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SS7 is an out-of-band signaling system used by the carriers to set up telephone calls. It is a protocol standard defined by the ITU. Network elements in the public-switched telephone network use SS7 to exchange information used not only to set up calls but to control the network. Part of SS7's call setup process is to create a circuit for the call through the telephone network and then place the call on the circuit.

SS7 is a message-based system that operates on a network that is separate from the digital lines that carry calls. This differs from early telephone signaling systems in which signals were transmitted as multifrequency tones in the same channels as calls. In addition, SS7 enables advanced services that were not possible in the older system, such as toll-free numbers, caller ID, call forwarding, call waiting, and local number portability.

SS7 messages sent between telephony switches set up and terminate calls and indicate the status of terminals involved in calls. These signals are carried over a separate data network known as CCS (Common Channel Signaling). The protocol used by CCS is SS7 (Signaling System 7). The entire system is called the IN (Intelligent Network). See "Telecommunications and Telephone Systems" for a more detailed explanation.

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