SIST EN IEC 62541-1:2026

OPC unified architecture - Part 1: Overview and concepts (IEC 62541-1:2025)

IEC 62541-1:2025 presents the concepts and overview of the OPC Unified Architecture (OPC UA). Reading this document is helpful to understand the remaining parts of the IEC 62541 series. Each of the other parts is briefly explained along with a suggested reading order. This first edition cancels and replaces IEC TR 62541-1 published in 2020

OPC Unified Architecture – Teil 1: Übersicht und Konzepte (IEC 62541-1:2025)

Architecture unifiée OPC - Partie 1: Vue d'ensemble et concepts (IEC 62541-1:2025)

L'IEC 62541-1:2025 décrit les concepts et donne une vue d'ensemble de l'Architecture unifiée OPC (OPC UA). La lecture du présent document est utile pour comprendre les autres parties de la série IEC 62541. Chacune des autres parties est brièvement expliquée avec un ordre de lecture suggéré. Cette première édition annule et remplace l'IEC TR 62541-1 paru en 2020.

Enotna arhitektura OPC - 1. del: Pregled in koncepti (IEC 62541-1:2025)

IEC 62541-1:2025 predstavlja koncepte in pregled Enotne arhitekture OPC (OPC UA). Branje tega dokumenta je koristno za razumevanje preostalih delov serije IEC 62541. Vsak od drugih delov je na kratko pojasnjen skupaj s predlaganim vrstnim redom branja. Ta prva izdaja razveljavlja in nadomešča IEC TR 62541-1, objavljen leta 2020.

General Information

• Status: Published
• Public Enquiry End Date: 30-Mar-2024
• Publication Date: 12-Apr-2026
• ICS: 25.040 - Industrial automation systems
• ICS: 25.040.40 - Industrial process measurement and control
• ICS: 35.240.50 - IT applications in industry
• Technical Committee: MOV - Measuring equipment for electromagnetic quantities
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 24-Feb-2026
• Due Date: 01-May-2026
• Completion Date: 13-Apr-2026
• Ref Project: EN IEC 62541-1:2026 - OPC unified architecture - Part 1: Overview and concepts

Relations

• Referred By: SIST EN IEC 62541-2:2026 - OPC unified architecture - Part 2: Security Model (IEC 62541-2:2026)
• Effective Date: 10-Mar-2026
• Referred By: SIST EN IEC 62541-5:2026 - OPC unified architecture - Part 5: Information model (IEC 62541-5:2026)
• Effective Date: 10-Feb-2026
• Referred By: SIST EN IEC 62769-7:2021 - Field Device Integration (FDI) - Part 7: Communication devices (IEC 62769-7:2021)
• Effective Date: 10-Feb-2026
• Referred By: SIST EN 62769-7:2015 - Devices and integration in enterprise systems; Field Device Integration - Part 7: Communication Devices (IEC 62769-7:2015)
• Effective Date: 10-Feb-2026
• Referred By: SIST EN IEC 62769-1:2023 - Field Device Integration (FDI®) - Part 1: Overview (IEC 62769-1:2023)
• Effective Date: 10-Feb-2026
• Referred By: SIST EN IEC 62541-11:2026 - OPC Unified Architecture - Part 11: Historical Access (IEC 62541-11:2025)
• Effective Date: 10-Feb-2026
• Referred By: SIST EN IEC 62541-23:2026 - OPC unified architecture - Part 23: Common reference types (IEC 62541-23:2025)
• Effective Date: 10-Feb-2026
• Referred By: SIST EN IEC 62769-6:2021 - Field Device Integration (FDI) - Part 6: Technology Mapping (IEC 62769-6:2021
• Effective Date: 10-Feb-2026
• Referred By: SIST EN 62769-3:2015 - Devices and integration in enterprise systems; Field Device Integration - Part 3: FDI Server (IEC 62769-3:2015)
• Effective Date: 10-Feb-2026
• Referred By: SIST EN 62769-1:2015 - Devices and integration in enterprise systems; Field Device Integration - Part 1: Overview (IEC 62769-1:2015)
• Effective Date: 10-Feb-2026
• Referred By: SIST EN IEC 62541-3:2026 - OPC unified architecture - Part 3: Address space model (IEC 62541-3:2025)
• Effective Date: 10-Feb-2026
• Referred By: SIST EN 62769-6:2015 - Devices and integration in enterprise systems; Field Device Integration - Part6: Technology Mapping (IEC 62769-6:2015)
• Effective Date: 10-Feb-2026
• Referred By: SIST EN IEC 62541-14:2026 - OPC unified architecture - Part 14: Pubsub (IEC 62541-14:2026)
• Effective Date: 10-Feb-2026
• Referred By: SIST EN IEC 62541-22:2026 - OPC unified architecture - Part 22: Base network model (IEC 62541-22:2025)
• Effective Date: 10-Feb-2026
• Referred By: SIST EN IEC 62541-6:2026 - OPC unified architecture - Part 6: Mappings (IEC 62541-6:2025)
• Effective Date: 03-Feb-2026

