Common Channel Signaling
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In telephony, Common Channel Signalling (CCS) or in the US Common Channel Interoffice Signalling (CCIS) is the transmission of signaling information (control information) on a separate channel to the data, and, more specifically, where that signalling channel controls multiple data channels.
For example, in the PSTN one channel of a communications link is typically used for the sole purpose of carrying signalling 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 tear-down traffic for numerous voice and data channels.
The logical alternative to CCS is Channel Associated Signalling (CAS), where each bearer channel has a signalling channel dedicated to it.
CCS offers the following advantages over CAS, in the context of the PSTN:
- Faster call setup.
- No interference 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 related to the use of in-band signalling with CAS.
- 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.
CCS is distinct from in-band or out-of-band signalling, which are to the data band what CCS and CAS are to the channel.
