SS7

(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.

SS7 Protocol Stack
The SS7 protocols do not conform precisely to the OSI model, but the comparison helps put them into perspective.

IN, AIN, INAP and MAP
IN, 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.

TCAP
TCAP (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 TUP
ISUP (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.

SCCP
When 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.

MTP
MTP-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.

The Definitive Reference

Travis Russell was the first to cover SS7 in all of its glorious detail. Since 1995, his books have served as the definitive references on the subject. (Image courtesy of McGraw-Hill.)

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SS7 - Signalling System 7