Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!sunybcs!ugkamins@marvin.cs.buffalo.edu
From: ugkamins@marvin.cs.buffalo.edu (Dr. R. Chandra)
Newsgroups: comp.sys.amiga
Subject: Re: 9600 bps Modems
Message-ID: <7699@cs.Buffalo.EDU>
Date: 10 Jul 89 05:48:18 GMT
References: <11167@mcdphx.phx.mcd.mot.com> <380027@hpdml93.HP.COM>
Sender: nobody@cs.Buffalo.EDU
Reply-To: ugkamins@marvin.cs.buffalo.edu (Dr. R. Chandra)
Organization: SUNY/Buffalo Computer Science
Lines: 49
In-reply-to: stephen@hpdml93.HP.COM (Stephen Holmstead)

In article <380027@hpdml93.HP.COM>, stephen@hpdml93 (Stephen Holmstead) writes:
=>_baud_ refers to "symbols per second" in the transfer rate.  In older
=>modems (ie. 1200bps, 300bps, etc), each symbol comprised one bit worth
=>of data (one frequency was used to represent a ZERO and another was used
=>to represent a ONE).  Thus, 1200 baud equals 1200 bps.  However, when 
=>people began increase the transfer rate, it only worked when there was a 
=>direct connection.  However, when trying to use the telephone system, 
=>the maximum transfer rate is about 2400 baud.  This is due to the 
=>multiplexing done on the telephone lines.  It sounds like when you are 

Well, just as you said you HAD to jump in and correct, I feel I must
do the same for(?) you.  BAUD refers to "transitions per second."
Virtually all the literature I have read states that the maximum data
transfer rate attainable on a PSTN line is 1200 baud TOTAL, meaning
600 baud in each direction.  Sorry, but unless you are not talking
about Bell 212A, each direction is going at 600 baud twice over, once
for FSK and the other for PSK.  Such arrangements are referred to as
dibits, or "two bits."  Data rates (bps) that are faster are
accomplished by means of AM, multiple carriers, and dividing the phase
transitions into more phase angles.  The best explanation I've seen so
far, which also jives with other sources I've seen too, is _C
Programmer's Guide to Serial Communications_ by Joe Campbell, pub. by
Howard W. Sams & Co.

It is also quite true that communications companies multiplex their
traffic to achieve higher rates, both voice and data (the latter with
PS/VAN's).

As stated in this book, it is not (necessarily) the fault of
multiplexing of the phone "conversations," but the historical (to be
read conservative) nature of PSTN service.  It is meant to be
optimized for clear communication for voice, not faithful reproduction
of sound, or transmittal of data.  (The major constraint is
bandwidth--from about 300 to 3K Hz.  Fitting two carriers for 212A
plus their associated sidebands when modulated is a miracle to me.)

I agree with you, though.  I *HATE* when people blur the distinctions
between baud rate and data rate.
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ugkamins@sunybcs.UUCP   (UnderGraduate john i. KAMINSki)
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