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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