement for WLANs. However, owing to variable protocol capacity, the advisable threshold also is variable, limiting the efficacy of fixed threshold based CAC. If the admission threshold is tuned to reflect the current protocol capacity, better throughput can be obtained while avoiding WLAN overload. In this paper, we propose a feedback based scheme to tune the utilization threshold for admission. We select the performance metric used for feedback, derive the system model from empirical data, and use control theory to design a feedback controller. Further, we present an analytical model and heuristics using which the controller can be adapted to work under diverse operating scenarios. Simulation results of proportional and proportional-integral controllers in OPNET suggest that the feedback based CAC is able to avoid WLAN overload and achieve high throughput despite large changes in protocol capacity.