Difference between MMS and GOOSE

Much is said about communication systems in IEC 61850.  Some can get quite complex but it is useful to first cover the fundamentals - what is the difference between client-server (MMS) type communication methodologies and Publisher-Subscriber mechanisms as GOOSE and Sampled Values.

MMS is like a telephone call - a one-to-one exchange of information/commands based on one party dialling the other to establish the connection with appropriate exchange of pleasantries to start such as "hello" and "this is Fred". You could be next to each other or on opposite sides of the world - i.e. it is routable beyond just your local domain. MMS therefore uses IP address to route the message at Layer 3 and having established the direct connection then uses MAC Address learning tables in the switches to restrict the message path by the most direct beans between the client and server, like initially asking "where are you?" and then having established that just using the direct path.

GOOSE and Sampled Values are like a radio transmission - the message is sent out from an antenna to "everywhere" and doesn't identify who, or indeed if anyone is listening - its job is to send (publish) the message.  If your radio is turned on and tuned to that channel you can hear the message (subscribe) with out any exchange of other "pleasantries". However you have to be within the radio transmission area to hear it, i.e. it is not routable beyond the signal strength coverage of the radio as its local domain. GOOSE is not (inherently) routable since it only uses Layer 2 switching based on MAC Address in the message - it does not use IP address. The MAC address a special broadcast to a generic MAC Address - it doesn't use the MAC address tables in the switches to learn the best route between source and destination as in MMS.

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