MAC for real-time applications over random-access time-varying channels
Knightly, Edward W.
Doctor of Philosophy thesis
This thesis introduces mechanisms for support of real-time applications in high-density random-access networks. Random-access Wireless LANs are increasingly being used to support real-time applications such as voice over IP. Providing service to real-time applications requires not only efficient utilization of the scarce physical resource, i.e., bandwidth, but also incorporation of precise knowledge of network parameters and characteristics in mechanisms for proper dimensioning and/or admission control. The contribution of this thesis is two fold. First, Opportunistic Auto Rate (OAR), a multi-rate media access protocol, is devised which exploits the inherent variations in channel conditions to increase the throughput while maintaining long-term temporal fairness. The key mechanism of the OAR protocol is to opportunistically send multiple back-to-back data packets whenever the channel quality is good. As channel coherence time typically exceeds multiple packet transmission times for both mobile and non-mobile users, OAR achieves significant throughput gains as compared to state-of-the-art auto-rate adaptation mechanisms. Moreover, over longer time scales, OAR ensures that all nodes are granted channel access for the same time-shares as achieved by single-rate IEEE 802.11. The thesis describes mechanisms to implement OAR on top of any existing auto-rate adaptation scheme in a nearly IEEE 802.11 compliant manner. It also analytically studies OAR and characterizes its gains in throughput as a function of the channel condition. An extensive set of ns-2 simulations is performed to study the impact of such factors as node velocity and channel conditions on the throughput of OAR. Second, the thesis presents a delay analysis of CSMA/CA that yields the media access delay distribution of a multi-rate system as a function of key performance factors such as network load and channel condition. The model is developed under assumption of fixed probability of collision in the network. The accuracy of the model is validated by comparing the numerical results with those obtained via ns-2 simulations. By linear approximation of the computed distribution over the small-delay regime, a computationally simple result applicable to on-line admission control is achieved. This admission control mechanism along with the devised opportunistic multi-rate MAC protocol enables support of real-time applications in the CSMA/CA networks.
Engineering, Electronics and Electrical