# SIGTRAN deployment architecture: SG, ASP, AS

> A SIGTRAN deployment links TDM SS7 and IP signalling through a Signalling Gateway (SG), which relays messages over SCTP associations to Application Server Processes (ASPs). Traffic for a routing key is served by a logical Application Server (AS), realised by multiple ASPs in override, load-share or broadcast mode for redundancy.

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

How a SIGTRAN network is built: the signalling gateway bridging TDM and IP, application servers and their processes, SCTP associations, and the redundancy modes (override, load-share, broadcast) that give carrier-grade availability.

## The end-to-end picture

A typical SIGTRAN deployment bridges two worlds. On the TDM side, classic SS7 links reach the Signalling Gateway from legacy STPs and switches. On the IP side, the SG maintains SCTP associations to the operator's IP-based signalling nodes — SMSC, HLR/HSS, MSC-server, SCP. The SG relays each SS7 message across, translating between the TDM MTP layers and the M3UA (or SUA) adaptation over SCTP, so a TDM peer and an IP node exchange identical MAP or ISUP messages without either being aware of the transport change.

*SIGTRAN bridging topology*

```text
  TDM SS7 world                 IP world
 ┌──────────┐   MTP links   ┌──────┐  SCTP assoc  ┌─────────────┐
 │ STP / MSC │──────────────│  SG  │══════════════│  ASP (SMSC) │
 └──────────┘   (E1/T1)     └──────┘   M3UA       └─────────────┘
                                │  ╲  SCTP assoc  ┌─────────────┐
                                │   ╲════════════│  ASP (HLR)  │
                                                  └─────────────┘
     One AS (routing key) may be served by several ASPs.
```

## Associations and routing contexts

The link between an SG and an ASP is an SCTP association — typically multi-homed across two IP paths for resilience. Over one association, an SG can carry traffic for several Application Servers, each identified by a routing context. When an SS7 message arrives from the TDM side, the SG matches it to a routing key (by destination point code, service indicator, and optionally SSN), selects the AS, and forwards it to an active ASP with the corresponding routing context so the receiver routes it internally.

## Redundancy modes

Multiple ASPs backing one AS provide high availability, and the traffic mode decides how they share load. Override runs one ASP active with the others standing by to take over on failure — an active/standby model. Load-share distributes traffic across all active ASPs, using a key (such as the SLS or an SCCP parameter) to keep related messages on one ASP for sequencing. Broadcast sends to all active ASPs. These modes are how SIGTRAN reproduces the mated-STP resilience of classic SS7 in an IP deployment.

*M3UA/SUA traffic (redundancy) modes*

| Mode | Behaviour | Analogous to |
|---|---|---|
| Override | One ASP active; another takes over on failure | Active/standby |
| Load-share | Traffic spread across active ASPs by a key | Active/active with affinity |
| Broadcast | Every message sent to all active ASPs | Full replication |

## State management and failover

The SG and ASPs keep each other's state synchronised. ASPs signal ASP-Up/Active to enter service and are monitored by SCTP heartbeats and M3UA BEAT messages; SSNM messages propagate TDM-side destination status (available, unavailable, congested) to the IP nodes so they stop or resume sending appropriately. If an ASP or a path fails, SCTP multi-homing and the AS traffic mode combine to move traffic to a healthy ASP or path within sub-second timescales, preserving signalling continuity.

## Design considerations

A sound SIGTRAN design pairs SGs and ASPs for no single point of failure, multi-homes every SCTP association across independent paths, and plans routing keys so traffic maps cleanly to application servers. Because SIGTRAN carries the same SS7 application layer, the same security posture applies: inbound signalling from interconnect still needs FS.11-style firewalling, and the SCTP associations themselves must be restricted to known peers and protected on the IP network.

## Frequently asked questions

### 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 SCTP (via M3UA or SUA) to IP application servers on the other, translating transport while leaving the SS7 application messages unchanged.

### What is an SCTP association in a SIGTRAN deployment?

It is the reliable, usually multi-homed transport connection between a signalling gateway and an application server process. One association can carry traffic for several application servers, distinguished by routing context, and multi-homing gives it path failover.

### What are the SIGTRAN redundancy modes?

Override (one ASP active, others on standby), Load-share (traffic spread across active ASPs with affinity for sequencing), and Broadcast (every message to all active ASPs). They let multiple ASPs back one Application Server, reproducing mated-STP resilience over IP.

### How does SIGTRAN achieve high availability?

By combining SCTP multi-homing (path-level failover) with multiple ASPs per Application Server in override or load-share mode (node-level failover), plus paired signalling gateways. Together these remove single points of failure so signalling survives path, process and gateway faults.

### Does moving to SIGTRAN change my SS7 security requirements?

The application-layer risks are unchanged, so FS.11 signalling firewalling still applies to inbound interconnect traffic. SIGTRAN adds IP-layer duties: restrict SCTP associations to known peers and protect them on the network. It changes the transport, not the need for signalling security.

## Sources

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

## Related

- [SIGTRAN overview](https://www.broadnet.me/reference/sigtran-overview)
- [M3UA: MTP3-User Adaptation](https://www.broadnet.me/reference/m3ua-mtp3-user-adaptation)
- [SCTP: Stream Control Transmission Protocol](https://www.broadnet.me/reference/sctp-stream-control-transmission-protocol)
- [SS7 security threats & GSMA FS.11](https://www.broadnet.me/reference/ss7-security-threats-fs11)
- [Signalling & CDR platform](https://www.broadnet.me/platforms/signaling-cdr)