Common-channel signaling

In telephony, common-channel signaling (CCS), in the US also common-channel interoffice signaling (CCIS), is the transmission of signaling information (control information) on a separate channel than the data, and, more specifically, where that signaling channel controls multiple data channels.

For example, in the public switched telephone network (PSTN) one channel of a communications link is typically used for the sole purpose of carrying signaling for establishment and tear down of telephone calls. The remaining channels are used entirely for the transmission of voice data. In most cases, a single 64kbit/s channel is sufficient to handle the call setup and call clear-down traffic for numerous voice and data channels.

The logical alternative to CCS is channel-associated signaling (CAS), in which each bearer channel has a signaling channel dedicated to it.

CCS offers the following advantages over CAS, in the context of the PSTN:

The most common CCS signaling methods in use today are Integrated Services Digital Network (ISDN) and Signalling System No. 7 (SS7).

ISDN signaling is used primarily on trunks connecting end-user private branch exchange (PBX) systems to a central office. SS7 is primarily used within the PSTN. The two signaling methods are very similar since they share a common heritage and in some cases, the same signaling messages are transmitted in both ISDN and SS7.

A CCS network provides signaling message transfer for participating Common Channel Signaling Switching Offices (CCSSOs), databases, and operator systems. It is a packet-switched communication network that allows call control and transaction messages from the voice and data networks to be transferred on communications paths that are separate from the voice and data connections.

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