MMS

MMS is the application-layer protocol on the IEC 61850 station bus. It carries client/server reads, writes, controls, and reporting between IEDs and station-level applications — gateways, HMIs, engineering workstations.

Where it fits

IEC 61850 defines an Abstract Communication Service Interface (ACSI) — a vendor-neutral data model and service vocabulary. IEC 61850-8-1 maps that abstract interface onto MMS frames over TCP/IP. The result is the protocol the station bus actually speaks.

Less time-critical than GOOSE or Sampled Values — millisecond-and-up rather than sub-millisecond. Richer in data types — supports complex structured payloads, file transfer, named variable lists, reports.

Why it has its own security part

MMS runs over TCP, so IEC 62351-4 wraps it in TLS and adds ACSE-level authentication that binds identity to the MMS association rather than the underlying socket. The 2018 edition added end-to-end signing of selected payload fields so a man-in-the-middle who terminates TLS still cannot silently rewrite a control command.

Why it is not on the WAN

MMS is the station-bus protocol. Northbound from the substation to the control centre, the protocol is typically DNP3 (UK distribution and North America) or IEC 60870-5-104 (European transmission). MMS stays inside the substation perimeter.