insteresting stuff you probably knew already..

Apropos of waking up early and being bored, and hating to not actually
KNOW stuff..I did some research.

...on the encapsulation of IP over ADSL..


This is broadly true for all UK setups.

Ip is carried over PPP: PPP is a point to point protocol that carries
the name/passwd combo to allow authentication. PPP is what the final
link in the logging in process to your ISP.

PPP is carried over ATM. Hnence PPPoA. ATM is what BT's backhaul uses to
transfer the signal from your exchange to your ISP. It appears from
reading Kitz if your login is (E-Mail Removed) then the packets should
be routed to that ISP's ATM interface.


ERgo, your router does indeed talk ATM. As well as IP and PPP.

Finally, the short hop from you to the exchange is carried over
ADSL..strictly the protocol has another name at that level, and there
are a few more bits in there, but the ADSL part is only between you and
the DSLAM at the exchange.

None of this matters a jot if its all working, but sometimes its useful
to know what does what, if its NOT working..


Some other countries use other encapsulations. PPPoE is PPP over
Ethernet, and that uses ATM as a sort of ethernet bridge. For example.

>Some other countries use other encapsulations. PPPoE is PPP over Ethernet,
>and that uses ATM as a sort of ethernet bridge. For example.

Long time ago BT changed things to allow PPPoE as well.
Apparently it is a bit more stable, as it uses more control characters, but
of course a wee bit slower.
If you want to use it you need to lower the MTU from 1500 to 1492.
Regards,
Martin

The Natural Philosopher wrote:
> Apropos of waking up early and being bored, and hating to not actually
> KNOW stuff..I did some research.
>
> ..on the encapsulation of IP over ADSL..
>
>
> This is broadly true for all UK setups.
>
> Ip is carried over PPP: PPP is a point to point protocol that carries
> the name/passwd combo to allow authentication. PPP is what the final
> link in the logging in process to your ISP.
>
> PPP is carried over ATM. Hnence PPPoA.

To be picky PPPoA is PPP over AAL5.

ATM is what BT's backhaul uses to
> transfer the signal from your exchange to your ISP. It appears from
> reading Kitz if your login is (E-Mail Removed) then the packets should
> be routed to that ISP's ATM interface.

I believe most ISPs use IP stream so BT terminate the ATM and the ISP
gets L2TP packets through the central over ethernet (possibly - but it's
not ATM whatever the carrier).

>
> ERgo, your router does indeed talk ATM. As well as IP and PPP.

Yep - well, I suppose it's the modem that does ATM as it's in the dsl
specs. The sync speed shown is ATM bitrate.

Andy.