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Gilles Berger-Sabbatel, Andrzej Duda, Olivier Gaudoin, Martin Heusse, and Franck Rousseau

Fairness and its Impact on Delay in 802.11 Networks

In Proceedings of IEEE GLOBECOM 2004. Dallas, USA, November 29-December 3, 2004

Monday 29 November 2004

We 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.

P.S.

@inproceedings{berger-globecom2004,
	author = {Berger-Sabbatel, Gilles and Duda, Andrzej and Gaudouin, Olivier and Heusse, Martin and Rousseau, Franck},
	title = {Fairness and its Impact on Delay in 802.11 Networks},
	booktitle = {Proceedings of the IEEE Global Telecommunications Conference (GLOBECOM'04)},
	address = {Dallas, USA},
	abstract = {We 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.},
	volume = 5,
	isbn = {0-7803-8794-5},
	doi = {10.1109/GLOCOM.2004.1378897},
	pages = {2967--2973},
	month = Nov # {~29--} # Dec # {~3,},
	year = 2004
}

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