Also known as Common Channel Signaling System 7 (CCS7), SS7 protocols were developed by AT&T in 1975 for use in the public switched telephone network (PSTN). The protocols were adopted and enhanced by the ITU-T in 1981, and are defined in the Q.7XX series of Recommendations. SS7 in a common channel signaling (CCS) system that uses channels separate from the communications channels for signaling and control purposes. In the United States, at least, SS7 operates not only over separate channels, but generally over a physically distinct network or subnetwork. SS7 significantly speeds call setup and call completion processes, allows access to databases that enable toll-free calling and number portability, and supports the CLASS (Customer Local Access Signaling Services) services such as caller ID, name ID, selective ringing (or priority ringing), selective call forwarding, call block (or call screen), repeat dial, call trace, and automatic call-back (call return). SS7 is fully deployed in all major TDM-based interexchange carrier (IXC) networks.While SS7 is largely deployed in the major incumbent local exchange carrier (ILEC) networks in developed countries, older central office (CO) switches do not support it. See also all of the features listed above. See also CCS, CLASS, CO, ILEC, IXC, and signaling and control.
(Signaling System 7) The protocol used in the public switched telephone system (the "intelligent network" or "advanced intelligent network") for setting up calls and providing services. SS7 is a separate signaling network that is used in Class 4 and Class 5 voice switches. SS7 Controls the Network The SS7 network sets up and tears down the call, handles all the routing decisions and supports all telephony services such as 800 numbers, call forwarding, caller ID and local number portability (LNP). The voice switches that carry the telephone conversations are known as "service switching points" (SSPs). The SSPs query "service control point" (SCP) databases using packet switches called "signal transfer points" (STPs). The SCP databases provide static information such as the services a customer has signed up for and dynamic information such as ever-changing traffic conditions in the network. Because the signaling network is separate, a voice circuit is not tied up until a connection is made between both parties. Different Versions There is an international version of SS7 standardized by the ITU, and national versions determined by each country. For example, ANSI governs the U.S. standard for SS7, and Telcordia (Bellcore) provides an extension of ANSI for its member companies. See Q.7xx, AIN, Class 4 switch, Class 5 switch and CCIS.