Date: Tue, 17 Nov 1992  02:21:52 EST
From: FZC@CU.NIH.GOV
Newsgroups: comp.dcom.telecom
Subject: Re: Dial Tone on CATV
Reply-To: TDARCOS@MCIMAIL.COM
Organization: TELECOM Digest
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 857, Message 4 of 11

In TELECOM Digest 12-852, on "15 NOV 92 04:03" Randy Gellens
<MPA15AB!RANDY@TRENGA.tredydev.unisys.com> writes:

> With all the talk about cable companies wanting to compete with LECs
> for local dial tone, I started wondering: how can a cable company
> offer dial tone...I guess they could put everyone on a different
> frequency, but that would probably use up all of the cable, and
> then some.   Don't they want to leave room for TV?

I'm not a hardware engineer but the answer seems -- to me -- to be
quite obvious.  One TV channel takes 6MHZ of bandwidth.  One telephone
channel takes up about 10KHZ of bandwidth if I remember correctly.
This implies that if you separated each channel by an equal amount of
spacing (to make it easy) that you can put 300 telephone calls on a
single channel, simply by raising or lowering the frequency inband on
that particular channel.

For receiving telephone calls, it's really simple; there can be a
dedicated channel that does nothing but send out the equivalent of
Cellular phone "MIN/ESN" signals, telling a local phone to (1) ring
(2) select a certain channel as the place where the call is
originating from (3) start a "guard tone" while the call is in
progress at a certain frequency above the regular channel.  In fact, I
suspect that a number of features used in cellular telephones may be
useful for use in running telephone calls over cable TV.

To make outgoing calls requires some synchronization, i.e. you could
have a "guard tone" in the empty space between calls as an indicator
that a channel is empty.  The dial unit listens starting at a certain
channel for a guard tone and if it finds one, climbs up by 10KHZ to
the next channel, until it either finds an empty channel or reaches
the top of the band.  When it gets to the top of the band, it starts
over at the bottom until it recycles.  Assuming that a full search of
the entire band of 300 channels can be done in, say, three seconds, if
it can't find an open channel, it doesn't give a dial tone.

Now, once you find an open channel, you send a guard tone and in that
band a signal to request a dial tone by sending your id code.  (Funny
this is sounding more-and-more like cellular).  If you get your code
back and dial tone, you know you've siezed an outgoing trunk and can
proceed to dial; you pass the dial tone on to the telephone.  If you
don't get your code back, then someone else has simultaneously grabbed
the line; if the line does have dial tone now, you know that another
party has the line because they got it first, so you go back and climb
"up" the channel list looking again.

There are other options, such as assigning a specific channel to a
specific line and thus when the user sends a guard tone or whatever
signal is used to indicate "off hook" he gets a dial tone if there are
dial-tone senders available.  What can be done is on the pole, they
could reserve a whole coaxial cable - all 50 channels or so, which
would allow 15,000 simultaneous telephone calls -- to run from the
general location to the cable company's hub end.  So what can happen
is something like this:

1.  Your phone is assigned a certain frequency on your wire.
2.  When you pick up the phone, your "network interface block"
    sends a guard tone along that frequency's signalling band
    to request a dial tone.
3.  The local sender converts your channel into the open slot
    for this area from the 15,000 open channels available.
4.  The hub end sees if you're authorized and returns a dial
    tone.
5.  You get the dial tone and dial a number.

All of this happens within about one or two seconds from the time you
pick up the phone.

An article I read about four or five months ago in {Forbes} stated
that for cable companies to offer telephone service, they would
probably have to spend as much as $500 per subscriber to install the
necessary equipment.  On the other hand, the same article claims,
using ordinary single pair, the phone company could run as many as
three TV channels.  I find it hard to believe that someone could run
18MHZ of signal through a pair of wire, or even two pair.


Paul Robinson   TDARCOS@MCIMAIL.COM   These opinions are mine alone.