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From: WITLICKI@WILLIAMS.BITNET
Newsgroups: comp.dcom.modems
Subject: Re: baud vs. bit/sec.
Message-ID: <8704220426.AA03311@ucbvax.Berkeley.EDU>
Date: Tue, 21-Apr-87 23:26:33 EST
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Posted: Tue Apr 21 23:26:33 1987
Date-Received: Thu, 23-Apr-87 04:00:32 EST
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In reference to the article Keith Petersen posted about Baud vs. Bits/sec.

> .....  This is nothing new, texts on the subject generally avoid
>        the term "baud"

  Yes, but computer companies use the word continually.

  ...Rather than do a point by point response to what I feel is a very
muddy, although perhaps technically correct article, let me give
you a few of my explanations... (hopefully a little more clearer then
the recursive definition of 'bit rate' in the glossary).

  We first have to separate our wires.  The RS232 wire between your
modem and your terminal or computer talks in binary (0 or 1) - the
data rate here is _Bits Per Second_.
  The Phone line which is between your modems talks in _Baud_.
  Now comes in the common confusion.  Computer and terminal makers
typically give the speed of their RS232 interfaces in terms of
'Baud' - this is wrong - it should be Bits Per Second.
  At modem speeds of less than 2400 baud the modulation techniques used
usually result in 1 baud = 1 bit per second, hence the casual and
incorrect use of terms.
  Definition: A Sine Wave is a signal at one frequency and one frequency
only.  (no harmonics if you think in terms of music).  On a telephone
channel which is bandwidth limited (by the phone company) to 3000 Hertz
- this is highest frequency (== fastest) which you can push through the
wire.  Because of signal degradation and the chaining of telephone
company equipment, the maximum signal frequency used in practice is
a few hundred Hertz lower.  If you are still confused - think
in terms of music - a higher frequency == a higher pitch.
  The Nyquist theorem says that for a sine wave of 2400 cycles per
second (I never did like renaming cycles per second into Hertz, next
thing you know, phonograph records will go at 33 1/3 Edisons, but I
date myself and I digress...) ... for a a sine wave of 2400 hz you
can have 1200 bits per second.
   Shannon, on the other hand, said hey fellows, we got us a Noiseless
Channel.  We don't have to push sine waves through it, we'll push
voltage levels through it which you can sample at each interval.
Heck, with no Noise at all, say we have at least 65,536 voltage levels
in each sample of a 600 baud channel and we can do 9600 bits per second...
(to use the example in the original piece).  Life is not so simple.
We do have noise.  The Nyquist Theorem does apply (...  but...).
   In the case of 9600 BPS over a voice grade channel, Phase Modulation
uses each sampling interval (baud rate time interval) to push several
bits of information over the line.

   I hope this helps some.

   Quite frankly, I think it's time to drop the term Baud from
the data communications consumer's terminology (who's the consumer?,
who's the engineer? who's the user? why do I have to go through
this to set a few a dip switches and use the thing?).  Computer systems
people have serial interfaces which talk at Bits Per Second.
I don't give a hoot what sideways modulation method and baud rate
you use over the phone line.  I want to talk in terms of 'I have a 9600
Bits Per Second Asynch. Serial Interface' and 'Does it talk to a Hayes
or a Bell 212A' and 'Can I use a voice grade line or do I order a
Type 3002 unconditioned line or what?'.

- randy

P.S. I am in the process of writing some introductory material
on data communications and would appreciate any pointers to
published stuff anyone has seen which explains bits per second,
parity, etc. for the user/system manager who shouldn't be subjected
to the Nyquist/Shannon intimidation.