[9537] in Commercialization & Privatization of the Internet
Re: Internet in a box
daemon@ATHENA.MIT.EDU (Rick Spanbauer)
Sun Jan 9 09:02:53 1994
Date: Sun, 9 Jan 1994 09:01:03 -0500
From: rick@sbcs.sunysb.edu (Rick Spanbauer)
To: com-priv@psi.com
Cc: rick@sbcs.sunysb.edu
To answer your "how much bw in a TV channel question": it is relatively
easy to fit ~ 4 mbps into a 6 mhz tv channel using a form of frequency
shift keying. FSK makes non coherent detection fairly easy using off
the shelf discriminator chips, eg the motorola MC13055/MC13155. To
get to the 14,400/3000 = ~5 bits/hz spectral efficiency that POTS modems
manage would be very expensive at 10 mbps. Slicing up the TV channel
and trying to fit in 200 or so subchannels per tv channel would eat up
a great deal of bw just in guard channels between the subchannels. All
proposals I've seen for pushing data over cable tv plants use QPSK signalling,
which is reasonably easy to do digitally even at IF. Note that there
are issues which may limit the maximum signalling rate on cable tv, ie
reflections, ingress/egress noise, etc. CableLabs has an experiment going
out West in which they are attempting to characterize a few cable systems
so as to help manufacturers design appropriate adaptive equalizers to
handle problems with microreflections.
I disagree with the comment that "you can't sell the same pipe to more
than 200 people". This is already done on the Internet, eg capacity is
usually added based on average demand, not peak. So it would be with a
cable TV based data modem. Adding capacity is even easier if the modem
is frequency agile and can be moved as needed between several channels in
order to load balance. Rick Spanbauer, State U of NY/Stony Brook
