For a long time the incumbent local exchange carriers (ILECs) in the USA-occupied territories (also known as RBOCs, Regional Bell Operating Companies) would not offer SDSL, only ADSL. But now that Phone Goddess AT&T aka Ma Bell has practically been reconstituted to her pre-1984 form, the new AT&T Internet services department does offer SDSL in addition to ADSL.
Does this news mean that the reconstituted-as-AT&T RBOCs have deployed DSLAMs with SDSL capabilities? Nope. Instead the trick is elsewhere: AT&T Internet services department serves SDSL not through their own ILECs, but through Covad! AT&T Interner services order SDSL transport from Covad who then turn around and order the loop from AT&T ILEC. Very peculiar indeed.
From talking to some AT&T SDSL customers and looking at their router configuration it seems that the IP layer is provided by AT&T Internet services and Covad only provides ATM transport. The encapsulation over Covad's SDSL/ATM appears to be RFC 1483, although it isn't clear whether it is routed or bridged.
We are looking for a brave volunteer who would like to try to make an open source connection to an AT&T SDSL circuit using our Hack-o-Rocket or OSDCU.