[Gllug] ADSL modem -- WTF?

[Russell Howe]

On Mon, Jul 19, 2004 at 11:06:47PM +0100, Chris Bell wrote:
>    I have just received an unofficial reply from a Demon staff member giving
> the following information:
> 
>    The slight disadvantage of using standard 1500 frame sizes with BT has
> been sorted, the problem went away when they upgraded their system, probably
> last year. Any possible remaining differences would not be noticed.
>    BT made changes some time ago that allowed Demon to offer their customers
> connections using PPPoA or PPPoE, arranged so that a greater range of
> customer equipment (mainly the cheaper end) is then able to connect, but
> they recommend PPPoA if available because PPPoE is inherently less efficient
> and some implementations have problems negotiating the correct MTU. In both

This pretty much ties in with what I thought - that the BT equipment can
do PPPoE, but it's up to the ISP to handle it

> cases the data is carried over ATM between the customer provided equipment
> and the DSLAM, so ethernet frames are encapsulated in ATM, and PPP inside
> the ethernet frames. The PPP can be terminated on something behind the modem
> over the ethernet connection, dependant on the customer's available
> facilities.

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.

There are various AAL<n> things which are ATM Adaptation Layers (?) and
are numbered. Basically different ways of using ATM signalling, I think.
Maybe one of those looks sufficiently like ethernet to run PPPoE over?

>    If anyone has any additional information I would be delighted to hear it.

My info is sketchy at best :)

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.

-- 
Russell Howe       | Why be just another cog in the machine,
rhowe at siksai.co.uk | when you can be the spanner in the works?
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