Effective Capacity Theory for Modeling Systems with Time-Varying Servers, with an Application to

Abstract. Many network applications rely on stochastic QoS guarantees. With respect to loss-related performance, the Effective Bandwidth/Capacity theory has proved useful for calculating loss probabilities in queues with complex input-and server-processes and for formulating simple admission control tests to en-sure associated QoS guarantees. This success has motivated the application of the theory for delay-related QoS too. However, up to now this application has been justified only heuristically for queues with variable service rate. The the-sis fills this gap by establishing rigorously that the Effective Bandwidth/Capacity theory may be used for the asymptotically correct calculation and enforcement of delay tail-probabilities in systems with variable rate servers too. Subsequently, the thesis applies the general results to IEEE 802.11 WLANs, by representing each IEEE 802.11 station as an On/Off server and employing the Effective Ca-pacity function for this model. Comparison of analytical results with simulation validates the effectiveness of the On/Off IEEE 802.11 model for loss- and delay-related QoS. Finally, the thesis uses the Effective Capacity of an IEEE 802.11 station in yet another way, namely as a design tool. Indeed, the specific form of the IEEE 802.11 Effective Capacity function highlights the role of certain param-eters of the IEEE 802.11 backoff window distributions. These parameters, when appropriately tailored, allow better delay-related (and loss-related) performance, while maintaining the standard saturation throughput of IEEE 802.11 WLANs