Performance Study of IEEE 802.11 DCF and IEEE 802.11e EDCA

One of the key factors for the wide acceptance and deployment of IEEE 802.11 Wireless Local Area Networks (WLANs) is the simplicity and robustness of the Medium Access Control (MAC) protocol. Based on the well-known carrier sense paradigm, with an exponential backoff mechanism devised to minimize the probability of simultaneous transmission attempts by multiple stations, the protocol is able to work in presence of interference, which is very critical for networks operating in unlicensed spectrum. In fact, interfering sources are simply revealed by the carrier sense mechanism in terms of channel occupancy times, or by the acknowledgement mechanism in terms of collisions. However, the simplicity and the robustness have often been traded off with the efficiency of the access protocol, in terms of radio resources which are wasted or underutilized. In this chapter, we provide a detailed analysis of the 802.11 distributed access protocol, by examining the protocol parameters which most critically affect the protocol efficiency. We quantify the protocol overheads due to control information (i.e., physical headers, frame headers, acknowledgement and other control frames) and to the distributed management of the channel grants (i.e., collisions and idle backoff slots). Then, we consider the distributed channel access extensions, defined in the recently-ratified 802.11e standard in order to support service differentiation among stations with different Qualityof-Service (QoS) requirements. Finally, we attempt to show how these parameters affect the resource repartitioning among the stations and how they can coexist with legacy DCF stations. The IEEE 802.11 defines a basic service set (BSS) as the number of stations controlled by a single coordination function, where a coordination function is the 802.11 terminology for medium access control (MAC). There are two types of BSSs. One is the infrastructure BSS, and the other is the independent basic service set (IBSS). An infrastructure BSS is composed of a single access point (AP), (a bridge between an infrastructure, i.e., a wireline network typically, and the wireless link), and a number of