Channel-associated signaling (CAS), also known as ''per-trunk signaling'' (PTS), is a form of digital communication
signaling
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
. 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
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 ov ...
) or a separate band (
out-of-band signaling
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.
Classif ...
) to the payload. Call setup will be comparatively slower since signaling channels are shared thus congestion can be faced.
CAS potentially results in lower available bandwidth for the payload. For example, in the
PSTN
The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local teleph ...
the use of out-of-band signaling within a fixed bandwidth reduces a 64 kbit/s
DS0
Digital Signal 0 (DS0) is a basic digital signaling rate of 64 kilobits per second (kbit/s), corresponding to the capacity of one analog voice-frequency-equivalent communication channel. The DS0 rate, and its equivalents E0 in the E-carrier system ...
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
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 ...
(CCS).
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The most common implementation of CAS is
robbed-bit signaling In communications 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 circui ...
.
References
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Telecommunications standards