M3UA (RFC 4666) is the SIGTRAN adaptation layer that presents MTP3 service over SCTP, so SCCP and ISUP users run over IP unchanged. It connects application server processes (ASPs) to a signalling gateway (SG), routing messages by point code via routing keys and routing contexts, and manages ASP state with dedicated message classes.
What M3UA does
M3UA replaces MTP3 in the SS7 stack, offering its users — SCCP and ISUP — the same service (message transfer to a point code, plus network-management indications) but over an SCTP association instead of TDM links. This lets an IP-based node participate in an SS7 network as if it had a point code, exchanging MAP or ISUP with legacy TDM peers through a signalling gateway that bridges the two transports.
SG, ASP and AS
M3UA defines three roles. The Signalling Gateway (SG) terminates classic SS7 on the TDM side and relays messages over M3UA/SCTP on the IP side. An Application Server Process (ASP) is an IP endpoint that handles signalling — a physical process such as an SMSC or MSC instance. An Application Server (AS) is the logical entity serving a range of traffic (a routing key); it is realised by one or more ASPs for redundancy and load-sharing.
| Entity | Meaning |
|---|---|
| SG | Signalling Gateway — bridges TDM SS7 to M3UA/IP |
| SGP | Signalling Gateway Process — an instance within an SG |
| ASP | Application Server Process — an IP signalling endpoint/process |
| AS | Application Server — logical entity for a routing key, served by ASPs |
Routing keys and routing context
M3UA routes to an Application Server using a routing key — a set of parameters (such as destination point code, service indicator, and optionally SSN or circuit ranges) that identifies which traffic belongs to which AS. Each routing key is bound to a numeric Routing Context that is carried in messages so the receiver knows which AS a message is for. This is how one SCTP association between an SG and an ASP can carry traffic for several application servers unambiguously.
Message classes
M3UA messages are grouped into classes. Transfer messages (the DATA / Payload Data message) carry the actual MTP3-user payload. SSNM conveys SS7 network-management status (destination available/unavailable/congested) so IP nodes learn about TDM-side reachability. ASPSM and ASPTM manage the ASP's state — bringing it up and making its traffic active. Together these let M3UA present a faithful MTP3 service and keep the distributed state consistent.
| Class | Name | Examples |
|---|---|---|
| Transfer | MTP3-user payload | DATA (Payload Data) |
| SSNM | SS7 Signalling Network Mgmt | DUNA, DAVA, SCON, DUPU (dest. status) |
| ASPSM | ASP State Maintenance | ASP Up, ASP Down, Heartbeat (BEAT) |
| ASPTM | ASP Traffic Maintenance | ASP Active, ASP Inactive |
| MGMT / RKM | Management / Routing Key Mgmt | ERROR, NTFY, REG REQ/RSP, DEREG |
ASP states and redundancy
An ASP progresses through states: it starts DOWN, moves to INACTIVE on an ASP-Up exchange, and to ACTIVE on ASP-Active, at which point it carries traffic. Multiple ASPs backing one AS give redundancy through traffic-mode selection: Override (one ASP active, others take over on failure), Load-share (traffic distributed across active ASPs), or Broadcast. This is how M3UA delivers high availability equivalent to mated STPs in the TDM world.
ASP → SG ASP Up (ASPSM) → state: INACTIVE
SG → ASP ASP Up Ack
ASP → SG ASP Active (RC=100) (ASPTM) → state: ACTIVE
SG → ASP ASP Active Ack
... DATA (Payload) now flows for routing context 100 ...
ASP ↔ SG BEAT / BEAT Ack (heartbeat keeps the link monitored)Frequently asked
What does M3UA replace?
M3UA replaces MTP3. It presents the same message-transfer and network-management service to SCCP and ISUP, but over an SCTP association instead of TDM MTP2/MTP1 links, so those SS7 users run over IP without modification.
What is a routing key in M3UA?
A routing key is the set of parameters — destination point code, service indicator, and optionally SSN or circuit ranges — that identifies which traffic belongs to a given Application Server. It is bound to a numeric routing context carried in messages so the receiver routes to the correct AS.
What is the difference between an ASP and an AS?
An Application Server Process (ASP) is a physical IP signalling endpoint or process. An Application Server (AS) is the logical entity serving a routing key, realised by one or more ASPs. Several ASPs back one AS to provide redundancy and load-sharing.
What are the M3UA ASP states?
An ASP is DOWN until an ASP-Up exchange moves it to INACTIVE, then an ASP-Active exchange moves it to ACTIVE, where it carries traffic for its routing contexts. ASP-Inactive and ASP-Down move it back. Heartbeat (BEAT) messages keep the association monitored.
What is IPSP mode in M3UA?
IPSP (IP Server Process) mode lets two M3UA endpoints communicate directly, peer-to-peer, without a signalling gateway between them — useful when both nodes are IP-native. It contrasts with SG mode, where an ASP talks to a signalling gateway that bridges to TDM SS7.
Source specifications
The published standards this page is grounded in. Opens in a new tab.