Overview

oSIST prEN IEC 62541-1:2024 (OPC Unified Architecture - Part 1: Overview and Concepts) presents the high-level concepts and structure of the OPC UA series. This Part is primarily non‑normative and is intended to help readers understand the architecture, terminology and the organization of the remaining parts of the IEC 62541 set. It summarizes design goals, integrated models and services, and key system concepts such as sessions, the AddressSpace model, and the security model.

Why it matters: OPC UA provides a vendor-neutral framework for interoperable information exchange in industrial automation and IT applications in industry. Part 1 gives architects, integrators and engineers the conceptual foundation to read and apply the normative parts of the specification.

Key Topics

The document outlines the core topics covered across the OPC UA family. Major headings include:

• Structure of the OPC UA series - explanation of the parts and suggested reading order
• Design goals - principles guiding interoperability and scalability
• Integrated models and services - how information models and services work together
• Security model - overview of authentication, authorization and secure channels
• Integrated AddressSpace model and object model - conceptual view of nodes, objects and references
• Sessions and client/server concepts - client, server applications and sessions
• Publish-Subscribe and redundancy - pub/sub architecture and server-to-server interactions
• Global services - discovery, certificate management, key/credential management, authorization services, device onboarding, Alias Names and Security Key Service (SKS)

Each topic is introduced at a conceptual level to prepare readers for more detailed parts that define models, services, mappings and profiles.

Applications

oSIST prEN IEC 62541-1:2024 is useful for:

• Solution architects selecting OPC UA for industrial interoperability and digital integration
• Developers and integrators designing OPC UA clients, servers and gateways
• Project managers and systems engineers planning secure, scalable automation systems
• Standards committees and certification bodies assessing conformity and interoperability

Practical benefits include a consistent vocabulary, a clear view of how AddressSpace and services interact, and guidance on security, discovery and deployment patterns used in industrial automation systems.

Related Standards

Part 1 references and orients readers to the other parts of the OPC UA series, including (selection):

• IEC 62541-2 (Security)
• IEC 62541-3 (Address Space Model)
• IEC 62541-4 (Services)
• IEC 62541-5 (Information Model)
• IEC 62541-6 (Mappings)
• IEC 62541-7 (Profiles)
• IEC 62541-8..21 (Data Access, Alarms & Conditions, Programs, Historical Access, Discovery & Global Services, Aggregates, PubSub, Safety, State Machines, Alias Names, Role-Based Security, Dictionary References, File Transfer, Device Onboarding)

For implementation or compliance work, readers should proceed to the normative parts listed above after studying this overview.

Standards Content (Sample)

SLOVENSKI STANDARD
01-maj-2026
Enotna arhitektura OPC - 1. del: Pregled in koncepti (IEC 62541-1:2025)
OPC unified architecture - Part 1: Overview and concepts (IEC 62541-1:2025)
OPC Unified Architecture – Teil 1: Übersicht und Konzepte (IEC 62541-1:2025)
Architecture unifiée OPC - Partie 1: Vue d'ensemble et concepts (IEC 62541-1:2025)
Ta slovenski standard je istoveten z: EN IEC 62541-1:2026
ICS:
25.040.40 Merjenje in krmiljenje Industrial process
industrijskih postopkov measurement and control
35.240.50 Uporabniške rešitve IT v IT applications in industry
industriji
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 62541-1

