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From: ms6b+@andrew.cmu.edu (Marvin Sirbu)
Newsgroups: comp.protocols.iso.x400
Subject: Re: DATA Compression and X400 standards
Message-ID: <Qb=4CfO00VADA1N41e@andrew.cmu.edu>
Date: 29 Oct 90 15:07:51 GMT
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Approved: usenet@ICS.UCI.EDU
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Urs,

You miss the point that Vint was trying to make.  Shannon's theory of
information says that the more you know about the message set, the more
effectively you can compress it.  Thus, if I send a multi-media message,
I want to use two dimensional run length encoding to compress the image
portion, but a very different scheme (LZW?) to compress the text.
With image alone, I would use a different encoding table if the image is
scanned at 600 dpi than I would use if it is scanned at 200 dpi.  In fact,
using an inappropriate encoding scheme can actually _increase_ the number of
bits a message takes.
The inefficiency of doing ecoding only at lower layers is well illustrated
by the problem of telephone circuit encoding.  If I intend to use the circuit
only for voice traffic, I can easily encode it as 16 kbps or 8 kbps.  If
I want the channel to carry any kind of 3300 Hz bandwidth information
(e.g. modem traffic as well) than the best I can do is ADPCM at 32 kbps.

While it may appear simpler to use a single compression scheme at a layer
below the application, such an approach may sacrifice substantial
potential efficiency gains in transmission.


Marvin Sirbu
CMU