Thanks wombat, this has turned out to be an interesting discussion.
but the phrase "fall back" from Zyxel is interesting
Im not sure either as the details are scant, but the Calix mention of PTM to me is the key.
ADSL2+ will run over PTM. PTM also has a big advantage of far less (framing) overheads than ATM, so this will help throughput speeds a bit too.
There's no need for different modems either - take for example my Zyxel can be used on adsl1/2/2+ or VDSL from the one DSL socket and its not the only one that can do this Billion and TPlink to name a few.
Yes theres currently 2 different (software) interfaces:-
[1]An ATM interface for ADSL and
[2] a PTM interface for VDSL.
But if you ran ADSL2+ over PTM then surely the router should be able to auto-sense VDSL/VDSL2/ADSL2+ in exactly the same way that a router can auto-sense ADSL1/ADSL2/ADSL2+ over ATM?
Obviously there would still need to be a configuration change on the DSLAM but that happens anyhow now with adsl1/adsl2/adsl2+ etc.
Certain US ISPs are already upgrading their ATM networks to PTM (such as AT&T) and switching their standard ADSL users away from ATM and over to PTM framing.
With BT, on FTTC enabled exchanges a PTM network is already there. There's line cards available to support VDSL & ADSL2+. They'd perhaps have to rethink the PSDmasks but I really dont see that being too complicated.
The only fly in the ointment that I can think of, is LLU... and if it would cause the likes of Sky to spit out their dummies.
They are already unhappy about no direct access to the DSLAMs in the cab for VDSL so how much of a stink will they kick up when it comes to ADSL too?
There's already one line of thought that the LLU providers demands to provide access is what could be causing in part some delay to the roll out of vectoring.