Data Compression

Jay Shoup (jashoup@indiana.edu)
Fri, 5 Apr 1996 14:36:06 +500


On 04-05-1996 someone wrote:

>V.42 is data compression. Actually, keep it on all the time, as it's
>smart enough to realize when it's not doing any good, and 'escape'. So it
> doesn't hurt.

No exactly true

V42 is NOT compression, it is error correction, V42Bis is compression
and error correction. (Fine point I understand)

If you are using PPP's built in compression (Van-Jacobson), V42Bis may
actually slow your data rate. The problem is that if the data is already
compressed and can not be compressed anymore, any attempt on the part
of the modem to further compress or to analyze to see if it can be compressed
actual expends time. This TIME delay acts as if it is slowing the data
throughput. This is also true if you are sending data that has been compressed
by products such as Pkware's ZIP tools.

> But to get it's advantages, you have to have a higher baud rate
> setting between the modem and the computer.

True

> With video/audio data like CU-seeme, it's useless, but for other
> peripheral data, who knows?

Not true. Video data is not random. It has very obvious patterns.
Compression works by looking at repeatable patterns and reducing the
information being sent in those patterns. Some forms of compression
are better than others, MPEG for example is a "better" compression
algorithm than is V42Bis for video, but V42Bis does still give some
compression on non-pre-compressed video. The same holds true for
audio.

Jay!
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Jay Shoup
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Indiana University
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