# SS7 stack overview: MTP, SCCP, TCAP, MAP, ISUP

> SS7 (Signalling System No. 7) is the out-of-band signalling stack that controls calls, SMS and mobility in TDM and 2G/3G core networks. It layers MTP1–3 for transport, SCCP for global-title addressing, TCAP for transaction dialogues, and user parts — MAP for mobile services, ISUP for call setup — on top.

- Canonical URL: https://www.broadnet.me/reference/ss7-stack-overview
- Language: en
- Last updated: 2026-07-18
- Publisher: BroadNet (https://www.broadnet.me)

How the SS7 stack is layered — MTP1-3 transport, SCCP addressing, TCAP transactions, and the MAP and ISUP user parts — and which layer does what for SMS, calls and mobility.

## Why SS7 exists

SS7 is a common-channel signalling system: the signalling that sets up, manages and tears down calls travels on a separate logical channel from the voice or data it controls, rather than in-band on the same circuit. Standardised by the ITU-T in the Q.700 series from the 1980s, it became the control plane of the global PSTN and, through the MAP application, of GSM/UMTS mobile networks — carrying call setup, SMS, roaming, authentication and location management between switches, databases and service centres.

## The layered stack

SS7 maps loosely onto the OSI model. The Message Transfer Part (MTP levels 1–3) provides the physical, link and network transport. SCCP adds richer addressing (global titles and subsystem numbers) and connectionless/connection-oriented service on top of MTP3 — together MTP3+SCCP are often called the Network Service Part (NSP). TCAP provides transaction-oriented dialogues for non-circuit applications. The user/application parts sit at the top: MAP for mobile services, CAP for CAMEL/intelligent-network, and INAP, while ISUP and TUP ride directly on MTP3 for trunk call control.

*SS7 layers and their role*

| Layer | Spec | Role |
|---|---|---|
| MTP-1 | Q.702 | Physical layer — E1/T1 timeslots (64 kbit/s signalling links) |
| MTP-2 | Q.703 | Data link — link alignment, error detection/retransmission, flow control |
| MTP-3 | Q.704 | Network — message routing by point code, load-sharing, network management |
| SCCP | Q.711–Q.714 | Global-title/subsystem addressing, connectionless & connection-oriented service |
| TCAP | Q.771–Q.775 | Transaction dialogues and component (operation) handling |
| MAP | TS 29.002 | Mobile Application Part — SMS, roaming, authentication, HLR/VLR |
| ISUP | Q.761–Q.764 | ISDN User Part — trunk call setup/teardown (rides directly on MTP3) |

## Where SMS and calls sit

A mobile-terminated SMS uses the full stack above SCCP: MAP operations (SendRoutingInfoForSM, MT-ForwardSM) are carried as TCAP components, addressed by SCCP global title to the HLR and serving MSC, and transported by MTP3 (or SIGTRAN over IP). A voice trunk call instead uses ISUP directly over MTP3 — no SCCP or TCAP — because it is circuit-associated signalling tied to a specific voice timeslot.

## SS7 today: TDM and SIGTRAN

The classic SS7 stack runs over TDM links (MTP1/2). Modern deployments carry the same MTP3/SCCP/TCAP/MAP payloads over IP using SIGTRAN — SCTP transport with M3UA replacing MTP3, or SUA replacing SCCP — so operators keep the SS7 application layer while retiring TDM. The signalling semantics are identical; only the transport under the application changes, which is why SS7 knowledge remains essential even in packet cores.

## Frequently asked questions

### What is the difference between MTP, SCCP and TCAP?

MTP (levels 1–3) is the transport that gets a message to the right signalling point by point code. SCCP adds global-title addressing so you can reach a node without knowing its point code. TCAP structures request/response operations into transaction dialogues on top of SCCP.

### Is MAP part of SS7?

MAP (Mobile Application Part) is an SS7 application/user part defined by 3GPP TS 29.002. It runs as TCAP operations over SCCP and MTP3 (or SIGTRAN) and carries GSM/UMTS services — SMS, roaming, authentication and location management — between the HLR, VLR, MSC and SMSC.

### Does a voice call use SCCP and TCAP?

No. Trunk call setup uses ISUP directly over MTP3, because it is circuit-associated signalling bound to a voice timeslot. SCCP and TCAP are used by non-circuit applications like MAP (SMS, roaming) and INAP/CAP (intelligent network).

### Is SS7 still used in 2026?

Yes. SS7/MAP still carries SMS, roaming and mobility for 2G/3G and legacy interworking, and its application layer is transported over IP via SIGTRAN in modern cores. It coexists with Diameter and SIP/IMS rather than being fully replaced.

### What replaced SS7 in LTE and 5G?

In LTE the equivalent control signalling moved to Diameter (mobility, authentication) and SIP/IMS (voice, SMS over IP). SS7/MAP remains for 2G/3G access and inter-operator interworking, which is why signalling firewalls still police SS7 traffic.

## Sources

- [ITU-T Q.700 — Introduction to SS7](https://www.itu.int/rec/T-REC-Q.700)
- [3GPP TS 29.002 — Mobile Application Part (MAP)](https://www.3gpp.org/dynareport/29002.htm)

## Related

- [MTP: the Message Transfer Part](https://www.broadnet.me/reference/mtp-message-transfer-part)
- [SCCP and global title translation](https://www.broadnet.me/reference/sccp-and-global-title)
- [MAP operations reference](https://www.broadnet.me/reference/map-operations)
- [SS7 vs SIGTRAN](https://www.broadnet.me/reference/ss7-vs-sigtran)
- [SS7 signalling security](https://www.broadnet.me/answers/ss7-signaling-security)
- [SS7 firewall (operators)](https://www.broadnet.me/operators/ss7-firewall)