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IP: Trade-offs vs wishful thinking (compression and UWB)
From: David Farber <dave () farber net>
Date: Thu, 10 Jan 2002 05:33:34 -0500
Date: Wed, 09 Jan 2002 20:08:24 -0800 To: dave () farber net From: Lars Poulsen <lpoulsen () ucwireless com> Subject: Trade-offs vs wishful thinking (compression and UWB) Lay people and the press generally fail to understand that marvellous technical advances are not miracles: They are born out of trade-offs. In the case of data compression, the tradeoffs involve - file size (compression ratio) - processing time (CPU power) - size of storage buffer (dictionary) - latency Data compression is not magic: It is an analysis of data to remove redundancy, and is valid within a defined application space where the redundancy is understood. Indeed, if you start with data such as - bitmaps of pictures (TIFF or BMP) - english text (or any other language) - computer programs - HTML pages ... it is very easy to remove enough redundancy to reduce the file size by a factor anywhere between 4 and 10. In the case of pictures, it has been standard for the last 10 years to always store and move these files in compressed form. The rest of the above tend to be small files, compared to the aggregate of what we are moving around today. The large data streams that fill up our data pipes these days are already compressed: GIFs, MP3s, video streams. Any further gains must come from recognizing repetitions of the same files. Analyzing network usage, one can derive models that indicate that compression factors in the 10-100 range are possible with Gigabyte sized storage buffers; the true bottleneck is memory bandwidth between the comparators and the storage buffer. However, the operational behavior of such a compression engine would be very similar to that of a simple web cache (like squid) which is much simpler to build. Two severe limitations on a black box compression appliance, (which can serve an arbitrary bit stream, not just HTTP file access) are that it works only for a point-to-point link and it requires an error-free link. If data may have been lost, the dictionaries must be reset, and the compression factor goes back to 1:1 and then climbs slowly back until the dictionary has been rebuilt. For a T3 link (43 Mbps) and a Gigabyte dictionary, it would take several minutes to recover from such an event. I apply similar skepticism to all the great projections of unlimited bandwidth in unlicensed wireless. This is also a game of trade-offs: - power and sensitivity - distance - jam-resistance - speed When you push on one of these dimensions, it comes out somewhere else. We hope for co-existence between outdoor (fixed) broadband internet access, indoor 802.11, HomeRF etc. In principle, this can be achieved with spread spectrum of either the FH or DS variety, but only by sacrificing some amount to link speed for forward error correction at one level or another of encoding. The parameters can be tweaked in any direction, but everyone always wants to hear that you can get the good numbers in all dimensions simultaneously. Sorry, that just won't happen. We have come a long way in 10 years of DSP development, and there may be room for more stretch, but I am still waiting to see plausible confirmation of the practical implementation of UWB pulse modulation. As far as I can see, the pure pulse modulation shifts the receiver bottleneck to the problem of maintaining time synchro- nization to an unprecedented degree. IIRC, the Time Domain web site talks about synchronizing to picosecond accuracy. (But then again, I'm not a DSP or RF engineer, so I don't really understand either type of modulation.) I do believe, is that there will be progress when we focus concentrated effort on a new approach (such as shifting from multi-threshold level detection to time synchronization); I'm less sure that it will come in exactly the predicted way. / Lars Poulsen E-mail: <lpoulsen () UCwireless com> Senior Software Engineer Telephone: +1-805-964-5848 ext 279 UC Wireless, 323 David Love Place, Santa Barbara, CA 93117, USA
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- IP: Trade-offs vs wishful thinking (compression and UWB) David Farber (Jan 10)