signalling system no 7 (SS7)
Signaling System No 7 (SS7) is a signaling protocol developed for digital telephone networks and standardized by the ITU-T. This procedure, which is used in ISDN and in mobile radio networks( GSM, DCS), is an out-of-band signaling.
The signaling system operates completely separately from the user data transmission. The signaling system thus has its own network structure, which can be set up in parallel with the user data network and can also have the same structure, but it can also have a completely separate structure.
In general, the structure of an SS7 network is star-shaped in the subscriber area and meshed in the long-distance area. The end points of the local network are designated as Signalling End Point( SEP) and the transfer points in the long-distance area as Signalling Transfer Point( STP).
In terms of structure, the SS7 protocol has four layers. In the lower three layers, the protocol consists of a message transfer part( MTP) common to all applications and a user part( UP).
The transport part forms a transport system for the UP data. The user part controls the user channels and service characteristics. There are different Telephony User Parts( TUP) for circuit switched data networks the Data User Part (DUP) and for ISDN the ISDN User Part( ISUP) as well as the application parts for mobile networks, the Mobile Application Part ( MAP), the one for operations and maintenance, the Operations Maintenance Administration Part ( OMAP), and the one for intelligent networks the Intelligent Network Application Protocol( INAP).
The transmission over the signalling links( SL) is data packet oriented at 64 kbit/s. The SS7 protocol is described under ITU-T standards Q.700 to Q.795. It is used in ISDN, GSM and ATM.