Newsgroups: comp.dcom.modems
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!zaphod.mps.ohio-state.edu!van-bc!cynic!curt
From: curt@cynic.wimsey.bc.ca (Curt Sampson)
Subject: Re: Using High Speed Modems
Organization: Mad Artists' Technological Hangout
Distribution: na
Date: Thu, 23 May 1991 04:07:23 GMT
Message-ID: <1991May23.040723.18039@cynic.wimsey.bc.ca>
References: <28675@uflorida.cis.ufl.EDU> <6536@vela.acs.oakland.edu>

In article <6536@vela.acs.oakland.edu>
  rdthomps@vela.acs.oakland.edu (Robert D. Thompson) writes:

> 	Do modems with V.42 compression cause compressed files (e.g.
> 	PKZIP, LHARC, etc...) to become effectively larger when
> 	transferred?  In other words, is there negative compression
> 	when transferring a compressed file via V.42?

Well, V.42 is an error correcting scheme.  It does introduce a certain
amount of "compression" when it strips the start and stop bits,
generally giving you about a 15% increase in bps rates no matter what
you transfer.

V.42 is often paired with MNP-5, which is a compression scheme.  Yes,
MNP-5 can slow down your transfers if you transfer stuff that's
already compressed.

V.42bis, which has its own compression as well as error correction
(thus avoiding the need for MNP-5) will detect compressed files and
transfer them as is, rather than attempting to compress them and
expanding them in the process.

cjs
-- 
Curt Sampson            | "[Atari's Race Drivin'] is more fun as a perverse
curt@cynic.uucp         | sort of flight simulator than a driving game."
curt@cynic.wimsey.bc.ca |           --Kevin Nomura (chow@netcom.com)