Newsgroups: comp.dcom.modems
Path: utzoo!utgpu!cunews!hobbit.gandalf.ca!dcarr
From: dcarr@hobbit.gandalf.ca (Dave Carr)
Subject: Re: Uses of V.42 (bis?) data compression
Message-ID: <1991Apr12.132116.11546@hobbit.gandalf.ca>
Organization: Gandalf Data Ltd.
References: <10334@pitt.UUCP> <3908.280363d4@hayes.uucp>
Distribution: na
Date: Fri, 12 Apr 1991 13:21:16 GMT
Lines: 24

In <3908.280363d4@hayes.uucp> tnixon@hayes.uucp writes:

>I always advise people to compress before sending if possible.  An 
>offline compression program can make multiple passes over the data 
>and really optimize the compression, while a modem only has one shot 
>at it.  

CAN make multiple passes over the data.  In reality, most commercial 
or shareware compression programs make only a single pass.  I was
suprised to find out my favorite (PKZIP) does only one pass.  
LZW based algorithm are usually one-pass.  LHarc is one-pass.
Unix Compress is one-pass.

>Also, the compression acheivable in the modem is limited by 
>the ratio between the DTE-DCE speed ..

I agree.  So why do modems and PC get it together and get a decent
interface worked out.  Why not Ethernet ?

>and the carrier speed.  

I don't see this at all.  Perhaps the link level protocol.
Perhaps the line error rate.  But the compression should be
independant on the carrier speed.