The Autobaud pre-activation message protocol defines two bytes that encode some aspects of the SDSL flavor. Bits 3:2 of byte 1 encode the framer type, and encodings are defined for none, HDSL, and ATM over CAP. The latter is a mystery. It seems to be some kind of framing structure borrowed from ADSL/CAP and adapted for use with SDSL/2B1Q. Mindspeed's M289xx software manual also refers to it as Open CAP Frame Format and RADSL. RADSL stands for Rate-Adaptive DSL and is a confusing term because it can be interpreted to mean either the generic concept of rate-adaptive DSL or a reference to a particular ADSL flavor.

We've had no success so far finding a description of the ATM over CAP framing format, not even in its original ADSL/CAP incarnation, let alone in the SDSL/2B1Q adaptation. The name itself suggests, however, that it's a framing format that provides ATM cell delineation and thus eliminates the need for raw bit-oriented cell delineation a la I.432.1. The latter is exactly what the Nokia SDSL framing format provides — perhaps AOC and Nokia framing are one and the same?