1 Performance Analysis of the 802.11 Distributed Coordination Function under Sporadic Traffic M. Garetto and C.-F. Chiasserini Dipartimento di Elettronica, Politecnico di Torino, Italy Email: {garetto, chiasserini}@polito.it Abstract— We analyze the performance of the Dis- tributed Coordination Function (DCF) for 802.11 WLANs. We consider a fixed number of contending stations within radio proximity, and we investigate the important case in which stations operate under non-saturated conditions. We assume that the MAC queues of wireless stations receive from the upper layers a stationary arrival process of packets. We identify the fundamental problems that arise in building an analytical model of the system, and we propose different approaches to overcome these difficulties. Finally, we apply our modelling technique to study several important issues in 802.11 networks, such as the impact of bursty traffic and the system performance in a multirate environment. The accuracy of the analytical results is verified by simulation with ns-2. Index Terms— Stochastic processes/Queueing theory I. I NTRODUCTION In the last few years, IEEE 802.11 Wireless Local Area Networks (WLANs) [1] have emerged as a pre- vailing technology for wireless access. Such a success is mainly due to the ability of 802.11 to provide a high- speed wireless environment, and to apply to infrastruc- tured as well as ad hoc networking. Nevertheless, the 802.11 technology still presents some limitations, among others, the limited bandwidth of the communication channel and the lack of quality of service (QoS) support. High data-rates in 802.11 networks are possible but at the cost of a shorter transmission range, i.e., they can be employed only when the distance between transmitter and receiver is sufficiently small. It follows that an efficient sharing of the available bandwidth is of crucial importance. As for the QoS support, some enhancements to the current channel access schemes are under study. In particular, the IEEE 802.11e draft standard [2] aims at introducing service differentiation at the MAC layer so as to provide the desired QoS level for various classes of traffic. These facts clearly indicate that the core element of the 802.11 technology is the MAC protocol, since it determines the efficiency of using the radio resources and the network performance. Therefore, it is of fundamental importance to develop a model of the 802.11 MAC function that accurately represent the system behavior under realistic assumptions and enable us to thoroughly investigate the system behavior. The primary 802.11 MAC function is the so-called Distributed Coordination Function (DCF). The DCF is a random access scheme based on the Carrier Sense Multiple Access with Collision Avoidance protocol (CSMA/CA). The DCF has two operating modes: the ba- sic channel access mode and the RTS/CTS (Request-to- Send/Clear-To-Send) mode. In the following, we assume the reader to be familiar with the access procedures of the DCF; for the necessary background see, for example, [1], [4], [5]. A. Our Contribution In this work, we present an analytical model of the 802.11 DCF. As discussed in Section I-B, several ana- lytical studies of the DCF have appeared in the literature [4]–[16]. Our model differs from previous work in that: (i) it identifies the critical assumptions in the develop- ment of analytical models of 802.11 networks; (ii) it presents a fairly simple as well as accurate model of the DCF in presence of non-saturated traffic sources; (iii) it is general enough to account for different arrival processes and traffic patterns, in particular, it applies to the case of bursty traffic like that produced by the TCP protocol; (iv) it evaluates the system performance in a multirate environment; (v) it applies to the case where a station seizing the channel is entitled to transmit a burst of packets, as specified in the IEEE 802.11e [2]; (vi) it evaluates several metrics of interest, such as the network throughput, the packet loss probability, the distribution of the MAC queue length at the wireless stations, the average packet delay, and the round-trip- time (which, together with the packet loss probability, is the main variable affecting transport protocols such as TCP). B. Related Work There are several theoretical works investigating the performance of the DCF access scheme. In [3], the