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From: gnu@hoptoad.UUCP
Newsgroups: comp.dcom.modems
Subject: Re: Facsimile modems vs Regular modems?
Message-ID: <2234@hoptoad.uucp>
Date: Tue, 2-Jun-87 17:31:11 EDT
Article-I.D.: hoptoad.2234
Posted: Tue Jun  2 17:31:11 1987
Date-Received: Fri, 5-Jun-87 00:45:35 EDT
References: <306@gen1.UUCP>
Organization: Nebula Consultants in San Francisco
Lines: 34

In article <306@gen1.UUCP>, peter@gen1.UUCP (Peter CAMILLERI) writes:
>   So my question is what is the difference between a regular modem and
> a facs modem and can I use facs modems to create a cheap, high speed
> private network?

Facsimile (usually abbreviated "fax") modems are just half duplex
modems.  For applications which can handle half duplex, they are fine.
A friend of mine is selling 9600 baud fax modems on a PC-bus card,
and has file transfer software and such for them.  They are great if
you want to send many kbytes in one direction before you get ANYTHING
back from the other direction.

It's like the difference between a Bell 202 modem and a Bell 212
modem.  The 202 is 1200 baud, half duplex.  You could get them used for
$50 in about 1980.  Even seven years later it's hard to get a Bell 212
(full duplex 1200 baud) modem for $50.

The problem with general use is that these modems take anywhere from
1/4 second to a second to turn the line around and transmit in the
other direction.  This is rough on most applications, which assume
instant turnaround.  Even for old half-duplex terminals like the IBM 2741,
a quarter second pause after you hit return was noticeable; the timesharing
system I helped to manage tried to keep response into the 1/10th-second
range except at peak hours.

Note that most of the high speed "full duplex" 9600 baud modems are
faking it.  They use this kind of modem internally, but buffer the data
and figure out when to turn the line around.  So far none of them seem
to have gotten it right, at least for interactive use.

-- 
Copyright 1987 John Gilmore; you may redistribute only if your recipients may.
(This is an effort to bend Stargate to work with Usenet, not against it.)
{sun,ptsfa,lll-crg,ihnp4,ucbvax}!hoptoad!gnu	       gnu@ingres.berkeley.edu