On 27/08/2013 09:16, The Natural Philosopher wrote:

Its very similar to ADSL in that THESE days all the 'modems' are in
fact banks of kit sitting at the end of an ATM backhaul for voice type
circuits. I cant remember if the presentation is ISDN or not.

The difference with ADSL is that the DSLAM sits in the exchange. However
the IP session only starts once the ATM conversation has been backhauled
back to the ISP. So as far as the ISP is concerned the IP routing stops
in their central machine room.
IP is then tunnelled over te ATM to the dslam and then over DSL to the
actual router/modem in the users premises.

I think you should start with rfc1853 and work on from there if you want
to know what BT does.

But not why it does it!