[Gllug] ADSL modem -- WTF?

Chris Bell chrisbell at overview.demon.co.uk
Thu Jul 15 18:50:09 UTC 2004

On Thu 15 Jul, Russell Howe wrote:

   I am sure that any errors in the following will be corrected:

   The BT main backbone system was designed for minimum delay on speech, so
runs om ATM with a very small packet size. 1492 is a convenient multiple
which does not exceed the standard 1500 maximum ethernet packet size, and
will give a slightly better maximum rate. BT are upgrading their network so
that it is better able to handle 1500, so there should soon be no advantage
of using the smaller packet size.
   You must connect to BT using PPPoA, but some ADSL modems can be
configured to act as a PPPoA - PPPoE converter, allowing you to run PPPoE on
your final termination, i.e. your firewall.
   I understand that if your firewall is configured to run PPPoE and the
ADSL modem is just used to convert between PPPoE and PPPoA, then the ADSL
modem can have a non-routable IP address, and the connecting ethernet
interface on your firewall can carry the assigned IP address. The system
should just work as well as if there was any other hardware, including an
analogue modem, hub, switch, or simple router, also in the path.

Chris Bell

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