Retransmission-based Error Control for Continuous Media Traffic in Packet-switched Networks

Dempsey, Bert J., Department of Computer Science, University of Virginia
Weaver, Alfred, Department of Computer Science, University of Virginia
Liebeherr, Jorg, Department of Computer Science, University of Virginia

Distribution of digital audio and video, continuous media, over packet-switched networks has become increasingly feasible due to technology trends leading to powerful desktop computers and high-speed networks. Unlike reliable data transfers, transmission of continuous media streams is sensitive to network delays and has some tolerance for limited data loss. End-to-end protocols for continuous media traffic are now emerging, and an area of active research is error control in this context.
This dissertation provides a comprehensive and fundamental study of retransmission-based error control for the distribution of digital continuous media over packet-switched networks. While widely dismissed in the current literature, a retransmission-based approach is attractive since it imposes little overhead on network resources and since alternative techniques have notable drawbacks with respect to complexity, portability, and cost. It must be demonstrated, however, that retransmissions can provide significant error coverage while respecting delay constraints.
We define a novel delay-constrained retransmission scheme, Slack ARQ, and develop a simulation model to determine its feasibility for distribution of packet voice in a local area network (LAN). The evaluation uses a unique performance metric for retransmission effectiveness, incorporating both error and delay considerations that determine the overall transmission quality. This work is extended with an analytical end-to-end model for Slack
ARQ, from which analytical expressions for our retransmission performance metric are derived. A principle reason for the paucity of retransmission-based approaches in the literature has been the lack of methodologies for assessing their effectiveness in a delay-sensitive environment. Our analysis provides, without resorting to lengthy simulations, quantification of the effects of transmission parameters, such as the packetization interval in the protocol and the network delay distribution, on retransmission success. Numerical examples show the applicability of Slack ARQ in many realistic transmission scenarios.
We design and carry out an empirical investigation of packet voice distribution across a contemporary high-performance campus-wide network. This study is of interest since large multiple-segment LANs are likely candidates for near-term deployment of continuous media applications and since little empirical work has been done in this area. It is concluded that
the campus-wide network in this study can indeed support real-time packet streams, but that sporadic high delays in the network may threaten transmission quality. We discuss the implications of the empirical measurements for our modeling, and, by calculating empirical probabilities, we show that the measurement data substantially corroborates the results of
our simulation and analytical studies. That is, in total, the simulation, analysis, and empirical measurements presented in this dissertation conclusively demonstrate the feasibility of Slack ARQ in most packet-switched networks.
Finally, in Appendix A, we define a novel connection-oriented service that provides limited recovery from packet loss using delay-constrained retransmission within a next-generation transport protocol, the Xpress Transfer Protocol (XTP).We implement this lightweight service through minor modifications to an existing XTP implementation, and its performance is demonstrated in experimental network transfers.

PHD (Doctor of Philosophy)
distribution networks, digitial audio, digital video
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