Channel-associated signaling

Last updated November 21, 2019

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, 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) or a separate band (out-of-band signaling) to the payload. Call setup will be comparatively slower since signaling channels are shared thus congestion can be faced.

In telecommunication, signaling is the use of signals for controlling communications. This may constitute an information exchange concerning the establishment and control of a telecommunication circuit and the management of the network—in contrast to manual setup of circuits by users or administrators, for example the sending of a signal from the transmitting end of a telecommunication circuit to inform a user at the receiving end that a message is to be sent.

In telecommunications, in-band signaling is the sending of control information within the same band or channel used for data such as voice or video. This is in contrast to out-of-band signaling which is sent over a different channel, or even over a separate network. In-band signals may often be heard by telephony participants, while out-of-band signals are inaccessible to the user.

CAS potentially results in lower available bandwidth for the payload. For example, in the PSTN the use of out-of-band signaling 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, telephone systems introduced since the 1960s rely more on common-channel signaling (CCS).^[1]

In telecommunication, common-channel signaling (CCS), or common-channel interoffice signaling (CCIS), is the transmission of control information (signaling) via a separate channel than that used for the messages, The signaling channel usually controls multiple message channels.

The most common implementation of CAS is robbed-bit signaling.

In communication systems, robbed-bit signaling (RBS) is a scheme to provide maintenance and line signaling services on many T1 digital carrier circuits using channel-associated signaling (CAS). The T1 carrier circuit is a type of dedicated circuit currently employed in North America and Japan.

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References

↑ John G. Van Bosse and Fabrizio U. Devetak (2007). Signaling in telecommunication networks (2nd ed.). John Wiley and Sons. p. 111. ISBN 978-0-471-66288-4.

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[1] John G. Van Bosse and Fabrizio U. Devetak (2007). Signaling in telecommunication networks (2nd ed.). John Wiley and Sons. p. 111. ISBN 978-0-471-66288-4.