Channel Associated Signaling

Channel Associated Signaling (CAS) signaling is a form of digital communication signaling. As with most telecommunication signaling methods, it uses routing information to direct the payload of voice or data to its destination. With CAS signalling, this routing information is encoded and transmitted along the same link as the payload itself. This results in lower available bandwidth for the payload, normally reducing a 64 kbit/s DS0 to 54 kbit/s. Other out-of-band signaling such as the ISDN based PRI and BRI, or SS7, avoid this by reserving an independent path for all of the control signaling, separating it from the payload completely. Because of this, and the inherent security benifits of separating the control lines from the payload, most current telecom systems rely more on the out-of-band signaling. The majority of the PSTN network is run by SS7 trunking, while a majority of digital PBXs use ISDN variants such as PRI and BRI.

• It is sometimes referred to as Robbed-bit signalling, as it takes a bit off of every nth (usually 6th) data frame to transmit routing and control information.
• It is commonly used in T1 and other T-carrier digital lines as it requires synchronous signalling,
• and is commonly found in use by PBXs.