Everything worth knowing, nothing that isn't.
M3UA — the workhorse
M3UA (MTP3 User Adaptation) is the most widely deployed SIGTRAN adaptation layer in the world. It carries everything that MTP3 used to carry — TCAP-bearing MAP/CAP, SCCP-routed services — over SCTP, while preserving SS7 Point Code addressing. Most operator-to-operator SIGTRAN peerings use M3UA. If you are reading SIGTRAN docs and unsure which layer applies, default-assume M3UA.
M2UA — the SG link replacement
M2UA (MTP2 User Adaptation) is used when you need to expose what used to be a single TDM SS7 link as an IP link to a Signalling Gateway (SG). It preserves MTP2 semantics (per-link sequencing, flow control). M2UA appears in operator deployments where the legacy SG hardware speaks MTP2 and the new core speaks IP, and somebody needs to bridge the two. Outside that bridging use case, M2UA is rarely seen on greenfield deployments.
SUA — bypassing MTP3 entirely
SUA (SCCP-User Adaptation) connects SCCP-using applications (MAP, CAP, INAP, RANAP) directly to SCTP — there is no MTP3 in the stack at all, and there are no SS7 Point Codes on the IP side. SUA is the future-clean approach but adoption has been limited; most operators picked M3UA because it preserved the operational model they already understood. SUA is more common in IMS/HLR deployments and in some greenfield 4G/5G signalling cores.
Operational and compliance angle
An SS7 firewall must inspect at the right layer — a firewall configured to inspect M3UA-encapsulated SCCP/MAP will miss attacks delivered via SUA, and vice versa. BroadNet's signalling firewall (/operators/ss7-firewall) inspects all three adaptation layers in a single policy engine, with shared peer-reputation across the stack. For GSMA FS.07 alignment, the categorisation and rate-limiting rules apply regardless of adaptation layer.