Signalling System No. 7 (SS7) is a family of telecommunications signalling protocols that networks use to exchange control messages. It helps switches and operators route calls, set them up and end them, and support services such as number translation and SMS. SS7 is network infrastructure—not a phone app, handset feature, or consumer product.
What does SS7 do?
Signalling is the network’s control conversation: it carries instructions and information that help telecommunications systems decide where a call should go and how to handle it. The call’s voice or other user data is separate from that control exchange. This analogy is useful, but it is not a formal standards definition.
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SS7 signalling supports functions including call setup and teardown, number translation, local number portability, prepaid billing, and SMS. The exact protocols and procedures used can vary by network and national implementation.
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SS7 was widely applied in traditional public switched telephone networks (PSTNs). It was also adapted for public land mobile networks (PLMNs), including 2G and 3G, using protocols such as MAP and CAP. ITU-T Recommendation Q.3062 describes this evolution and notes that SS7-based signalling can be carried between networks over IP using SIGTRAN, an approach developed by the IETF. In IMS-based networks, including 4G VoLTE and ViLTE, signalling logic has moved to Diameter. ITU-T Q.3062 (September 2022)
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SS7, SIGTRAN and Diameter: what is the difference?
| Term | Role |
|---|---|
| SS7 | A family of signalling protocols and procedures historically used in fixed and mobile telecommunications networks. |
| SIGTRAN | An IP transport approach for carrying SS7-based signalling between networks; it does not, by itself, replace SS7’s trust model. |
| Diameter | Signalling used in IMS-based networks, including those supporting VoLTE and ViLTE, as described by ITU-T Q.3062. |
Why does SS7 raise security concerns?
SS7 was designed around operator-managed networks and an assumption that entities connected to them could be trusted. As interconnection expanded and conditions changed, that assumption became a security concern. ITU-T identifies possible attack categories including location tracking, subscriber fraud, interception of calls or messages, number spoofing, denial of service, infiltration, routing attacks, and telephone spam. These are documented types of risk; they do not mean every SS7 network is compromised or that every subscriber faces the same level of exposure. ITU-T Q.3062
Moving SS7 signalling over IP with SIGTRAN is a transport change, not a security fix. ITU-T also notes that the later use of Diameter has not eliminated the basic vulnerabilities associated with SS7.
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How are signalling messages and caller identity authenticated?
Authenticating signalling exchanges
ITU-T Recommendation Q.3062 defines requirements for authenticating signalling messages so operators can check whether exchanges are authentic using an accepted trust anchor. The recommendation states: “The goal of Recommendation ITU-T Q.3062 is to define the signalling requirements for authentication of signalling messages, in order for operators to be able to verify the authenticity of signalling exchanges based on an accepted trust anchor.” ITU-T Q.3062, September 2022
This is an operator-network security measure, not a setting an ordinary phone owner can enable.
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Authenticating calling-line identity
ITU-T Recommendation Q.3063 addresses authentication of calling-line identification in response to spoofing risks involving untrusted devices that interconnect with PSTN and PLMN networks. It outlines an approach using public-key infrastructure and calling-line identification certificates. This is a defined procedure for authenticating caller identity, distinct from the broader SS7 protocol family. ITU-T Q.3063, September 2022
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