CAS and CCS

At present there are two signaling principles used in private networks. The division in principles is based on how the signals are transmitted in relation to the speech channels they belong to. The division is sort of a division in old and new.

Channel Associated Signaling (CAS)

This is the traditional principle for signaling.

Each speech channel has a fixed and unambiguously defined signal path:

• either attached to the speech channel, that is, the signals are transmitted with the speech channel.

  This is called inband signaling.

• or associated with the speech channel, that is, the signals are transmitted in their own signal channel separate from the speech channel.

  This is called outband signaling.

In other words: Speech and signals travel together.

There is a big variety of different signaling techniques used in CAS, in some cases inband and outband signaling are combined.

Common Channel Signaling (CCS)

The speech channels use a common data link for transmission of all signals. A signal is transferred as a signal message to which an address is tied. The address states which speech channel the signal belongs to.

In other words: Speech and signals travel independently, both in time and media.

ISDN, DPNSS, and CCSS7 are examples of signaling systems using CCS.