NORME EUROPÉENNE
EUROPÄISCHE NORM February 2026
ICS 25.040
English Version
OPC unified architecture - Part 1: Overview and concepts
(IEC 62541-1:2025)
Architecture unifiée OPC - Partie 1: Vue d'ensemble et OPC Unified Architecture - Teil 1: Übersicht und Konzepte
concepts (IEC 62541-1:2025)
(IEC 62541-1:2025)
This European Standard was approved by CENELEC on 2026-01-23. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 62541-1:2026 E

European foreword
The text of document 65E/1039/CDV, future edition 1 of IEC 62541-1, prepared by SC 65E "Devices
and integration in enterprise systems" of IEC/TC 65 "Industrial-process measurement, control and
automation" was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2027-02-28
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2029-02-28
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 62541-1:2025 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standard indicated:
IEC 61508 (series) NOTE Approved as EN 61508 (series)
IEC 61784-3:2021 NOTE Approved as EN IEC 61784-3:2021 (not modified)
IEC 62541-3 NOTE Approved as EN IEC 62541-3
IEC 62541-4 NOTE Approved as EN IEC 62541-4
IEC 62541-5 NOTE Approved as EN IEC 62541-5
IEC 62541-6 NOTE Approved as EN IEC 62541-6
IEC 62541-7 NOTE Approved as EN IEC 62541-7
IEC 62541-8 NOTE Approved as EN IEC 62541-8
IEC 62541-9 NOTE Approved as EN IEC 62541-9
IEC 62541-10 NOTE Approved as EN IEC 62541-10
IEC 62541-11 NOTE Approved as EN IEC 62541-11
IEC 62541-12 NOTE Approved as EN IEC 62541-12
IEC 62541-13 NOTE Approved as EN IEC 62541-13
IEC 62541-14 NOTE Approved as EN IEC 62541-14
IEC 62541-15 NOTE Approved as EN IEC 62541-15
IEC 62541-16 NOTE Approved as EN IEC 62541-16 to be published
IEC 62541-17 NOTE Approved as prEN IEC 62541-17 to be published
IEC 62541-18 NOTE Approved as EN IEC 62541-18
IEC 62541-19 NOTE Approved as EN IEC 62541-19 to be published
IEC 62541-20 NOTE Approved as prEN IEC 62541-20 to be published
IEC 62541-21 NOTE Approved as prEN IEC 62541-21 to be published
IEC 62541-22 NOTE Approved as prEN IEC 62541-22 to be published
IEC 62541-23 NOTE Approved as prEN IEC 62541-23 to be published
IEC 62541-24 NOTE Approved as prEN IEC 62541-24 to be published

IEC 62541-1 ®
Edition 1.0 2025-12
INTERNATIONAL
STANDARD
OPC unified architecture -
Part 1: Overview and concepts
ICS 25.040  ISBN 978-2-8327-0828-6

IEC 62541-1:2025-12(en)
IEC 62541-1:2025 © IEC 2025
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms, definitions and abbreviated terms . 5
3.1 Terms and definitions. 5
3.2 Abbreviated terms . 9
4 Structure of the IEC 62541 series . 10
4.1 Series organization . 10
4.2 IEC 62541 series parts . 10
5 Overview . 12
5.1 Scope . 12
5.2 General . 12
5.3 Design goals . 12
5.4 Integrated models and services . 14
5.4.1 Security model . 14
5.4.2 Integrated AddressSpace model . 15
5.4.3 Integrated object model . 16
5.4.4 Integrated services . 16
5.5 Sessions . 16
6 Systems concepts . 17
6.1 Client Server Overview . 17
6.2 OPC UA Clients . 17
6.3 OPC UA Servers . 18
6.3.1 General . 18
6.3.2 Real objects . 18
6.3.3 Server application . 18
6.3.4 OPC UA AddressSpace . 19
6.3.5 Subscription entities . 19
6.3.6 OPC UA Service interface . 20
6.3.7 Server to Server interactions . 20
6.4 Redundancy . 21
6.5 Publish-Subscribe . 21
6.6 Synergy of models . 22
6.7 Global Services . 23
6.7.1 General . 23
6.7.2 Discovery Services . 23
6.7.3 Certificate management . 24
6.7.4 KeyCredential management . 24
6.7.5 Authorization services . 24
6.7.6 Device Onboarding . 24
6.7.7 Alias Names . 24
6.7.8 Security Key Service (SKS) . 24
7 Client/Server Service Sets . 25
7.1 General . 25
7.2 Discovery Service Set . 25
7.3 SecureChannel Service Set . 25
IEC 62541-1:2025 © IEC 2025
7.4 Session Service Set . 26
7.5 NodeManagement Service Set . 26
7.6 View Service Set . 26
7.7 Query Service Set . 26
7.8 Attribute Service Set . 26
7.9 Method Service Set . 27
7.10 MonitoredItem Service Set . 27
7.11 Subscription Service Set . 27
Bibliography . 29

