Path: utzoo!mnetor!uunet!portal!cup.portal.com!truett
From: truett@cup.portal.com
Newsgroups: comp.dcom.modems
Subject: Re: fax modems / 'normal' modems
Message-ID: <4517@cup.portal.com>
Date: 15 Apr 88 06:17:12 GMT
References: <834@spdcc.COM>
Organization: The Portal System (TM)
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XPortal-User-Id: 1.1001.2190


There are commercial companies that sell copies of the CCITT standards but the
simplest method I have found for getting them is to order them from the United
Nations Bookstore in New York.  The facsimile standards are in the volume
containing the Series T Recommendations.  The Group III encodings are in T.4
and the Group IV codings are in T.5.

The comment made by someone that Group IV requires T1 transmission is wrong.
The group I and II standards assumed an analog transmission method.  Group III
facsimile uses a digital method but incorporates error recovery features.
Group IV contains no error recovery features since it assumes a separate Link
Layer is handling that chore.

As for the differences between "fax" modems and "data" modems:  A fax modem
establishes a connection using V.25 signalling and an information exchange
protocol with the other end (defined in T.30) that is conducted at 300 bps.
It then switches to the highest speed the other end says it can accept, which
is usually either 4800, 7200, or 9600 bps.  This is a half-duplex, synchronous
data transfer conducted using V.29 signaling.

A data modem, however, establishes the connection using a V.21 sequence of
training tones which the originating modem responds to to let the answering
modem know it has agreed on a frequency.  This is quite different from the use
of a fixed 300 bps starting speed to exchange data about available higher
rates.  Most 9600 bps data modems are either using V.29 half-duplex or V.32
full-duplex.  There are also modems which use the 14.4 kbps V.33 standard but
run it at 9600 which it allows as a fallback.  Note that all 9600 modes are
synchronous.

Finally, the ANSI standards can all be acquired from CBEMA in Washington, D.C.

Truett Lee Smith, Sunnyvale, CA
UUCP:  truett@cup.portal.com