Fine those are the technical details don't loose the overall picture. Sorry
I should have been more rigerous with the details but I was just trying to
show a concept. I was just trying to get it clear that the DSL represents
the conection to the ISP. Not the gateway to the internet that is the ISP.

I was also noting that dial in modems have a need for a fixed continuious
64kb/s per modem (digital to analogue conversion) and require hardware to
decode the modems signals. This represents costs in hardware and possible
multiple "telco" circuits. This to me seems more expensive than having one
central ATM portal which handles all the DSL connections with one router
etc.

Also Saying that DSL runs at 8 Mb/s is misleading. You need something at the
other end which can serve 8MB/s. The rate at which the DSL runs has got an
indirect link to the internet gateway. This is determined by the ISPs desire
for quality of service. TCP/IP is packet based (atm more so) so that
"streams" of packets can easily be interleaved spreading the badwidth across
multiple clients. Just because you have a link to your ISP of 8MB/s does not
mean that your link to the internet is 8Mb/s you share the link to the
internet with other users.

I also note that note everybody is constantly using all their bandwith.
Usage is bursty - you download, you browse. So this means on average you may
use alot less than your total available bandwith.

Adam

----- Original Message -----
From: "Juha Saarinen" <juha@saarinen.org>
To: "Adam Bowden" <adam@datafreight.co.nz>; "Adsl@Unixathome. Org"
<adsl@unixathome.org>
Sent: Friday, October 06, 2000 10:20 AM
Subject: RE: clear/telecom deal

> 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.
> %->
> %->
>
>
> This message is part of the NZ Broadband mailing list.
> see http://unixathome.org/adsl/ for archives, FAQ,
> and various documents.
> To unsubscribe: send mail to majordomo@unixathome.org
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>

This message is part of the NZ Broadband mailing list.
see http://unixathome.org/adsl/ for archives, FAQ,
and various documents.
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Received on Fri Oct 6 10:58:52 2000