Figure 1 – OPC UA target applications. 13
Figure 2 – OPC UA system architecture . 17
Figure 3 – OPC UA Client architecture . 17
Figure 4 – OPC UA Server architecture . 18
Figure 5 – Peer-to-peer interactions between Servers . 20
Figure 6 – Chained Server example . 21
Figure 7 – Integrated Client Server and PubSub models . 23
Figure 8 – SecureChannel and Session Services . 26

IEC 62541-1:2025 © IEC 2025
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPC unified architecture -
Part 1: Overview and concepts
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
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the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 62541-1 has been prepared by subcommittee 65E: Devices and integration in enterprise
systems, of IEC technical committee 65: Industrial-process measurement, control and
automation. It is an International Standard.
This first edition cancels and replaces IEC TR 62541-1 published in 2020.

The text of this International Standard is based on the following documents:
Draft Report on voting
65E/1039/CDV 65E/1093/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
IEC 62541-1:2025 © IEC 2025
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
Throughout this document and the other Parts of the series, certain document conventions are
used:
Italics are used to denote a defined term or definition that appears in the "Terms and definitions"
clause in one of the parts of the series.
Italics are also used to denote the name of a service input or output parameter or the name of
a structure or element of a structure that are usually defined in tables.
The italicized terms and names are also often written in camel-case (the practice of writing
compound words or phrases in which the elements are joined without spaces, with each
element's initial letter capitalized within the compound). For example, the defined term is
AddressSpace instead of Address Space. This makes it easier to understand that there is a
single definition for AddressSpace, not separate definitions for Address and Space.
A list of all parts in the IEC 62541 series, published under the general title OPC Unified
Architecture, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
IEC 62541-1:2025 © IEC 2025
1 Scope
This part of IEC 62541 presents the concepts and overview of the OPC Unified Architecture
(OPC UA). Reading this document is helpful to understand the remaining parts of the IEC 62541
series. Each of the other parts is briefly explained along with a suggested reading order.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
3.1.1
AddressSpace
collection of information that a Server makes visible to its Clients
Note 1 to entry: See IEC 62541-3 for a description of the contents and structure of the Server AddressSpace.
3.1.2
Aggregate
function that calculates derived values from Raw data
Note 1 to entry: Raw data may be from a historian or buffered real time data. Common Aggregates include averages
over a given time range, minimum over a time range and maximum over a time range.
3.1.3
Alarm
type of Event associated with a state condition that typically requires acknowledgement
Note 1 to entry: See IEC 62541-9 for a description of Alarms.
3.1.4
Attribute
primitive characteristic of a Node
Note 1 to entry: All Attributes are defined by OPC UA and may not be defined by Clients or Servers. Attributes are
the only elements in the AddressSpace permitted to have data values.
3.1.5
Broker
intermediary program module that routes NetworkMessages from Publishers to Subscribers
Note 1 to entry: Brokers are building blocks of Message Oriented Middleware.
3.1.6
Certificate
digitally signed data structure that contains a public key and an identity
Note 1 to entry: Certificates are used to identity for example Clients, Servers, users, and certificate authorities.
IEC 62541-1:2025 © IEC 2025
3.1.7
Client
software application that sends Messages to OPC UA Servers conforming to the Services
specified in this set of specifications
3.1.8
Condition
generic term that is an extension to an Event
Note 1 to entry: A Condition represents the conditions of a system or one of its components and always exists in
some state.
3.1.9
Communication Stack
layered set of software modules between the application and the hardware that provides various
functions to encode, encrypt and format a Message for sending, and to decode, decrypt and
unpack a Message that was received
3.1.10
Complex Data
data that is composed of elements of more than one primitive data type, such as a structure
3.1.11
DataSet
list of named data values
Note 1 to entry: A DataSet typically consists of Event fields or Variable values.
