Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!uunet!seismo!dimacs.rutgers.edu!rutgers!cbmvax!grr
From: grr@cbmvax.commodore.com (George Robbins)
Newsgroups: comp.dcom.modems
Subject: Re: comp.dcom.modems lexicon
Message-ID: <19594@cbmvax.commodore.com>
Date: 7 Mar 91 10:55:24 GMT
References: <1991Mar5.225546.6672@panix.uucp> <19552@cbmvax.commodore.com> <3832.27d4dcf9@hayes.uucp> <1991Mar7.055236.16871@ims.alaska.edu>
Reply-To: grr@cbmvax.commodore.com (George Robbins)
Organization: Commodore, West Chester, PA
Lines: 56

In article <1991Mar7.055236.16871@ims.alaska.edu> floyd@ims.alaska.edu (Floyd Davidson) writes:
> Duplex vs. simplex originally was used to define radio
> communications that could be carried on in both directions at
> the same time (duplex), as opposed to communications that used
> the same spectrum and facilities for both directions, but not at
> the same time (simplex).  The terms were applied to two-way
> radios, for instance CB, police FM, etc.  Certainly the same usage
> could be applied to a two wire telephone circuit used for data.

Cough, choke, gasp...

The simplex/duplex distinction goes back to telegraphy, like Western
Union, and this usage probably predates the radio orgin you suggest.
I don't know when the full/half duplex distinction came in - it could
have been early or could have awaited telephone carrier system to
show up.

> I'm not so sure that Toby is correct in calling it a misuse of
> the terminology to use full/half duplex to refer to local echo.
> That is exactly how the term was derived as it relates to
> data modems.  But it certainly has become ambigiuos, and such
> use should be discouraged.

It's a misuse when all it refers to is enabling and disabling
local echo, while the modem is actually still modulating in
the full duplex mode.  Remember that things like teletypes didn't
really generate a "local echo", it's just the transmitter and
receiver were normally wired in series on the same circuit and
couldn't the receiver seeing what was trasmitted, unless you
manually bridged across it.

> Likewise I don't think a modem that can only transmit or receive
> is a simplex device.  It is a "read-only" or "transmit-only"
> device.

You're on very thin ice here.  You want to avoid confusing the
sense of the terms as used to describe radio equipment
from data communications modems and communication channels. 

A simplex modem is one that can only send or receive.  A simplex
data terminal is something like a stock ticker. 

>                                              Also I am willing
> to accept whatever definition CCITT wants to give any of these
> terms, and use that definition henceforth.

I don't have too much trouble with the CCITT trying to define these
terms precisely and non-ambiguously, however that doesn't particularly
effect the traditional definitions of the words, at least outside the
context of the standards community.

I would be impressed if any committe could elide infamous "baud rate"...
-- 
George Robbins - now working for,     uucp:   {uunet|pyramid|rutgers}!cbmvax!grr
but no way officially representing:   domain: grr@cbmvax.commodore.com
Commodore, Engineering Department     phone:  215-431-9349 (only by moonlite)