What is decadic line signalling in the CAS protocols?

Decadic register signalling causes one of the line signalling (ABCD) bits to temporarily change state (or pulse) a number of times to represent a particular digit (this is similar to loop disconnect dialling). When an application calls call_openout (and the out_xparms.destination_addr array has been filled with a valid DDI), the firmware will automatically send each decadic digit one by one. Additionally, at the incoming side, the decadic digits are detected by the firmware and the user is able to see what digits have been received using the call_details function. Therefore, as decadic digits are conveyed as part of the line signalling, no external resource is required to generate or detect anything on the voice channels.

Decadic dialling is inherently slower than tone based (DTMF and MFC) dialling and is, therefore, encountered less often.