06-10-2010, 03:19 PM
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Introduction
The packet transport service provided by representative packet-switched networks, including IP networks, is not reliable and the quality-of-service cannot be guaranteed. Packets may be lost due to buffer overflow in switching nodes, be discarded due to excessive bit errors and failure to pass the cyclic redundancy check at the page link layer, or be discarded by network control mechanisms as a response to congestion somewhere in the network. Forward Error Correction coding has often been proposed for end-to-end recovery from such packet losses. However, the use of FEC in this application provides a double-edged sword. From an end user’s perspective, FEC can help recover the lost packets in a timely fashion through the use of redundant packets, and generally adding more redundancy can be expected to improve performance provided this added redundancy does not adversely affect the network packet loss characteristics. On the other hand, from the network’s perspective, the widespread use of FEC schemes by end nodes will increase the raw packet-loss rate in a network because of the additional loads resulting from transmission of redundant packets. Therefore, in order to optimize the end-to-end performance, the appropriate tradeoff, in terms of the amount of redundancy added, and its effect on network packet-loss processes, needs to be investigated under specific and realistic modeling assumptions.
We provide a study of the overall effectiveness of packet-level FEC coding, employing interlaced Reed-Solomon codes, in combating network packet losses and provide an information- theoretic methodology for determining the optimum compromise between end-to-end performances and the associated increase in raw packet-loss rates using a realistic model-
based analytic approach. Intuitively, for a given choice of block length we expect that there is an optimum choice of redundancy, or channel coding rate, since a rate too high is simply not powerful enough to effectively recover packet losses while a rate too low results in excessive raw packet losses due to the increased overhead which overwhelms the packet recovery capabilities of the FEC code. The optimum channel coding rate results in an optimum compromise between these two effects.
Here we focus on evaluating the capability of FEC in recovering packet losses over IP networks using residual packet-loss rate as the performance measure. In terms of characterizing end-to-end performance, we assume that performance is directly proportional to the source coding rate, or network load, that can be supported for a fixed residual packet-loss rate. The analytic procedure developed is then used to determine the maximum load that can be supported as a function of coding parameters. By modeling the fully interleaved network transport channel as a block interference channel, we provide information theoretic bound on the performance achievable with FEC. This bound provides a useful context for assessing the efficacy of FEC in this application as a function of coding parameters.
