Fairness and its impact on delay in 802.11 networks

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Abstract
International audienceWe analyze in this paper the fairness of the 802.11 DCF (Distributed Coordination Function) access method. Short- term fairness is an important property of a MAC layer for obtaining short delays. We show that contrary to the common wisdom, a 802.11 cell with two hosts does not exhibit short- term unfairness. Many papers considered 802.11 as short-term unfair by referring to a study of the Wavelan CSMA/CA access method [1]. The confusion comes from the extrapolation of its results to 802.11. Actually, these two access methods are very different: the Wavelan CSMA/CA access method performs exponential backoff when the channel is sensed busy, whereas 802.11 does the same only after a collision. We propose a new fairness index: the number of inter- transmissions that other hosts may perform between two trans- missions of a given host. By means of this index we analyze the fairness of 802.11 for the case of two hosts and derive the probability distribution of the number of inter-transmissions. Our results show that even on the short term time scale the 802.11 DCF access method provides fairness to competing hosts. When compared with Slotted ALOHA, a multiple access randomized protocol with good fairness properties, 802.11 presents even better fairness. To validate the model, we compare the analytical results with experimental histograms obtained via simulations and measurements

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