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From: motcid!ivory!rupeb@uunet.uu.net (Bernard Rupe)
Newsgroups: comp.dcom.telecom
Subject: Re: Modems and Phone Rates
Message-ID: <1856@accuvax.nwu.edu>
Date: 5 Dec 89 19:08:01 GMT
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X-Telecom-Digest: Volume 9, Issue 554, message 6 of 11


In article <1798@accuvax.nwu.edu> jbayer@ispi.com (Jonathan Bayer)
writes:
>X-Telecom-Digest: Volume 9, Issue 551, message 1 of 11

>Sorry, you're wrong.  The telephone network is designed to work with
>human voices. As such the equipment multiplexes many conversations
>onto a single wire.  Human conversation has many gaps that the network
>can use to multiplex other conversations using the same frequency.  A
>modem is on continously, tying up a frequency full-time.  Assuming
>that a wire can handle 100 different conversations at one time, and
>further assuming that 10 % of the conversations is quiet, that means
>that with the proper equipment a single wire could handle 110
>conversations at the same time. However, you use modems and all of a
>sudden the network loses some of its excess capacity.

>...however you cannot deny that modems _do_ take up bandwidth that
>conversations do not.

Although I haven't been following this subject consistently, I think I
can shed some light on the situation.  Voice calls in today's network
are indeed restricted to 3400 Hz.  These calls are sampled at 8 kHz
and converted to a digital rate of 64 Kb/s.  These 64 Kb/s channels
are then multiplexed and sent into the telephone network.  Today's
technology does not take advantage of any silent passages in
conversation (although it could be done, it would be very expensive).

Modem data is converted into the same 64 Kb/s and is multiplexed into
the telephone network just like a voice call (otherwise, how could you
use a regular phone line for a modem call?).  The result, then, is
that a modem call and a voice call take up exactly the same bandwidth
in the telephone network.

Bernie Rupe  uunet!motcid!rupeb