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