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.

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.

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

• 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.

CCS Network Interconnections

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.

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.

There are generally two architecture configurations used for interconnection of CCS networks.

• Interconnection of two networks through a mated Signaling Transfer Point (STP) pair in the CCS Network Provider network to a mated STP pair in the Interconnecting CCS Network (ICN) via D-link quads

• Interconnection of a CCSSO in the ICN with a CCS Network Provider mated STP pair via A-links.

Other alternative architectures for CCS interconnection, such as the E-link architecture, the combined node architecture, or the A-Link Concentrator (ALC) architecture, may be available subject to business arrangements between the CCS Network Provider and the ICN.

GR-905, Common Channel Signaling Network Interface Specification (CCSNIS) Supporting Network Interconnection, Message Transfer Part (MTP), and Integrated Services Digital Network User Part (ISDNUP), details the interfaces between the out-of-band CCS architectures utilizing the SS7 protocol deployed by the CCS Network Provider networks and other Interconnecting CCS Networks (ICNs).

References

• 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.