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From: rdippold@cancun.qualcomm.com (Ron Dippold)
Newsgroups: comp.dcom.modems
Subject: Re: Using High Speed Modems
Message-ID: <1991May24.200315.7997@qualcomm.com>
Date: 24 May 91 20:03:15 GMT
References: <6536@vela.acs.oakland.edu> <1991May23.193607.14738@qualcomm.com> <1991May24.181546.3802@waikato.ac.nz>
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In article <1991May24.181546.3802@waikato.ac.nz> atc@waikato.ac.nz writes:
>In article <1991May23.193607.14738@qualcomm.com>, rdippold@cancun.qualcomm.com (Ron Dippold) writes:
>> 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?
>> 
>> First, V.42 is only error correction, V.42bis is compression.  However, V.42
>> is smart enough to realize when the compression is causing the data to expand
>> and will send the data un"compressed".
>> 
>So do all compressed files expand or only some?
>i.e. if i'm only interested in sending compressed files, then is v42bis a waste
>of time?
>
>Your answer keenly awaited as I have on order a v22bis/v42bis modem.

The bottom line is that it _can't_ hurt you.  And it is definitely a help in
most cases.  It depends on the modem's implementation, but many files that
are compressed also have sections that aren't compressed (the individual file
headers, etc.) and sometimes the V.42bis can compress other parts slightly
better depending on the packer.

I get about 1700 cps with V.42bis turned on and about 1500 cps with it turned
off on a typical transfer.

In addition, once you have V.42bis and the other side is using it, there are
times when it is not worth it to use a packer, the V.42bis will speed things
up enough.  So you just send that text file instead of having to pack it
first.  More and more modems in the 2400 bps range are showing up with V.42bis,
and there are plenty of high speed modems with it.  I'd say it's definitely
worth it.
-- 
Standard disclaimer applies, you legalistic hacks.     |     Ron Dippold