6. Messaging and Payloads in the SS7 Stack

Overview of the SS7 protocol stack. MTP (levels 1–3) provides reliable transport and routing of signaling messages, while higher layers like SCCP, TCAP, and various User Parts (ISUP, MAP, INAP, CAP, etc.) handle application-specific messaging.

The SS7 stack is often compared to the OSI model, with Message Transfer Part (MTP) layers 1–3 forming the transport foundation, and various application parts at Layer 4. MTP Level 3 routes messages based on point codes and link sets, providing network-layer functionality, while MTP Level 2 ensures reliable link transmission (error checking, flow control), and MTP Level 1 defines the physical signaling link (e.g. 64 kbps TDM channels). Above MTP, Signalling Connection Control Part (SCCP) and Transaction Capabilities Application Part (TCAP) offer additional routing and transaction support for non-circuit messages, and User Parts like ISUP, MAP, CAP, etc., implement the application-specific signaling. All SS7 messages (except low-level link keepalives) are carried in Message Signal Units (MSUs) at the MTP level, which contain a routing label and a payload for a higher protocol. The following sections describe the major SS7 protocol parts, how they construct and process messages, and the standards defining their message formats and payloads.