# SS7 vs SIGTRAN: TDM signalling over IP

> SS7 and SIGTRAN are not competitors — SIGTRAN is SS7 carried over IP. Classic SS7 runs the MTP3/SCCP/TCAP/MAP stack over TDM links (MTP1/MTP2). SIGTRAN keeps that same application layer but replaces the TDM transport with SCTP over IP and adaptation layers such as M3UA (for MTP3) or SUA (for SCCP).

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

How SIGTRAN relates to classic SS7: the same MTP3/SCCP/TCAP/MAP application layer, but carried over IP with SCTP and M3UA (or SUA) instead of TDM MTP1/MTP2 links.

## Same application, different transport

The signalling applications — MAP for SMS and roaming, ISUP for calls, CAP for intelligent network — are identical whether they run over TDM or IP. What changes underneath is the transport. Classic SS7 uses MTP1 (physical timeslots) and MTP2 (link reliability) to move messages between adjacent points. SIGTRAN discards those and uses SCTP over IP for reliable transport, with an adaptation layer that presents the same service the discarded layer used to provide.

*Classic SS7 vs SIGTRAN, layer by layer*

| Function | Classic SS7 (TDM) | SIGTRAN (IP) |
|---|---|---|
| Application | MAP / ISUP / CAP | MAP / ISUP / CAP (unchanged) |
| Transaction / SCCP | TCAP / SCCP | TCAP / SCCP — or SUA replaces SCCP |
| Network (MTP3) | MTP3 | M3UA (MTP3-User Adaptation) |
| Link (MTP2) | MTP2 | SCTP association |
| Physical (MTP1) | E1/T1 timeslot | IP / Ethernet |

## Why operators moved to SIGTRAN

TDM signalling links are fixed at 64 kbit/s, expensive to scale, and tied to circuit infrastructure. Carrying signalling over IP lets operators use commodity packet networks, scale capacity far beyond a single timeslot, converge signalling onto the same transport as their data core, and interwork with IP-native systems. SCTP was designed specifically for this: it gives SS7 the reliable, sequenced, redundant transport MTP2 provided, plus multi-homing for path resilience that TDM never had.

## The adaptation layers

SIGTRAN is a family of adaptation layers, each replacing a different classic layer, so operators can migrate at the boundary that suits them. M3UA is the common choice — it replaces MTP3 and lets SCCP/ISUP users run over IP between an application server and a signalling gateway. SUA goes higher, carrying SCCP-users (TCAP/MAP) directly over SCTP and skipping MTP3 entirely. M2PA replaces MTP2 for full IP-based STP links, and M2UA/IUA backhaul lower-layer signalling from a gateway.

*SIGTRAN adaptation layers*

| Layer | RFC | Replaces / carries |
|---|---|---|
| M3UA | RFC 4666 | MTP3 — carries SCCP/ISUP users over SCTP |
| SUA | RFC 3868 | SCCP — carries TCAP/MAP users directly |
| M2PA | RFC 4165 | MTP2 — peer IP links running full MTP3 |
| M2UA | RFC 3331 | MTP2 backhaul from gateway to controller |
| IUA | RFC 3057 | ISDN Q.921 user adaptation |

## How they coexist

In practice most networks run a hybrid. A Signalling Gateway terminates legacy TDM SS7 on one side and relays the same messages over SIGTRAN to IP-based application servers on the other, so a TDM peer and an IP node can exchange the identical MAP/ISUP messages transparently. This is why SS7 knowledge stays relevant: the application semantics, addressing and security concerns are unchanged — only the bottom of the stack has become IP.

## Frequently asked questions

### Is SIGTRAN a replacement for SS7?

Not exactly — SIGTRAN is SS7 transported over IP. The MTP3/SCCP/TCAP/MAP application layer is unchanged; SIGTRAN only replaces the TDM transport (MTP1/MTP2) with SCTP over IP plus an adaptation layer like M3UA. The signalling itself is still SS7.

### What is the difference between M3UA and SUA?

M3UA replaces MTP3, so SCCP and ISUP run over IP unchanged above it. SUA replaces SCCP itself, carrying SCCP-users such as TCAP/MAP directly over SCTP without MTP3 or M3UA. M3UA is the more common migration point.

### Why did operators move SS7 onto IP?

To escape the cost and 64 kbit/s ceiling of TDM signalling links, scale capacity, converge signalling onto packet infrastructure, and interwork with IP-native systems. SCTP also adds multi-homing and per-flow redundancy that TDM links lacked.

### Does SIGTRAN change SS7 security?

The application-layer risks — MAP location tracking, SMS interception — are identical, so FS.11 filtering still applies. SIGTRAN adds IP-layer concerns (secure the SCTP associations, restrict peers) but does not fix SS7's lack of message authentication; a signalling firewall is still required.

### What is a signalling gateway in SIGTRAN?

A Signalling Gateway sits at the TDM-to-IP boundary: it terminates classic SS7 links on one side and relays the same MTP3-user messages over SIGTRAN (M3UA/SCTP) to IP application servers on the other, letting legacy and IP nodes exchange identical SS7 signalling.

## Sources

- [IETF RFC 2719 — Framework Architecture for Signalling Transport](https://datatracker.ietf.org/doc/html/rfc2719)
- [IETF RFC 4666 — M3UA](https://datatracker.ietf.org/doc/html/rfc4666)

## Related

- [SIGTRAN overview](https://www.broadnet.me/reference/sigtran-overview)
- [SCTP: Stream Control Transmission Protocol](https://www.broadnet.me/reference/sctp-stream-control-transmission-protocol)
- [M3UA: MTP3-User Adaptation](https://www.broadnet.me/reference/m3ua-mtp3-user-adaptation)
- [SS7 stack overview](https://www.broadnet.me/reference/ss7-stack-overview)
- [SIGTRAN explained](https://www.broadnet.me/answers/sigtran-explained)