Monday, January 24, 2011

What are Client/Server and Publisher/Subscriber in IEC 61850?

The terms Client/Server (C/S) and Publisher/Subscriber (P/S) in IEC 61850 are describing (communication) roles a real device may have. A device can play any of the four roles - even at the same time.

From an information flow point of view (independent of C/S and P/S) there are different levels of relations: (#1) the Application Layer Protocol or communication view (see first slide); (#2) the system view as seen from an IEC 61850-6 (SCL - System Configuration Language) point of view (see second slide).

(#1) First slide: A server exposes the Data (in a LD/LN) that can be accessed by the client over a TCP/IP connection - may be over Ethernet. Or the client will receive event-driven reports from the server over TCP/IP (Ethernet). This is a 1:1 connection at protocol level. A server may communicate with many clients (one TCP/IP connection between each client and server). The connection is opened by the client. Note: a real device could play both roles - in MMS an association (connection) allows both devices to play both roles (this is not yet used in IEC 61850)!

The slide also shows the publisher and subscriber. The publisher sends multicast messages that are picked-up by subscribers. The subscriber is (at communication level) NOT subscribing to the subscriber. The message is just sent and any device that has a subscriber role picks-up the messages it wants to receive. Each multicast message has an identification (let's say number 277). The publisher does NOT know who is receiving the messages. This is like picking-up a newspaper at the red traffic light in the morning. You may also subscribe to the publishing house to get the newspaper delivered to your home every morning - this is the real publishing/subscribing).


A device that has a Server can model Data (in LD/LN), e.g. status of a cirsuit breaker. This Data can be used for publishing values (by a DataSet and a Control block). Strictly speaking: a Publisher does not expose a data model. This is done by a server. The publishing service makes use of a server (explicitly or implicitly). Explicitly means: data - dataset - control block - message. Implicitly means: message - you don't see the model; it may be defined in SCL only.

(#2) From a SCL (system) point of view we can model the flow of information from a source (right), through a server/publisher, message, client/server, ... to a sink (see next slide). In SCL we are describing the information exchange between a Data in logical nodes - clients/servers are not in the main focus. SCL provides - in my words - the wiring plan of a whole system (from a source to a sink).


The services defined in IEC 61850 ACSI are listed in the third slide:


The last slide shows that clients and servers can be cascaded ... this is outside the protocols - but can be specified with SCL! SCL is a VERY powerful specification language!!


Examples of these cascaded relations are presented and discussed during the hands-on training courses of NettedAutomation ... and much more.

Summary: each device implementing all four roles defined in IEC 61850 can communicate with each other device as a client, server, publisher, and subscriber.

No comments: