IEEE 802.11多重速率無線網路的效能評估與改善

Abstract

Due to the availability of affordable devices that are capable of transmitting at high data rates, wireless LANs (WLANs) have become increasingly popular. For example, the IEEE 802.11n standards now can support data rates up to 540 Mbps. These high rates are achieved through new modulation schemes that are optimized for the channel conditions bringing about a dramatic increase in throughput performance. Since the choice of which modulation scheme to use depends on the current state of the transmission channel, newer wireless devices often support multiple modulation schemes, and hence multiple data rates, with mechanisms to switch between them. Users are given the option to either select an operational data rate manually or to let the device automatically choose the most appropriate data rate to match the prevailing conditions. Although automatic rate adaption protocols have been studied widely for cellular networks, there have been relatively few proposals for WLANs. In this dissertation, we first showed 3 problems of the 802.11-based WLANs in which the wireless devices have the multi-rate capability: (1) the problem of power management inefficiency; (2) the problem of unfair channel time allocations; and (3) the problem of degraded network throughput. We then proposed a scheduling mechanism, called the Shortest Time First Scheduling, to improve these problems. In the second part of this dissertation, an analytical model, called the Rate-Adaptive Markov Chains, was proposed to study the saturation throughput and delay performance of a WLAN in which the mobile hosts have the multi-rate and automatic rate adaption capability. We also showed how to evaluate the throughput of a path for stationary STAs in multi-rate multi-hop ad hoc networks. In the future, we hope to develop a practical user mobility model such that the expected throughput of a dynamic route can be evaluated. Then, a routing protocol to select the most throughput efficient path from all possible paths can be designed. Simulations are also provided to verify the performance of the packet scheduling mechanism, the correctness of the analytical model and the mechanism used to evaluate path throughput.