Extended IFS and asynchronism influence on IEEE 802.11b medium access equity

This article presents a theoretical modeling of an ad hoc network scenario that shows that a great inequity can appear in medium access between nodes using the IEEE 802.11 DCF mode. Numerical results confirm that both asynchronism and Extended Inter Frame Spacing (EIFS) use perturbate the equity between mobiles in wireless networks. Key-words: IEEE 802.11b, performace evaluation, Mobile ad-hoc networks, interferences Modélisation de l’influence de l’asynchronisme et des interférences dans les réseaux 802.11b Résumé : Dans ce papier nous proposons une modélisation théorique d’une configuration ad hoc qui présente une très grande inégalité dans l’accès au médium entre les mobiles lorsque le mode DCF de 802.11 est utilisé. Les résultats numériques obtenus viennent corroborer l’idée que l’asynchronisme et l’utilisation des EIFS perturbent fortement l’équité entre les mobiles dans les réseaux 802.11. Mots-clés : IEEE 802.11b, évaluation de performace, Réseaux ad hoc, interférences Modeling of the IEEE 802.11b protocol 3 Introduction Most of the actually used wireless interface cards implement the IEEE 802.11b standard that defines physical and medium access layers for wireless networks. The first standard (802.11) was defined in 1997 while the actual one (802.11b), widely used nowadays, was specified in 1999. This protocol can operate in two modes: PCF (Point Coordination Function) mode shall be used when mobiles communicate using a base station and DCF (Direct Coordination Function) mode is used when mobiles communicate directly without the help of any fixed infrastructure. In order to understand and to evaluate the performance of these networks, a certain number of contributions have been written. These works can be classified in two main categories: a theoretical approach and a simulation based one. Performance evaluation of 802.11 based on a theoretical analysis mainly evaluate the characteristics (throughput, retransmissions, delay, RTS / CTS mechanism cost) of the DCF mode of the protocol [2, 9, 10, 3, 4, 7]. All these analysis are made in a configuration where all the nodes are in communication range of the others and show the same results. Nevertheless, some works [8, 11, 5], have shown that in real ad hoc configuration, in which some mobiles compete with more nodes than others for medium access, the MAC protocol equity was not preserved. All these works are based on simulations and no theoretical analysis has been proposed yet to evaluate the performance of the mobiles in these situations. In [5], a particular configuration showed a great inequity between the mobiles. As this scenario involves few emitters, it constitutes a good choice for a first step towards analysis of scenarios involving asymmetry. Moreover the effect of this asymmetry is accentuated by a mechanism of the 802.11 protocol that is triggered when interferences disrupt the carrier sense mechanism. In section 1, we will describe briefly the DCF mode of 802.11, then we will present the ad hoc scenario studied. In section 2, we describe how we modeled this scenario using discrete time Markov chains. Due to the asynchronism of the system, the Markov chain is infinite. We will describe how to reduce this number of states. We will then compute the transition probabilities in Section 3. Finally, results, that confirm that a great inequity between mobiles can appear, will be presented in section 4. 1 System description DCF mode of IEEE 802.11 [6] being part of the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol family, it associates a carrier sense mechanism to a random wait (backoff) before transmission mechanism. When a mobile wishes to transmit a frame, it first ensures that the radio medium is not occupied by another transmission by measuring the signal level on the radio channel. As long as the channel is occupied, transmission is deferred to prevent collisions. As soon as the medium becomes free, the random backoff mechanism is initiated. The emitter chooses a random number between 0 and a value of CWmin (initially equal to 31 in 802.11b). The