What is SS7?
Signaling System No. 7 (SS7) is a common-channel signaling system standardized for telephone networks such as PSTN and ISDN and for mobile networks. It carries control messages on a signaling network separate from user traffic. It uses . The signaling network is logically separate from the traffic channels and carries control messages for many calls.
Separating signaling from user traffic lets the network exchange digital call-control messages without occupying the voice channel. SS7 supports call setup and services such as toll-free number translation, calling-line identification, SMS signaling, and mobility procedures. Signaling links connect the nodes that originate, transfer, or terminate these messages.
SS7 network architecture
An SS7 network contains distinct types of signaling nodes connected by signaling links. Their roles determine whether a node originates, transfers, or terminates signaling traffic.
Network elements
- SP (Signaling Point): A node that can originate or terminate signaling messages, including telephone exchanges and databases. An SP is identified by a signaling point code (SPC) within the relevant signaling network.
- STP (Signal Transfer Point): A node that transfers SS7 messages between signaling points according to routing information. It normally provides transfer functions rather than user-part applications.
- SCP (Service Control Point): A service-control node that provides service logic and data to Intelligent Network applications. An SSP can query it for instructions such as toll-free number translation. In mobile networks, subscriber databases such as the HLR are separate nodes reached through signaling procedures.
- SSP (Service Switching Point): A telephone exchange equipped to handle SS7 signaling. An SSP can originate, terminate, or transit calls by sending SS7 messages to other SSPs or querying an SCP for instructions.
Signaling modes
The mode depends on the relationship between the signaling route and the traffic route it controls. The distinction describes the route arrangement, rather than a different message format.
- Associated mode: Signaling messages are sent over a direct signaling link between two signaling points that are also connected by the traffic channels being controlled. This reduces intermediate nodes. It requires links along each associated traffic route and can increase the number of links in larger networks.
- Quasi-associated mode: Signaling messages follow a separate, predetermined path through one or more STPs. This route replaces a direct link between the signaling points. The arrangement supports hierarchical designs in which STPs provide intermediate routing and reduces the need for a fully meshed signaling network.
The SS7 protocol stack
SS7 is a layered protocol suite. It has three MTP levels and user parts that provide signaling applications, forming a four-level functional architecture. The arrangement carries call-control, intelligent-network, and mobile-network messages across fixed and cellular networks.
- MTP Level 1 (Signaling Data Link): The physical layer defines the transmission medium and electrical or optical interface. In practice it uses 64 kbit/s channels in an E1 stream, while T1 variants use 56 or 64 kbit/s depending on the regional implementation. Other transport variants can use optical fiber or microwave PDH/SDH interfaces. For ATM transport, the signaling link uses the SAAL (Signaling ATM Adaptation Layer) stack, combining AAL5 with SSCOP and SSCF. It specifies line coding, framing, and signal levels.
- MTP Level 2 (Signaling Link): The data-link layer transfers MSU (Message Signal Units) between directly connected signaling points. It detects errors with the link check procedure and uses acknowledgments and retransmissions, preserves sequence, controls flow, and supervises link status with LSSU and FISU signal units.
- MTP Level 3 (Signaling Network): The network layer routes messages using the Destination Point Code (DPC). It selects links and shares load, reroutes traffic after failures (changeover/changeback), controls overload, and performs Signaling Network Management.
- User Parts (Level 4): A collection of application protocols that use the transport service of MTP or SCCP. Principal user parts include:
- ISUP (ISDN User Part): Provides call control in PSTN/ISDN networks, including call setup, modification, and release for voice and circuit-switched data.
- B-ISUP (Broadband ISUP): An extension of ISUP for broadband networks such as ATM and SDH, supporting broadband connection control.
- TUP (Telephone User Part): An earlier telephone user part used in traditional telephony. ISUP expanded support for ISDN and circuit-switched data, while TUP remains in some regions.
- SCCP (Signaling Connection Control Part): Adds application-level addressing (subsystem numbers) and offers connectionless and connection-oriented services above MTP-3.
- TCAP (Transaction Capabilities Application Part): Enables query/response transactions without establishing a dedicated circuit. Used by Intelligent Network (IN) and databases such as HLR/VLR.
- MAP (Mobile Application Part): Specific to GSM/UMTS networks. Handles roaming, authentication, SMS routing and location updates, transported over TCAP.
- BSSAP (Base Station Subsystem Application Part): Carries signaling between the base station controller (BSC) and the mobile switching center (MSC) in GSM.
- INAP (Intelligent Network Application Part) / CAP (CAMEL Application Part): Support value-added services such as prepaid billing, number translation and call screening.
- OMAP (Operations and Maintenance Application Part): Protocols for SS7 network operations, administration and maintenance.