Channel Associated Signaling (CAS), also known as Per-Trunk Signaling (PTS), 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 signaling, this routing information is encoded and transmitted in the same channel as the payload itself. This information can be transmitted in the same band (in-band signalling) or a separate band (out-of-band signalling) to the payload.
CAS potentially results in lower available bandwidth for the payload. For example, in the PSTN the use of out-of-band signalling within a fixed bandwidth reduces a 64 kbit/s DS0 to 56 kbit/s. Because of this, and the inherent security benefits of separating the control lines from the payload, most current telecom systems rely more on Common Channel Signaling.
