(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 NetworkThe 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 VersionsThere 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.
SS7 Protocol StackThe SS7 protocols do not conform precisely to the OSI model, but the comparison helps put them into perspective.IN, AIN, INAP and MAPIN, AIN and INAP protocols are used to initiate non-circuit related functions throughout the network (not dealing with connect/disconnect). AIN (Advanced Intelligent Network) is used in North America. INAP (Intelligent Network Application Part) is used in Europe, and other countries use proprietary versions of IN (Intelligent Network) standards based on the ITU. These protocols use TCAP to access remote devices.MAP (Mobile Application Part) enables cellular carriers to use the SS7 network and allows the cellphone's telephone number and serial number to be transmitted over the network. See IS-41.TCAPTCAP (Transaction Capabilities Application Part) is used to send database queries to a service control point (SCP). It is also used to send non-circuit related messages between switches. TCAP keeps track of multiple queries that are part of the same session.ISUP and TUPISUP (ISDN User Part) is used to connect and disconnect a call in North America. TUP (Telephone User Part) was the international counterpart of ISUP with different message types and parameters, but has been mostly replaced by ISUP.SCCPWhen TCAP is used to query a database, SCCP (Signaling Connection Control Point) is used in conjunction with TCAP to route the message to the appropriate database subsystem. SCCP also provides guaranteed end-to-end delivery and thus delivers both OSI layer 3 and 4 capabilities.MTPMTP-3 (Message Transfer Part-3) provides message routing and network management. The individual data links are continuously monitored. When a link fails, messages are rerouted, and the link is reset and resynchronized, which often solves the problem.MTP-2 is responsible for error-free transmission between two SS7 nodes (signaling points). Packet sequence numbers are also generated at this level to ensure that everything arrives properly.MTP-1 supports numerous digital interfaces including DS0A, DS1 and V.35.
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.