Design, analysis and optimization of CSMA/CA based wireless networks.

摘要

Recent years have witnessed a remarkable interest in wireless networks that need little or no infrastructure support. Popular examples of such networks include IEEE 802.11 based ad hoc and local area networks which use CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) as the random media access protocol. In contrast to cellular systems, little understanding exists regarding the design and optimization of these networks.; In this thesis, we start by considering single-hop networks and develop packet cycle time analysis as a convenient means of analyzing the throughput and delay characteristics of CSMA/CA based wireless networks. Our model captures the complex interactions that exist between the physical and media access layers, and higher layers (traffic). In particular, we will study the saturation throughput and develop M/G/1 processor sharing and G/G/1 queueing models. We then generalize the analysis to multi-hop networks and apply the model to help answer important network design questions such as determining the feasibility of a traffic matrix and identifying bottlenecks. We study in-depth the impact of Binary Exponential Backoff (BEB) on the throughput-fairness tradeoff and find that the initial contention window is the most influential parameter on the unfairness problem.; We then optimize CSMA/CA contention parameters and find that once the initial contention window is optimized, interestingly, BEB either offers no additional improvement or penalizes performance. We find that the optimal backoff policy is agnostic to the trans mission type, i.e., it chooses the same backoff window for a new transmission as well as a retransmission. Among other advantages, we show that such an optimization has several practical benefits with respect to selfish and automatic link adaptation, and, latency minimization in sensor networks. We conclude by presenting an adaptive distributed CSMA/CA protocol called WABA that outperforms the BEB protocol of IEEE 802.11 systems while achieving close to optimal performance.