PSTNSS7

PSTN SS7, or Signaling System No. 7, is a suite of signaling protocols used in the Public Switched Telephone Network to manage call setup, routing, control, and some supplementary services. It operates separately from the voice channels, enabling flexible and efficient network signaling for large-scale telephone systems.

The SS7 signaling network relies on dedicated signaling points and links. Core components include signaling transfer points (STPs), which route messages between signaling end points such as signal transfer points and signaling points (SSPs) at switches. The protocol stack begins with the Message Transfer Part (MTP), which provides reliable transport and link management. Above MTP are the Signaling Connection Control Part (SCCP) for routing and management, and the ISDN User Part (ISUP) for establishing, maintaining, and releasing telephone calls. The Transaction Capabilities Application Part (TCAP) allows applications to exchange non-circuit-related signaling information. Mobile networks also use SS7 applications such as the Mobile Application Part (MAP) for roaming and SMS, and CAP for intelligent network services.

Key functions of PSTN SS7 include setting up and tearing down calls, routing decisions, number portability and database queries, call routing to roaming networks, and short message handling in mobile contexts. It also supports advanced features through intelligent networks and value-added services, often via TCAP-based applications.

Security and evolution: SS7 signaling can be vulnerable to interception, spoofing, and toll fraud if improperly protected. Modern deployments increasingly use Signaling System No. 7 over IP (Sigtran) to transport SS7 signaling across IP networks, with additional security controls and network segmentation to reduce risk.

Overview: Developed by ITU-T and widely deployed since the 1980s, PSTN SS7 remains foundational for traditional telephone networks and for many mobile signaling functions, even as operators migrate toward IP-based signaling architectures.