Information conveyed using the ABCD bits is referred to as 'line signalling'. Line signalling is used to allow the near and far end to know the state of a particular call. An example of some line signalling states are;
- Isilans (wait for answer)
- Forward/backward release
Aculab digital network access cards are able to interpret line signalling states (ABCD bits) and return a generic event (for example, EV_INCOMING_CALL_DET, EV_WAIT_FOR_ACCEPT, EV_CALL_CONNECTED, EV_REMOTE_DISCONNECT) to an application.
'Register signalling' refers to the way in which DDI and/or CLI information is conveyed. There are three ways in which DDI and CLI information can be conveyed for CAS protocols. These are; decadic pulses, DTMF (dual tone multi-frequency) tones or MFC (multi-frequency compelled) tones.