[Gllug] ADSL modem -- WTF?

Chris Bell chrisbell at overview.demon.co.uk
Tue Jul 20 08:53:09 UTC 2004

On Mon 19 Jul, Russell Howe wrote:

> Well DSL uses an ATM base, right? The framing over the DSL signal is
> ATM, AFAIK. For PPPoE to work, that ATM layer would have to emulate
> ethernet, so you get PPPoEoA almost. ATM has lots of acronyms associated
> with it and I don't really know what any of them mean. There's something
> called LANE (LAN Emulation) which I guess emulates a broadcast network
> over ATM, but I guess that's overkill for running PPPoE sessions over
> DSL.
   ATM is the system designed to carry low data rate (speech) with minimal
delay, so has (had?) a very small packet size (something like 48 bytes plus
header), it was adapted and adopted by BT. The data that interests us is
sent over the ATM system, now modified to work better when the MTU is 1500.
There are other users sending different data at much higher rates, so I
guess the system may now handle even larger or perhaps variable size frames
without problems.
   PPPoA sets up a persistent point to point data link which can carry data
using ordinary ATM packets, it seems that PPP can then be encapsulated
within the data stream, although this is an additional overhead and so is
less efficient.

> Since I want to terminate my DSL on my firewall, so long as it's using
> an ethernet interface to communicate with the modem, it's going to have
> to receive PPPoE frames.
> Hopefully with some ATM equipment I can switch to receiving PPPoA frames
> on my firewall and lose the MTU issues.

   As I understand it the ADSL modem converts between your system (normally
either a PCI card, a PCMCIA card, or ethernet) and ATM packets which are
carried on high frequency carriers on the local pair. Negotiation is only
necessary to set up the initial direct link between your modem and the BT
equipment, and the data is then forwarded as ATM packets. It is not
necessary to further encapsulate PPP within those packets. The ATM section
is only a link in the point to point chain, the data stream can still be
terminated behind your modem.

Chris Bell

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