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.
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 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).
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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