Study On Contention Window Control In WLANs

In recent years, wireless LANs are becoming increasingly prevalent and have been widely used for their mobility, flexibility, easy extension and low cost. Wireless LANs have been interpreted as an integral part of future's personal communication systems. As the most widely used standards, IEEE 802.11x protocols are the most widely used standards in wireless LANs.IEEE 802.11 standards use CSMA/CA mechanism in MAC layer. Each node is scheduled to access the wireless channel through the contention window control algorithm, which is named as binary exponential back-off algorithm. But using this mechanism, wireless LANs achieve not only lower throughput comparing with physical rate, but also poor fairness, especially when the number of contending nodes is large. Furthermore, some other functions have been added to support QoS and multi-rate, so the impact of the contention window control on these aspects also should be focused. The main results of the dissertation are as follows:1) Contention window parameters control in IEEE 802.11 wireless LANs: Most of current contention window control algorithms are complicated to realized in practical environment. So this dissertation studies the impact of contention window parameters(including the minimum contention window and the maximum contention window) on the saturation throughput of wireless LANs when using binary exponential back-off algorithm. CWPA(Contention Window Parameters Adaptation), which may be easy to implement, is presented. In CWPA, only the minimum contention window and maximum contention window are adapted to improving the performance of wireless LANs. The simulation results show that, comparing with the MAC mechanism of IEEE 802.11 standards, CWPA obtains higher throughput and better fairness.2) The impact of contention window on the derterministic proportional QoS: In wireless LANs, IEEE 802.11e can't properly support deterministic proportional QoS because of the binary exponential back-off algorithm used in it. To solve this problem, this dissertation presents a distributed algorithm of contention window control. In our algorithm, each node estimates the number of consecutive idle slots in wireless LANs, and then controls the contention window based on the throughput ratio of each node. By converging the average number of consecutive idle slots to its approximately optimal value through contention window control, deterministic proportional QoS in wireless LANs can be supported. Simulation results also demonstrate that the proposed method can support deterministic proportional QoS.3) The impact of contention window control on the performance of IEEE 802.11 multi-rate wireless LANs: Collisions have negative impact on the performance of wireless LANs, and it can't be eliminated by rate adaptation. This dissertation studies the impact of contention window on the saturated throughput in IEEE 802.11 multi-rate wireless LANs. The ineffectiveness of rate adaptation on collisions is investigated, and present to improve the performance of multi-rate wireless LANs by decreasing collisions through the contention window control. Simulation results show that, combining rate adaptation with contention window control can achieve much better network performances, including throughput and fairness, than only using rate adaptation in IEEE 802.11 multi-rate wireless LANs.