It's a service framework for subscriber based circuit switched connections. The modulation format is separate and I doubt they expected to keep such a meager encoding for the long predicted life of the network.

The error was in not seeing packed switched networks as the clear winner not in choosing a modulation scheme that was designed to bring maximum bandwidth over existing copper infrastructure.

In any case, the strength was that you could bond up to 24 channels in a North American PRI package, which is what a lot of businesses considered their "T1" service, or used as a mixed group of DID and LOOP channels for calling.

Interestingly, in the end, they used bonded ADSL to provide an encapsulated 24 port PRI "circuit" to the customer using just 4 wires.