3.1.12
DataSetMessage
payload of a NetworkMessage created from a DataSet
Note 1 to entry: The DataSetMessage is an immutable payload of the NetworkMessage handed off to the Message
Oriented Middleware (transport layer) for delivery by the Publisher. The Subscriber receives the DataSetMessage as
the payload of a NetworkMessage from the Publisher with additional headers that may be supplied by the Message
Oriented Middleware along the way.
3.1.13
Discovery
process by which Client obtains information about Servers, including endpoint and security
information
3.1.14
Event
generic term used to describe an occurrence of some significance within a system or system
component
3.1.15
EventNotifier
special Attribute of a Node that signifies that a Client may subscribe to that particular Node to
receive Notifications of Event occurrences
3.1.16
Information Model
organizational framework that defines, characterizes, and relates information resources of a
given system or set of systems.
Note 1 to entry: The core AddressSpace model supports the representation of Information Models in the
AddressSpace. See IEC 62541-5 for a description of the base OPC UA Information Model.
IEC 62541-1:2025 © IEC 2025
3.1.17
Message
data unit conveyed between Client and Server that represents a specific Service request or
response
3.1.18
Message Oriented Middleware
infrastructure supporting sending and receiving NetworkMessages between distributed systems
Note 1 to entry: An OPC UA Application may support different types of Message Oriented Middleware
infrastructures and protocols like AMQP, MQTT, or UDP with IP multicast. Other types like DDS or XMPP can also
be integrated into the OPC UA PubSub model.
3.1.19
Method
callable software function that is a component of an Object
3.1.20
MonitoredItem
Client-defined entity in the Server used to monitor Attributes or EventNotifiers for new values
or Event occurrences and that generates Notifications for them
3.1.21
NetworkMessage
DataSetMessages and header to facilitate delivery, routing, security, and filtering
Note 1 to entry: The Publisher hands off the NetworkMessage to the Message Oriented Middleware (transport layer)
to deliver DataSetMessages to the Subscribers.
Note 2 to entry: The term message is used with various connotations in the messaging world. The Publisher can
like to think of the message as an immutable payload handed off to the Message Oriented Middleware for delivery.
The Subscriber often thinks of the message as not only that immutable payload from the sender, but also various
annotations supplied by the Message Oriented Middleware along the way. To avoid confusion the term
DataSetMessage is used to mean the message as supplied by the Publisher for a DataSet and the term
NetworkMessage is used to mean the DataSetMessage plus sections for annotation at the head and tail of the
DataSetMessage.
3.1.22
Node
fundamental component of an AddressSpace
3.1.23
NodeClass
class of a Node in an AddressSpace
Note 1 to entry: NodeClasses define the metadata for the components of the OPC UA object model. They also
define constructs, such as Views, that are used to organize the AddressSpace.
3.1.24
Notification
generic term for data that announces the detection of an Event or of a changed Attribute value;
Notifications are sent in NotificationMessages.
3.1.25
NotificationMessage
Message published from a Subscription that contains one or more Notifications
IEC 62541-1:2025 © IEC 2025
3.1.26
Object
Node that represents a physical or abstract element of a system
Note 1 to entry: Objects are modelled using the OPC UA Object Model. Systems, subsystems, and devices are
examples of Objects. An Object may be defined as an instance of an ObjectType.
3.1.27
Object Instance
synonym for Object
Note 1 to entry: Not all Objects are defined by ObjectTypes.
3.1.28
ObjectType
Node that represents the type definition for an Object
3.1.29
OPC UA Application
Client, which calls OPC UA Services, or a Server, which performs those Services, or an OPC
UA Publisher or an OPC UA Subscriber.
3.1.30
Publisher
entity sending NetworkMessages to a Message Oriented Middleware
Note 1 to entry: A Publisher can be a native OPC UA Application or an application that only has knowledge about
the Message Oriented Middleware and the rules for encoding the NetworkMessages and DataSetMessages.
3.1.31
PubSub
OPC UA variant of the publish subscribe messaging pattern
3.1.32
Profile
specific set of capabilities to which a Server may claim conformance
Note 1 to entry: Each Server may claim conformance to more than one Profile
Note 2 to entry: The set of capabilities are defined in IEC 62541-7
3.1.33
Program
