CAS stands for Common Associated Signaling and the signaling operates by “robbing” the least significant bits of information from voice channels and using this to send clocking and framing information. CAS is also known as robbeb bit signaling.
At the opposite of the ISDN, T1/E1 CAS uses in-band signaling which are:
- Loop-start
- Ground-start
- E&M wink-start (Feature group B, FGB)
- E&M wink-start (with acknowledge)
- E&M double wink (Feature group D, FGD)
- E&M Feature Group D Exchange NA, FGEANA
- E&M immediate-start
E&M signaling is often the preferred optino for CAS because it avoids glare, it provides answer/disconnect supervision and it can receive Automatic Number Identification (ANI) with FGD and send ANI with FGD-EANA. In other words, you can have 1 channel-group for incoming calls and 1 channel-group for outgoing calls.
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