Common channel signaling (CCS) is signaling in which a group of voice-and-data channels share a separate channel that is used only for control signals. This arrangement is an alternative to channel associated signaling (CAS), in which control signals, such as those for synchronizing and bounding frames, are carried in the same channels as voice and data signals.
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.