Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!ukma!uflorida!mephisto!prism!gitpyr!davidc
From: davidc@pyr.gatech.EDU (David Carter)
Newsgroups: sci.electronics
Subject: Re: Data Over Voice
Summary: No magic, just filters and FDM
Message-ID: <9703@pyr.gatech.EDU>
Date: 8 Dec 89 19:23:38 GMT
References: <8912061817.AA28752@en.ecn.purdue.edu>
Distribution: usa
Organization: Georgia Institute of Technology
Lines: 33

In article <8912061817.AA28752@en.ecn.purdue.edu> steele@en.ecn.purdue.edu (Richard Steele) writes:
>Here at Pudue U., we now have access to something called data over voice
>(DOV) units.

Here at Georgia Tech, I thought the service was called Voice Over Data.

>Question:  How would something like this work?  The phone company _does_
>need to make some changes to their central switch...
>So, what magic is involved here?

The box contains both a 9600 bps modem and low-pass and high-pass filters.
A similar device is installed at the CO.  I was also under the impression
that you must be reasonably close to your CO for this to work.

The service uses Frequency Domain Multiplexing -- it gives your phone a band
from 0 to 4KHz.  Everything higher is filtered out so you don't hear the modem
noise.  Otherwise, your phone is working the way it always did, on an analog
line.  Similarly, the modem is kept from hearing everything below some
frequency (probably a little bit higher than 4KHz).  That's all there is to
it.  If you could listen to the phone line before it goes into the box, you'd
hear both your voice calls and the modem noise.

There is probably just a bit more circuitry to enable you to still get
incoming ring without blowing up the modem, and to get a dialtone when you 
pick up the phone.  But I don't know the details.  I've only heard about
the service here (available to certain dorm residents); I haven't actually
used it.

I think users had to place a very short phone call to the CO (maybe a *nn
or #nn type thing) in order to activate the modem each time it is used.


David Carter		davidc@pyr.gatech.edu