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 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:
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.
CCS messages provide for transaction-based services and for call control signaling between network nodes. The architecture for a CCS network is generally based on the geographical location of signaling points, the number and sizes of signaling points, the expected traffic load, and the services to be provided by the network.
Regardless of the type of architecture selected, the CCS network will consist of a combination of the following components:
As CCS networks have continued to evolve, and network interconnection architectures and the services they support have grown more complex, it has become necessary to identify the type of behavior all network providers are expected to exhibit when specific requirements do not exist.
Providers of Interconnecting CCS networks and other interconnecting networks need information about the processes and interfaces that need to be supported to ensure that call originations routed via other interconnecting networks receive at least the same quality of service as intranetwork calls.
Telcordia GR-905, Common Channel Signaling Network Interface Specification (CCSNIS) Supporting Network Interconnection, Message Transfer Part (MTP), and Integrated Services Digital Network User Part (ISDNU), contains this important information and includes information for providers of Legacy Network Gateway and Legacy Selective Router Gateway systems to support the delivery of emergency calls between legacy origination networks and IP-enabled PSAPs, as well as emergency calls routed via Emergency Services IP Networks (ESInets) and legacy PSAPs that are served by Selective Routers. It provides explicit references to ANSI and ATIS Standards and ITU-T Recommendations.