: a question on what I buy when I get a broadband connection.]

["David Farber" <dave () farber net>]

---------------------------- Original Message ----------------------------
Subject: Re: [IP] a question on what I buy when I get a broadband connection.
From:    "David I. Emery" <die () dieconsulting com>
Date:    Tue, February 27, 2007 9:05 pm
To:      "David Farber" <dave () farber net>
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On Wed, Feb 28, 2007 at 07:56:26AM +0900, David Farber wrote:
> I buy a , say, 6 meg cable broadband. Just what am I buying? Is it a
> constant guaranteed 6 meg, a burst 6 megs or what? What are the true
> economics?

        At the risk of injecting a lay opinion into an august forum, at
least in the US it is very clear that the infrastructure is built
(especially cable modem plant, which is somewhat Ethernet like) with
VASTLY less bandwidth than what would be required to support anything
like a constant guaranteed 6 meg to even a significant minority of
subscribers.

        Basically one or more 6 MHz wide CATV channels are allocated to
256 QAM downstream transmissions from the HFC node to all the users on
that node and  all downstream IP traffic gets muxed together onto the
resultant signal (around 40 Mb/sec depending on exact FEC and symbol
rate used).   This obviously limits all the users who share that channel
to share the same 40 Mbs bandwidth for their traffic, and in typical
CATV systems only at most two or three RF channels are available for
this stream,   This means that for the hundreds of subscribers who share
the cable plant connected to a HFC node (individual fiber back to the
end end basically)  only a few can be seeing anything like 6 Mbs
continuous traffic at once.

        Most cable modems and routers rate limit a given subscriber to a
burst max of something less than the full bandwidth of the downstream
channel, but occasionally more than the steady state rate in brief
bursts - however usually clamping to the steady state subscribed rate
quite quickly.

        As for the policy level issues as opposed to the limits of the
hardware, most cable modem ISPs have clauses in their AUPs which limit
"excessive use of system resources or interference with other users"
which have been regularly interpreted as limiting continuous high rate
flows of packets sustained for long periods.   Some monitor and enforce
unwritten bandwidth per month caps, others go after heavy users by
looking at other statistics.   And few seem to spell out publicly
exactly what their limits really are and how they apply them (or when
they change them).

        It has always been my understanding that the actual cost of
bandwidth to the Internet at large is such that a user sustaining
anything like the nominal maximum for a month would be highly
unprofitable for the ISP, but not being in the business I cannot cite
exact numbers.

        It basically is all you can eat provided your appetite and
capacity to eat are normal... and pricing is set accordingly.

-- 
   Dave Emery N1PRE,  die () dieconsulting com  DIE Consulting, Weston, Mass
02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."




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