Common Channel Signaling

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Template:Context Common Channel Signaling (CCS) is the transmission of signaling information out of the information band.

In CCS, one channel of a communications link is be used for the sole purpose of carrying signaling for establishment and tear down of calls. The remaining channels are used entirely for the transmission of voice or data. In most cases a single 64kbit/s channel is sufficient to handle the call setup and teardown traffic for numerous voice and data channels.

Compare with Channel Associated Signaling (CAS), generally implemented using robbed-bit signaling, where the signalling information for each channel is transmitted within the channel itself and as a result, reduces the amount of bandwidth available within the channel (in some cases by as much as 12.5%).

CCS offers the following advantages over CAS: Faster call setup. No interferrance between signalling tones by network and frequency of human speech pattern. Greater trunking efficiency due to the quicker set up and tear down, thereby reducing traffic on the network. No security issues with in band signalling. CCS allows the transfer of additional information along with the signalling traffic providing features such as caller ID.

The most common CCS signalling methods in use today are ISDN and SS7.

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