Channel Associated Signaling (CAS), also known as per-trunk signaling (PTS), is a form of digital communication signaling

Signalling (telecommunications)

In telecommunication, signaling has the following meanings:*the use of signals for controlling communications...

. 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 signaling

In-band signaling

In telecommunications, in-band signaling is the sending of metadata and control information in the same band or channel used for data.-Telephone:...

) or a separate band (out-of-band signaling) 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 telephone systems rely more on Common Channel Signaling

Common Channel Signaling

In telephony, Common Channel Signaling , in the US also Common Channel Interoffice Signaling , is the transmission of signaling information on a separate channel from the data, and, more specifically, where that signaling channel controls multiple data channels.For example, in the public switched...

 (CCS).

The most common implementation of CAS is robbed bit signaling

Robbed bit signaling

In communication systems robbed-bit signaling is a scheme to provide maintenance and line signaling services on many T1 digital carrier circuits using Channel Associated Signaling ....

.