Optimization of IEEE 802.11 DCF based on Bayesian estimation of the number of competing terminals
Friday, July 1, 2005 - 9:00am - 10:00am
Xiaodong Wang (Columbia University)
The performance of the Distributed Coordination Function (DCF) of the IEEE 802.11 protocol has been shown to heavily depend on the number of terminals accessing the distributed medium. The DCF uses a carrier sense multiple access scheme with collision avoidance (CSMA/CA) where the backoff parameters are fixed and determined by the standard. While those parameters were chosen to provide a good protocol performance under light loads, they fail to provide an optimum utilization of the channel in many scenarios. In particular, under heavy load scenarios, the utilization of the medium can drop tenfold. Most of the optimization mechanisms proposed in the literature are based on adapting the DCF backoff parameters to the estimate of the number of competing terminals in the network. However, existing estimation algorithms are either inaccurate or too complex. In this paper we propose an enhanced version of the IEEE 802.11 DCF that employs an adaptive estimator of the number of competing terminals based on sequential Monte Carlo methods. The algorithm uses a Bayesian approach, optimizing the backoff parameters of the DCF based on the predictive distribution of the number of competing terminals. We show that our algorithm is simple yet highly accurate even at small time scales. We implement our proposed new DCF in the ns-2 simulator and show that it outperforms existing methods. We also show that its accuracy can be used to improve the results of the protocol even when the nodes are not in saturation mode. Moreover, we show that there exists a Nash equilibrium strategy that prevents rogue nodes from changing their parameters for their own benefits, making the algorithm applicable in a complete distributed fashion without having to modify the standard or the existing base stations.