Common Channel Signaling

Common Channel Signaling

In telephony, Common Channel Signaling (CCS), in the US also Common Channel Interoffice Signaling (CCIS), is the transmission of signaling information (control information) on a separate channel from the data, and, more specifically, where that signaling channel controls multiple data channels.[1]

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.[2]

The logical alternative to CCS is Channel Associated Signaling (CAS), in which each bearer channel has a signaling channel dedicated to it.

CCS offers the following advantages over CAS, in the context of the PSTN:[3]

  • Faster call set-up time
  • No falsing interference between signaling tones by network and speech frequencies
  • Greater trunking efficiency due to the quicker set up and clear down, thereby reducing traffic on the network
  • No security issues related to the use of in-band signaling with CAS
  • Can transfer additional information along with the signaling traffic, providing features such as caller ID

The most common CCS signaling methods in use today are Integrated Services Digital Network (ISDN) and Signaling System 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.

CCS is distinct from in-band or out-of-band signaling, which are to the data band what CCS and CAS are to the channel.


  1. ^ (Ronayne 1986, p. 141).
  2. ^ (Ronayne 1986, p. 145).
  3. ^ (Ronayne 1986, p. 142).
  • Ronayne, John P. (1986). "The Digital Network". Introduction to Digital Communications Switching (1st edition ed.). Indianapolis: Howard W. Sams & Co., Inc. ISBN 0-672-22498-4. 

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