Thursday, June 11, 2015

XMPP - IEC 61850-8-2 Defines Additional Communication Mapping

IEC TC 57 has published a first draft for an additional mapping of IEC 61850 information models and communication messages.

IEC 61850-8-2 (57/1583/CD):
Communication networks and systems for power utility automation - Part 8-2: Specific Communication Service Mapping (SCSM) – Mapping to Extensible Messaging Presence Protocol (XMPP)

Closing date for comments is 2015-09-11

The new mapping defines (relies on) the following definitions:

Service mapping (unchanged)
The abstract (client-server) services of IEC 61850-7-2 are mapped to MMS as defined in the existing IEC 61850-8-1 Ed2.

Message Encoding (new concrete encoding)
The encoding of the messages uses ASN.1 XER (XML encoding rule) – in addition to ASN.1 BER according to IEC 61850-8-1 Ed2. The encoding defines an XML schema – contained in the draft.

Model mapping (unchanged)
As in IEC 61850-8-1 Ed2. This applies to the flattening of the object identification and adding Functional Constraint (FC=ST or MX) in the path name and using “$” for “.”:


Underlying Transport (new T-Profile)
The transport (exchange) of the XER encoded messages uses a new approach: using XMPP.

This new transport mechanism and encoding will be used between all kinds of utility Distributed Energy Resource devices and related power management systems, over any communication infrastructure including public networks.

The coming IEC 61850-8-2 can be understood as an (encoding and transport) extension of the existing IEC 61850-8-1.

It is very crucial that most parts of implementations and tools can be re-used! Re-Use is one of the basic approaches used in IEC 61850! Don’t start always from scratch – use what is available and add something.

So, to read the frequency of Bay5 is almost the same .. using the reference
encoded in ASN.1 BER (IEC 61850-8-1) and in ASN.1 XER (IEC 61850-8-2).

See also example of encoding.

A second document explains the needs and background for an additional mapping:

IEC 61850-80-3 TR (57/1584/DTR):
Communication networks and systems for power utility automation -
Part 80-3: Mapping to Web protocols – Requirements and technical choices

It describes the requirements and the technical principles for a new specific communication service mapping (SCSM) based on Web Protocols.
For more information about the candidate technologies which have been analyzed but not selected as well as about the selection process used for choosing the technology, national committees are invited to consult document 57/1585/INF which is circulated in parallel:

Accompanying document to 57/1584/DTR, Proposed IEC TR 61850-80-3

It mainly describes the technical solutions which have been investigated but finally not selected for the SCSM of the IEC 61850 based on Web Protocols.

  1. IEC 61400-25-4 Annex A (Web services)
  2. DPWS (Devices Profile for Web Services)
  3. REST (Representational State Transfer)
  4. XML messaging over Websocket
  5. ACSI XML Messaging
  6. OPC UA

The finally chosen solution  “MMS XER payload over XMPP as transport” was recognized after several years of work as the preferred solution – especially from a fast time-to-market point of view.

What does XMPP provide?

XMPP (RFC 6120) is a middleware messaging and presence protocol supporting decentralized architectures and provides:

  • Registering resources in publicly reachable servers
  • Resolving resources based on names
  • Security (authentication, integrity, confidentiality) for the communication with the XMPP server

This fits well to the information models defined in IEC 61850.

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