| IEEE 802.11-based wireless local area networks (WLAN) [1] have experienced an unprecedented growth in deployment. In spite of its current prevalence, the IEEE 802.11 protocol displays several intrinsic weaknesses. If not addressed in a timely and effective manner, these weaknesses will ultimately limit its success, as the technical challenges posed by a multitude of new applications constantly emerge.;Among the difficulties that WLAN faces, support for QoS is an issue that demands special attention and an immediate solution. In the past few years, the 802.11e enhanced distributed channel access (EDCA) function has been prescribed as a mechanism to meet QoS requirements. However, the actual efficacy of EDCA has not been thoroughly investigated. To develop a comprehensive understanding of the protocol behavior, therefore, we propose a novel multi-dimensional Markov chain model in this thesis, which faithfully captures all the major QoS features of EDCA. After applying this analytical framework to examine the impact of the primary QoS features on network performance (e.g., capacity, system delay, etc.), we make critical observations and gain important insights.;Another problem addressed in the thesis is the severe performance degradation widely observed in a WLAN network that contains stations of different rates. This anomaly is primarily caused by peculiarities of the distributed coordination function (DCF) protocol. More specifically, the IEEE 802.11 DCF has attempted to maintain fair channel access among all the stations, which ultimately lets slow stations drag down the aggregate throughput of the network. To fulfill this fairness aim but at lesser cost, we introduce in this thesis a new medium access control protocol called CoopMAC. As readily demonstrated by simulation and experimentation results, the CoopMAC protocol can maintain the fair channel access that legacy 802.11 strives to ensure, while substantially improving system throughput and delay performance. In addition, our measurement results confirm that a cooperative MAC can provide benefit for not only the stations being helped, but also the ones who offer the cooperation. Indeed, all the key observations and reflections repeatedly echo a well-known saying: It is more blessed to give than to receive (Act 20:35). |
