Although I've already said that we do not generally treat IDSL as a flavor of SDSL/2B1Q, an interesting question is whether or not one could treat it this way. In other words, would it be possible to implement a compliant IDSL (and hence ISDN) terminal using the HDSL/SDSL bitpump if one wanted to? The bitpump can operate at the 80 kbaud symbol rate used in ISDN, the 2B1Q line code should be the same, and one certainly could implement an external framer for the ISDN frame format just like any other SDSL flavor.

The main difficulty would be the ISDN startup sequence, which is apparently quite different from the HDSL one. It seems to me that the bitpump hardware can probably support it, but modifying the BT software to implement a different activation sequence would probably be a real pain.

It's interesting to ponder in any case. Also see how this problem is handled by Netopia 4652.