[IP] more on Users: The Usual Suspects Again

[Dave Farber <dave@farber.net>]

------ Forwarded Message
From: "Steven M. Bellovin" <smb@research.att.com>
Date: Tue, 31 Dec 2002 16:08:37 -0500
To: dave@farber.net
Cc: ip <ip@v2.listbox.com>
Subject: Re: [IP] Users: The Usual Suspects Again

I suspect that the problem is technological, rather than the cable
companies bowing to the RIAA or trying to have their cake and eat
it, too.  Briefly, IP over cable is *not* Ethernet, and is not
symmetric; as a consequence of the physical design, there is
inherently much more bandwidth downstream than upstream, and adding
more downstream bandwidth is much cheaper than adding upstream
bandwidth.  Note that the article quoted the companies themselves
as talking about upload rates, not download rates.

Let me give more details.  The original cable plants were built as
trees, with unidirectional amplifiers at the nodes.  The arcs
closest to the root are fiber; those nearer the leaf edges are
coax.  To convert a cable system to two-way traffic, a high-pass
filter is put in front of the existing repeaters, while upstream
repeaters are gated by low-pass filters.  In other words, the
current TV channels, from 1 to whatever, continue to flow as before,
along with data traffic from the Internet.  Data traffic to the
Internet has to share a single low-frequency band.

Now -- if more people start downloading stuff, the cable company
has an easy problem:  they take away the Home Gerbil Channel or
some such, and allocate that frequency band to IP.  But the only
cure for too much upstream traffic is to split the tree, and move
the fiber further down, as well as installing more "fiber nodes".
That's an expensive business, and I don't think they're done paying
for the initial two-way conversion yet.

Could they have deployed a different two-way scheme, way back when?
Sure.  But they didn't, and they made that choice well before
Napster was on the horizon.  Right now, we're stuck with an asymmetric
data path.  (Disclaimer:  this is the way things worked about 5
years ago, when dealing with IP over cable was part of my day job.
I don't know if things have changed since then.)


        --Steve Bellovin, http://www.research.att.com/~smb (me)
        http://www.wilyhacker.com (2nd edition of "Firewalls" book)



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