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 controls multiple message channels. 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 messages. In most cases, a single 64 kbit/s channel is sufficient to handle the call setup and call clear-down traffic for numerous bearer (voice and data) channels. The technical alternative to CCS is channel-associated signaling (CAS), in which each bearer channel has a dedicated signaling channel. CCS offers the following advantages over CAS, in the context of the PSTN: Faster call set-up time Greater trunking efficiency due to the quicker set up and clearing, thereby reducing traffic on the network Can transfer additional information along with the signaling traffic, providing features such as caller ID Signaling can be performed mid-call The most common CCS signaling methods in use 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.