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Re: PPP & Compression: Useful or Useless?



>>turn off (some) modem compression?  This is because I've heard that
>>the protocols in the modem may actually slow things down when handling
>>already compressed data.
>
>This is true for MNP5 compression. V.42bis compression disables itself when
>data can't be compressed further. At least it is supposed to do so.

Again almost true; MNP5 is stupid and will try to compress data regardless
of whether or not it will actually decrease the size of the file.  V.42bis
will selectively compress files (ie. if compressing a gzip file doesn't
make it smaller it won't try to compress it) however it still takes a
certain amount of time to make that decision.

>From memory (and this is going back a bit to when our backbone was a 14k
modem :) when we were running two ZyXEL modems back to back over a very
clean analog leased line the best performance we got was with compression
on but error checking off.  Though this was for general net traffic.  

If you were maintaining a mirror of exclusivly pre-compressed files I would
suspect that you might get marginally faster times without compression
(assuming use of V.42bis) but whether the gains would be worth the hassle
would be debatable.

Try it and see.

Adam



----------------- Earthlight Communications Limited ----------------
P.O. Box 5301	                Adam Shand 	       (fax) +64 3 477 5463
Dunedin, New Zealand       Systems Manager    (voice) +64 3 479 0303
------------------ http://larry.earthlight.co.nz/ ------------------


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