Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!mips!cs.uoregon.edu!ns.uoregon.edu!milton!hayes.ims.alaska.edu!floyd
From: floyd@ims.alaska.edu (Floyd Davidson)
Newsgroups: comp.dcom.modems
Subject: Re: comp.dcom.modems lexicon
Message-ID: <1991Mar7.055236.16871@ims.alaska.edu>
Date: 7 Mar 91 05:52:36 GMT
References: <1991Mar5.225546.6672@panix.uucp> <19552@cbmvax.commodore.com> <3832.27d4dcf9@hayes.uucp>
Organization: University of Alaska, Institute of Marine Science
Lines: 95

In article <3832.27d4dcf9@hayes.uucp> tnixon@hayes.uucp writes:
>With regard to the discussion on "duplex" vs. "full duplex", etc.:
>
>I serve on the Vocabulary Special Rapporteur's Group in Study Group
>XVII, and so try to pay attention to these things. I sometimes point
> [...]
>per second".  "Full duplex" never has been an official term, so we 
> [...]
>Personally, I don't see any real harm in saying "full duplex" when 
>you mean "duplex", in terms of two-way simultaneous transmission on 
>the phone line.  There's really no confusion here.  I would prefer 
>that glossaries and lexicons mention that the correct term is simply 
>"duplex".
>
>I _do_, however, object to the use of "half duplex/full duplex"
>when, from context, the meaning seems to be "local echo/no local
>echo"; that is a serious misapplication of the terminology. 
>
>The issue of "duplex" vs. "simplex" is interesting.  The standards 
>committees generally use "simplex" when the modem is truly capable 
>of only transmission OR reception, and "half-duplex" when it is 
>capable of alternating between transmission and reception, but not 
>performing both at the same time.  I suppose we could call it 
>  [...]
>In article <19552@cbmvax.commodore.com>, grr@cbmvax.commodore.com
>(George Robbins) writes: 
>
>> The opposite of duplex is simplex, i.e. able to transmit one way, over
>> on set of wires, period.  Given this, and the fact that half-duplex
>> technology probably preceeded full duplex, and the full was added to
>> to make the distinction, I think it's still a meaningful modifier.
>
>Actually, the first modems standardized by the CCITT were V.21 (300 
>baud duplex) and V.23 (600 or 1200 baud forward channel with 75 baud 
>backward channel).  V.23 can be implemented with or without the 
>reverse channel, and the direction of transmission on the reverse
>channel can be either linked or not linked to the forward channel 
>direction.

I don't want to add any confusion to this subject, but it might do
well to note the difference in the historical derivation of the
duplex/simplex and full/half duplex terminology.

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.

Full/Half Duplex originated with teletype circuits on telephone
carrier circuits.  And it did directly relate to local echo.
Both a full dux and a half dux circuit used a two way (duplex)
modem at the point where a digital signal was converted to an
analog signal to be sent over the telephone carrier system.  An
half dux circuit used the same digital circuit for transmit and
receive.  (Generally this was a 60ma neutral loop.)  By
definition a half dux circuit HAD local echo, and that was what
made it half duplex.  And that was a function of the terminal
equipment on the digital side, not a function of the modem.
The digital side of the modem did have to be arranged to not
send what it received (we used to call it a "reflection" if
it did).

The terms are confusing today because historically they came
from two different areas of technology (telephone people have
always had their own terminology that is distinctly different
from the rest of the world of electronics).

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.

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.  A device that can transmit or receive, but not at the
same time is a simplex device.  A device that is essentually
a duplex device (can send and receive at the same time), but
which is arranged for local echo (and hence can't be used in
both directions at the same time) is using a half duplex mode
of operation.

I'll accept failure in not wanting to add confusion to the
subject.  I probably have added just that.  Also I am willing
to accept whatever definition CCITT wants to give any of these
terms, and use that definition henceforth.

Floyd
 
-- 
Floyd L. Davidson  |  floyd@ims.alaska.edu   |  Alascom, Inc. pays me
Salcha, AK 99714   |    Univ. of Alaska      |  but not for opinions.