DSL is the "transport layer" between the exchange and customer -- it uses a
frequency band over the normal voice band (think it's 32 or 50kHz to 1MHz),
which in turn is split up in 256 4kHz "bins" because Telecom uses Discrete
Multi-Tone (DMT) modulation of the DSL signal. Some "bins" are used for
upstream data, others for downstream, and you can configure the modems to
use different "bins" or not use some of them at all to avoid noisy parts of
the frequency spectrum.

You've actually got ATM running over the DSL signal to your modem, but all
IP traffic is encapsulated in PPP frames, which the AAL (ATM Adaptive Layer)
in the modem chops up into 48+5 byte ATM cells (these are reassembled into
PPP frames at the ISP). You don't actually have to use PPP for the
connection to work... from what I can understand, you could run IP over ATM,
but PPP is the only protocol with authentication capabilities (and perhaps
error correction/link control as well?) so it's used despite the additional
overhead it introduces.

Seeing that each ADSL modem is capable of up to 8Mbps downstream, and close
to 1Mbps upstream, a T-1 or E-1 (1.544Mbps and 2.048Mbs respectively) would
be inadequate. Even E-3 circuits (45Mbps) couldn't support that many users
per circuit. ATM with 155Mbps and faster circuits is better suited for this,
but I can't imagine any single ISP in NZ having the deep pockets to fund a
national network with that kind of bandwidth.

Then there's the international bandwidth issue... you need lots of the
stuff, which is why Telecom e.g. has bought in satellite capacity for
instance.

Disclaimer: this is based on my understanding of things, which could be
wrong (the things, not my understanding ;-)), and there are many more
factors in the DSL equation than I have mentioned above.

-- Juha

%-> -----Original Message-----
%-> From: owner-adsl@unixathome.org [mailto:owner-adsl@unixathome.org]On
%-> Behalf Of Adam Bowden
%-> Sent: Friday, 6 October 2000 9:49 a.m.
%-> To: adsl@unixathome.org
%-> Subject: Re: clear/telecom deal
%->
%->
%-> IMHO:
%->
%-> This is what I thought happens - Just a guess (probably get flamed )
%->
%-> DSL represents the connection between client and telecoms atm
%-> network. Which
%-> seems to establish some form of "ppp" type connection across
%-> telecoms ATM
%-> network. This represents the connection between client and ISP. Telecom
%-> charges the dsl user on a usage based system.
%->
%-> If the ISP did not connect to the client through the ATM
%-> network then they
%-> would probably need ISDN T-1 circuits to take the incoming
%-> modem calls and
%-> the appropriate modem hardware. Ie they need a modem and 1/24th
%-> of an T-1
%-> ISDN connection for each client. This is probably more
%-> expensive than the
%-> ATM type solution because they need hardware and fixed
%-> bandwidth per client
%-> ie each modem requires 64kbs.
%->
%-> Now I assume what ISPs rely on is that not everybody thrashes
%-> there internet
%-> connection so they will "concentrate" all their clients going
%-> through one
%-> Internet pipe. The speed of this pipe only needs to be good enough to
%-> maintain their customers perception of Quality of service.
%->
%->

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Received on Fri Oct 6 10:20:14 2000