From: Steven M. Bellovin (smb@research.att.com)
Date: Sat Mar 04 2000 - 12:37:25 PST
In message <3.0.6.32.20000304105946.007f7420@pop.sprynet.com>, David Honig writ
es:
> What really concerns me is that their business model
> trashes the Everyone's-a-Publisher potential of the net.
> Maybe there is unlimited free web space out there today, but its
> not under your control, and it limits what you can do (eg.,
> CGIs, freedom of expression, etc.)
...
>
> Well lets see, the cable/dsl ISPs own the equiptment on both ends of the
> last mile.. what physics and the FCC don't constrain it seems that they do.
As I said, there is physical reality involved, not the business model. I
hadn't wanted to post a detailed expanation on something that's mostly
off-topic, but....
A standard one-way cable system is set up as a tree. There's fiber from the
head end to "fiber nodes". (For all practical purposes, bandwidth on the
fiber is unlimited.) From the fiber node, there are many lengths of coax;
these in turn feed repeaters, which each feed several other branches of coax.
You end up with a moderately bushy setup, with (as I recall) at least three
levels of repeaters.
To convert a cable system to two-way, you don't make it into a giant CSMA/CD
Ethernet. You can't -- the distances involved are too great. Instead, every
repeater is coupled to a high-pass filter; at the same points, a repeater
pointing the other way (upstream) is coupled to a low-pass filter.
The standard TV signals continue to propagate downstream as they did before.
The old repeaters, fed by high-pass filters that allow through everything from
channel 1 and up, to about 1Ghz, carry standard TV signals. One or more
channels -- that is, one or more *TV* channels -- carry IP service downstream.
Cable modems broadcast on frequencies below channel 1; these signals feed
through the low-pass filter and are sent upstream. Eventually, they reach the
CMTS (cable modem termination system) at the fiber node; at that point, they
travel on the fiber where bandwidth is plentiful. Cable modems have to request
time slots to transmit; the CMTS knows the distance (in nanoseconds) to each
cable modem, and uses that for allocations.
If you want to add more downstream bandwidth, it's easy -- just allocate
another TV channel to the Internet instead of the Do-it-yourself Gerbil
Network. But what do you do to add more bandwidth upstream? Given the design
of the two-way cable plants, your only choice is to split the fiber nodes.
That generally calls for many more fiber nodes, and hence the replacement of a
lot of coax with fiber. Yes, it can be done, but it's expensive.
Bottom line -- there is certainly a business decision about how you allocate
your limited upstream bandwidth. But it is much more limited than downstream
bandwidth, and increasing it is much more expensive.
--Steve Bellovin
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