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The Signaling System No. 7 (SS7) is a globally used set of telephony signaling protocols that enables call setup, routing, billing, and information exchange between telephone networks.
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The SS7 protocol suite is organized into a stack, structured in levels, similar to the OSI model:
- MTP Level 1 (Physical Layer): This is the physical and electrical connection for the network, handling the delivery of signaling messages via the transmission medium.
- MTP Level 2 (Link Layer): Ensures error checking and message sequence control by detecting and correcting errors in the transmission of signaling information.
- MTP Level 3 (Network Layer): Provides message routing between exchanges and manages message distribution, ensuring the integrity of signaling message transfer.
- SCCP (Signaling Connection Control Part): Adds enhanced routing capabilities and supports connection-oriented and connectionless network services, facilitating a variety of telecommunications services beyond basic call control.
- TCAP (Transaction Capabilities Application Part): Enables the deployment of non-circuit-related queries and transactions, supporting features like number translation, roaming, and database queries.
- ISUP (ISDN User Part): Handles the setup and tear down of voice and data calls over the public switched telephone network (PSTN).
- MAP (Mobile Application Part): Utilized in mobile networks for functions such as roaming, location updating, handovers, and short message service (SMS) delivery.
The SS7 stack is essential for communication between network elements in telecommunications networks, supporting efficient and robust call processing and management.