| Summary: | 碩士 === 國立臺灣師範大學 === 工業教育學系 === 98 === In recent years, IEEE 802.11 Wireless Local Area Networks (WLANs) have become increasingly popular with the wide deployment of infrastructures and the prevalence of mobile/handheld devices. Many public areas have been providing IEEE 802.11 WLANs such as library, metro station and campus where are called hot spot. More and more smart-phones were sold, it makes many people now can easier and quickly access the Internet than before. Basically, IEEE 802.11 WLANs employ an infrastructure mode in which an Access point (AP) acts as a bridge between mobile stations and network servers of wired domain. However, IEEE 802.11 Medium Access Control (MAC) layer employs a contention-based channel access mechanism, named Distributed Coordination Function (DCF) for its distributed and simple manner. With DCF, all IEEE 802.11 nodes with packets to send including AP and each mobile station generally have the same channel-access probabilities. In the wireless networks, it leads to a momentous throughput degradation of downlink. In this thesis, we propose a cross-layer adaptive algorithm which dynamically adjusts the minimum contention window size (CWmin) in MAC layer of AP, according to the average ratio between downlink and uplink throughput and channel environments to achieve per-flow fairness. In case that uplink and downlink transmissions are with different bandwidth demands for various applications, our algorithm also can efficiently find the suitable CWmin which provides weighted fairness based on their resource requirements. The simulation results demonstrate that our scheme can effectively provide both per-flow fairness and weighted fairness in a diverse and time-varying WLAN environment.
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