# What is SS7?

> SS7 (Signalling System No. 7) is the ITU-T-defined control-plane signalling system that 2G and 3G mobile networks use for call setup, mobility management, and SMS routing. Its layered stack — MTP, SCCP, TCAP, and MAP — carries the SendRoutingInfoForSM and MT-ForwardSM operations that deliver every cross-operator SMS to a handset. On 4G and 5G, Diameter and HTTP/2 signalling take over, but SS7 still underpins the majority of SMS delivered today.

- Canonical URL: https://www.broadnet.me/answers/what-is-ss7
- Language: en
- Last reviewed: 2026-07-22
- Publisher: BroadNet (https://www.broadnet.me)

## The SS7 stack, layer by layer

MTP (Message Transfer Part) is the transport; SCCP (Signalling Connection Control Part) adds global-title routing; TCAP (Transaction Capabilities) handles dialogues; and MAP (Mobile Application Part) is the mobile application layer that actually carries SMS and location operations. Over IP, SS7 rides on SIGTRAN. See /answers/ss7-map-explained and /answers/what-is-sigtran.

## Why SS7 is a security concern

SS7 was designed for a small, trusted club of state telcos, so peers largely trust each other with no per-message authentication. That trust is abused for location tracking and SMS interception — most notoriously via SendRoutingInfoForSM harvesting. GSMA FS.07 and FS.11 define the signalling-firewall controls that close these holes; BroadNet operates one at /operators/ss7-firewall.

## Related

- [All answers](https://www.broadnet.me/answers)
- [Glossary](https://www.broadnet.me/answers)
- [SS7 MAP — what it is and why messaging still depends on it](https://www.broadnet.me/answers/ss7-map-explained)
- [SIGTRAN — SS7 over IP explained](https://www.broadnet.me/answers/what-is-sigtran)
- [Diameter S6a — the 4G equivalent of MAP](https://www.broadnet.me/answers/diameter-s6a-explained)
- [GSMA FS.07 and FS.11 — the SS7 signalling firewall standards](https://www.broadnet.me/answers/gsma-fs07-fs11-signaling-firewall)
- [SS7 Firewall](https://www.broadnet.me/operators/ss7-firewall)
- [Signaling & CDR Intelligence](https://www.broadnet.me/platforms/signaling-cdr)