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UBC Theses and Dissertations

Per-session weighted fair scheduling for real time multimedia in multi-rate wireless local area networks Fallah, Yaser Pourmohammadi


Supporting real time multimedia applications is of utmost importance for future wireless data networks. In particular, it is crucial to support such applications in widely deployed and fast growing wireless local area networks (WLAN) that are based on IEEE 802.11 standard. However, achieving this goal requires features and mechanisms that have not been offered by the original IEEE 802.11 standard. To address this issue, several Quality of Service (QoS) enabling features have been added to the Medium Access Control (MAC) layer in the new IEEE 802.11e standard. Nevertheless, the new standard does not mandate a specific QoS solution, and intentionally leaves such task to developers and equipment vendors. Devising mechanisms that can efficiently provide the required QoS in WLANs has proved to be a challenging task. This is mainly due to the fact that WLANs such as 802.11e are based on the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) access method, which is inherently a distributed mechanism with random uplink or downlink access. Moreover, the physical layer of a WLAN allows each station to use a different transmission rate. The transmission rate could also dynamically change from one packet to the next, for the same station. The general problem of packet loss in wireless networks is also present in WLANs. The existing solutions and the prioritized contention based mechanism provided by the standard are found to be inadequate for providing the required services, especially in heavily loaded networks. Considering the issues mentioned above, we present a solution in this thesis that employs the controlled access mechanisms of the 802.11e standard to provide per-session guaranteed QoS to multimedia sessions. We introduce a framework that centralizes the task of scheduling uplink and downlink flows in the access point through the concept of virtual packets. We propose a new queuing structure that works with a fair generalized processor sharing based scheduler, integrated with a traffic shaper, for scheduling controlled (polling) and contention access durations. To address the issues of physical channel impairment and variable rate operation of a WLAN, we extend our scheduling framework to provide throughput or temporal fairness in multirate WLANs. Through analysis and experiments, we demonstrate that our solution provides guaranteed fair access for multimedia sessions over WLANs.

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