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From: motcid!segal@uunet.uu.net (Gary Segal)
Newsgroups: comp.dcom.telecom
Subject: Re: Baud per Hertz
Message-ID: <8843@accuvax.nwu.edu>
Date: 10 Jun 90 18:21:20 GMT
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rpw3%rigden.wpd@sgi.com (Rob Warnock) writes:

[Very in depth and useful analysis of noise and it's effects on analog
signals deleted]

>In article <8772@accuvax.nwu.edu> codex!peterd@uunet.uu.net (Peter
>Desnoyers) writes:

>| However, a baud is not a bit. By the Nyquist theorem, you can only get
>| 2f bauds per second. In practice high-speed modems such as V.32 run at
>| about 2500-3000 bauds/sec over lines with a 3000Hz bandwidth.

>| >Still, in the real world 7 baud on 5 Hz is very good!

>| 9600bps over 3000 Hz is a good deal better, and is quite common.

>Oops! You fell in the trap, too! Those 9600 b/s modems use 4
>bit/symbol modulation, and so actually run at 2400 baud. And 2400 baud
>on 2700 Hz (3000 - 300) is not as good as 7 baud on 5 Hz.

True, V.32 modems can run at 4 bits/baud (16 symbols), but most are
usually run at 5 bits/buad (32 symbols) with trellis coding.  Trellis
coding provides a type of forward error correction (one extra bit for
every four data bits) at a very low level of data transfer.  (Is this
OSI layer 0.5??? :-).  In general, trellis coding gives the modem a
performace gain of about one to two db over the uncoded signal.  Note
that both run at 2400 baud, so that trellis coding sends 12,000 bps,
of which 20% is error correction.


Gary Segal	...!uunet!motcid!segal		+1-708-632-2354
Motorola INC., 1501 W. Shure Drive, Arlington Heights IL, 60004
The opinions expressed above are those of the author, and do not consititue
the opinions of Motorola INC.