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From: tnixon@hayes.uucp
Newsgroups: comp.dcom.modems
Subject: Re: Using High Speed Modems
Message-ID: <3958.283bf0b4@hayes.uucp>
Date: 23 May 91 16:41:24 GMT
References: <28675@uflorida.cis.ufl.EDU> <6536@vela.acs.oakland.edu>
Distribution: na
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 44

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?

Any compression scheme will expand data that does not have enough 
redundancy in it, simply because of the overhead required to pass 
the data through the encoding scheme.  This can happen if the data 
is pre-compressed, or simply random by nature (e.g., object code).

V.42bis provides the ability for the transmitting modem to signal 
the receiving modem that it wants to switch from sending the 
original data (in the clear) to sending the encoded (compressed) 
data, and vice-versa.  It is also possible, but not required, for 
V.42bis modems to monitor their transmitter performance, and 
determine when the compressed data requires more bits than the 
uncompressed data.  V.42bis modems that incorporate such a
performance monitoring function can use the mode-switching function
to send the uncompressed stream instead of the compressed stream, if
that happens to be more efficient at the time. 

I must point out that the parameters of this performance monitoring 
function and the decision points as to when to change modes are NOT 
a subject of V.42bis -- not a part of the standard.  The reason this
was left open is that it's NOT a compatibility issue -- the modems
WILL accurately transfer data, even if they never switch modes when
performance degrades.  The monitoring of performance and switching
of modes is a PERFORMANCE issue, and whenever possible the standards
committees try to leave these things open for the creativity of
implementors to shine through a differentiate well-engineered
products from those that aren't.  Therefore, this is an area where
you will see performance differences between manufacturers (and it 
is not the only such area in V.42bis).

	-- Toby

-- 
Toby Nixon, Principal Engineer    | Voice   +1-404-840-9200  Telex 151243420
Hayes Microcomputer Products Inc. | Fax     +1-404-447-0178  CIS   70271,404
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