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Reply To: Introduction to VOIP



Go to for SS7 and other definitions:


(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.

The SS7 network sets up and tears down the call, handles all the routing decisions and supports all modern telephony services such as 800 numbers, call forwarding, caller ID and local number portability (LNP). The voice switches known as “service switching points” (SSPs) query “service control point” (SCP) databases using packet switches known as “signal transfer points” (STPs).

Accessing databases using a separate signaling network enables the system to more efficiently obtain static information such as the services a customer has signed up for and dynamic information such as ever-changing traffic conditions in the network. In addition, a voice circuit is not tied up until a connection is actually made between both parties.

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 AIN, Class 4 switch, Class 5 switch and CCIS.


An ITU standard for realtime, interactive voice and videoconferencing over LANs and the Internet. Widely used for IP telephony, it allows any combination of voice, video and data to be transported. H.323 specifies several video codecs, including H.261 and H.263, and audio codecs, including G.711 and G.723.1. Gateways, gatekeepers and multipoint control units (MCUs) are also covered. See MGCP and SIP.