Progressively reliable packet delivery for interactive wireless multimedia

In this dissertation, we propose a progressively reliable end-to-end network protocol for delivery of delay-sensitive visual multimedia over a wireless channel with a time-varying bit error rate and limited bandwidth. Interactive applications like Web-based image browsing which operate over a wireless access link to the Internet require immediate delivery of image data to the receiver in order to support genuine interactivity. Wired Internet connections incur roundtrip delays approaching the interactive latency bound. Reliable protocols that retransmit lost or corrupt packets over an Internet connection that also includes a noisy wireless link will be unable to deliver an image by the interactive latency bound. We derive a lower bound on the latency incurred by an ideal retransmission-based ARQ protocol in a noisy bit error rate (BER) wireless channel, and show that full reliability will cause the transport latency to far exceed the interactive latency bound at 1% BER. We also show that, even by adding powerful Reed-Solomon forward error correction (FEC) codes with redundancy rates that double or triple the bandwidth, the latency penalty due to ARQ retransmissions is too high to achieve the interactive latency bound at 3% BER.

Applications that accept unreliable packet delivery, e.g. packet loss and packet corruption, have a more reasonable chance of achieving the interactive latency bound over a noisy channel. Practical complexity and delay constraints on FEC and compression cause the traditional approach of aggressive compression and aggressive FEC to be unable to deliver images quickly enough at severe BER's. Under these conditions, we show that the joint source/channel coding approach of error-resilient image coding/decoding combined with forwarding of corrupt packet data can continue to support interactive image display. We demonstrate that error-tolerant image coding can reconstruct images at 3% BER and simultaneously compress images down to 0.75 bits/pixel via lossy quantization only.

We propose a progressively reliable end-to-end protocol designed for rapid yet asymptotically reliable delivery of delay-sensitive imagery. A possibly noisy initial version of a packet is forwarded to the receiver quickly to allow the end user to interact immediately with an initially noisy image. For bursty multimedia applications like Web-based image browsing, the noisy still-image needs to be cleaned of any persistent artifacts. Therefore, the protocol follows its initial delivery with multiple increasingly reliable deliveries of each packet, leveraging off of the retransmission mechanism of the protocol. We call this progressively reliable protocol "Leaky ARQ". We identify through X server simulation three additional performance-enhancing functions for Leaky ARQ: delaying retransmissions by many seconds; cancelling out-of-date retransmissions; and fine-grained scheduling of application data through the use of flows. Finally, we show how Type-II Hybrid ARQ protocols, also called packet combining or memory ARQ protocols, can be modified to implement ARQ.