Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!motcid!koch
From: koch@motcid.UUCP (Clifton Koch)
Newsgroups: comp.dcom.modems
Subject: Re: Inexpensive 9600 baud modems
Message-ID: <5637@navy22.UUCP>
Date: 6 Dec 90 16:56:00 GMT
References: <136548@pyramid.pyramid.com>
Organization: Motorola Inc., Cellular Infrastructure Div., Arlington Heights, IL
Lines: 45

From article <136548@pyramid.pyramid.com>, by lstowell@pyrnova.pyramid.com (Lon Stowell):
-> In article <5435@navy19.UUCP> benyukhi@motcid.UUCP (Ed Benyukhis) writes:
->>57 Kbps on the voice grade line that is band limited to 3.4 Khz is contrary
->>to both Shannon and Nyquist rules.
->>
->>
->>Ed Benyukhis
-> 
-> 
-> Maybe so, but the POTS line, with its 3.4 Khz limit can easily
-> handle 160 Kbit/second or so rates.  ISDN easily places 144 Kbit
-> USER rate on this 2-wire line, using 2B1Q signalling...

  You're comparing apples to oranges.  First, the 3.4 Khz limit is not imposed
by the wires themselves.  It's imposed by the transmission methods of voice
data.  Second, the rates you quote are accomplished by quite different
transmission methods.  They used direct digital data transfer rather than
encoding data into audio tones.
  
-> Don't confuse the Nyquist limit on "baud" or carrier signalling
-> interval over 3.4 KHz with the ability to encode and decode user
-> information as multiple bits/baud over the bandwidth limited
-> channel...   38.4 USER rate works quite nicely over V.32 modems
-> by using Trellis (5:4 redundancy) and QAM (4:1) signalling at a
-> baud rate of only 1200 on the analog channel...although the
-> "raw" bit rate is only 9600...and V.32bis will push this to 14.4
-> raw rate,,,should be able to use data compression to get 57.6
-> out of a V.32bis modem easily...

  Maybe if you send a bunch of nulls.  I haven't seen any compression 
algorithms that can manage that big of a compression on even simple text
files (on the fly, at least).  But we're not really talking compression
here.  The true data rate is the amount of worst case (i.e. uncompressable)
data you can send.  Most of my modem sessions are with already compressed
files, and I keep compression turned off because I get less throughput due
to the header overhead of the compression scheme.

  Nyquists theorm does limit the basic baud rate of the information sent,
but baud rate doesn't have much to do with data rate.  Several bits
can be encoded (relatively) easily within each transition change, but there
is a theoretical limit to this also.
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