PPPoA

I'm only going to consider PPPoE and PPPoA encapsulation, as those are the only two options AAISP offer.

On PPPoA, PPPoE, ATM and ADSL Farnz 2010

On a BT Wholesale line, you get to choose an encapsulation method either PPPoA or PPPoE; the other end can autodetect what encapsulation you're using, by looking at the incoming ATM cells.

On PPPoA, PPPoE, ATM and ADSL Farnz 2010

The question raised was what is the difference between PPPoE and PPPoA on ADSL?

On PPPoA, PPPoE, ATM and ADSL Farnz 2010

For PPPoA, the incoming ATM cells contain PPP directly.

On PPPoA, PPPoE, ATM and ADSL Farnz 2010

Incidentally, this means that PPPoE is always going to be slightly slower than PPPoA - when you're unlucky, it will use one ATM cell more than PPPoA for a packet, costing you some speed.

On PPPoA, PPPoE, ATM and ADSL Farnz 2010

Eagle-eyed readers will also recognise that this makes Be Retail's ethernet over ATM services use slightly more overhead than PPPoA services; there are 20 bytes of overhead for Be, to the 2 bytes of PPP overhead.

On PPPoA, PPPoE, ATM and ADSL Farnz 2010

If you're doing PPPoA, this 1502 bytes becomes 31 cells containing 48 bytes of payload, and a cell containing 14 bytes payload, 26 bytes padding, and the 8 byte AAL5 trailer.

On PPPoA, PPPoE, ATM and ADSL Farnz 2010

This post goes into the details of how PPPoE, PPPoA and ATM are interrelated on UK ADSL providers.

On PPPoA, PPPoE, ATM and ADSL Farnz 2010

Anyone who's looked at the options offered by their ADSL modem will have seen a huge variety of options; there's VC Mux, LLC, PPPoA, RFC1483/2684, PPPoE, CLIP and often others.

On PPPoA, PPPoE, ATM and ADSL Farnz 2010

This is also where the faulty advice to use a 1478 byte MTU with PPPoA comes from - when doing bulk transfers, most of your packets are MTU-sized.

On PPPoA, PPPoE, ATM and ADSL Farnz 2010