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> From: Mike Champion [mailto:mc@x...] >So, my somewhat ignorant question is: why would these WBXML folks go to >the trouble of defining yet another compression scheme for wireless XML? There are both theoretical and practical reasons for reconsidering compression in wireless environments. All have to do with the interaction between compression and error correction. Virtually all digital wireless systems employ some form of Reed-Solomon based forward error correction. For those not familiar, this involves coding a field of n information elements as n+p information elements such that when those n+p elements are transmitted if some number greater than n elements arrive, then the message can be reconstructed. Almost all systems employ this coding over bit fields. And some wireless applications employ it over packet fields in addition to whatever bit level correction is used. It has been observed (and I believe proved as well) that for transmissions on a randomly noisy channel, that transmission with forward error correction is closer to the Shannon limit for that noisy channel. In other words one comes closer to the total information carrying capacity of the channel by adding information, when that channel is noisy! Compression on a clear channel assumes that by coding using a fewer bits is optimal. But in the noisy wireless environment we've demonstrated a case where more bits is more efficient (in the information theoretical sense). Compression and error correction are often treated as discrete steps. One might find greater overall efficiencies if these bit-adding, and bit-reducing steps were treated holistically. On the practical side, there are several different types of Reed-Solomon error correction. Some will guarantee to deliver a correct message or fail. Others make a best effort, but will indicate whether a field might be incorrect. Applications that can tolerate some loss of data, or require as much of a message as it can reliably receive, might not want a compression method that require the entire stream to reliably decode it. That's a long answer to why one would want to reconsider compression for wireless. From a not particularly thorough reading of WBXML, I'm not sure that it's trying to deal with these issues. Maybe it should be.
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