executable Object that, when invoked, immediately returns a response to indicate that execution
has started, and then returns intermediate and final results through Subscriptions identified by
the Client during invocation
3.1.34
Reference
explicit relationship (a named pointer) from one Node to another
Note 1 to entry: The Node that contains the Reference is the source Node, and the referenced Node is the target
Node. All References are defined by ReferenceTypes.
3.1.35
ReferenceType
Node that represents the type definition of a Reference
Note 1 to entry: The ReferenceType specifies the semantics of a Reference. The name of a ReferenceType
identifies how source Nodes are related to target Nodes and generally reflects an operation between the two, such
as "A contains B".
IEC 62541-1:2025 © IEC 2025
3.1.36
Server
software application that implements and exposes the Services specified in this set of
specifications
3.1.37
Service
Client-callable operation in a Server
Note 1 to entry: Services are defined in IEC 62541-4. A Service is similar to a method call in a programming
language or an operation in a Web services WSDL contract.
3.1.38
Service Set
group of related Services
3.1.39
Session
logical long-running connection between a Client and a Server.
Note 1 to entry: A Session maintains state information between Service calls from the Client to the Server.
3.1.40
Subscriber
entity receiving DataSetMessages from a Message Oriented Middleware
Note 1 to entry: A Subscriber can be a native OPC UA Application or an application that has just knowledge about
the Message Oriented Middleware and the rules for decoding the NetworkMessages and DataSetMessages. A
Subscription in the OPC UA Client Server model has a different meaning than the Subscriber in the PubSub model.
3.1.41
Subscription
Client-defined endpoint in the Server, used to return Notifications to the Client
Note 1 to entry: Subscription is a generic term that describes a set of Nodes selected by the Client (1) that the
Server periodically monitors for the existence of some condition, and (2) for which the Server sends Notifications to
the Client when the condition is detected.
3.1.42
Underlying System
hardware or software platforms that exist as an independent entity. UA Applications are
dependent on an entity's existence in order to perform UA services. However, the entity is not
dependent on UA Applications.
Note 1 to entry: Hardware and software platforms include physical hardware, firmware, operating system,
networking, non-UA applications, as well as other UA Applications. A Distributed Control System, PLC/Device, and
UA Server are examples of an Underlying System.
3.1.43
Variable
Node that contains a value
3.1.44
View
specific subset of the AddressSpace that is of interest to the Client.
3.2 Abbreviated terms
A&E alarms and events
AMQP advanced message queuing protocol
API application programming interface
IEC 62541-1:2025 © IEC 2025
COM component object model
DA data access
DCS distributed control system
DDS data distribution service
HDA historical data access
HMI human-machine interface
JSON JavaScript object notation
LDAP lightweight directory access protocol
MES manufacturing execution system
MQTT message queue telemetry transport
OAuth2 open authorization
OPC open platform communications
PLC programmable logic controller
SCADA supervisory control and data acquisition
UA unified architecture
UADP UA datagram protocol
WSDL web services definition language
XML extensible markup language
XMPP extensible messaging and presence protocol
4 Structure of the IEC 62541 series
4.1 Series organization
This document is organized as a multi-part standard. Parts 1 through 5 describe core concepts
of OPC UA, therefore, readers are encouraged to read Parts 1 through 5 of the series before
reading the other Parts.
4.2 IEC 62541 series parts
At the time of publication of this document, the IEC 62541 series is composed of the following
parts:
– Part 1 (IEC 62541-1) – Overview and concepts
Part 1 (this part) presents the concepts and overview of OPC UA.
– Part 2 (IEC 62541-2) – Security Model
Part 2 describes the model for securing interactions between OPC UA Applications.
– Part 3 (IEC 62541-3) – Address Space Model
Part 3 describes the contents and structure of the Server's AddressSpace.
– Part 4 (IEC 62541-4) – Services
Part 4 specifies the Services provided by Servers.
– Part 5 (IEC 62541-5) – Information Model
Part 5 specifies the types and their relationships defined for Servers.
– Part 6 (IEC 62541-6) – Mappings
Part 6 specifies the mappings to transport protocols and data encodings supported by
OPC UA.
IEC 62541-1:2025 © IEC 2025
– Part 7 (IEC 62541-7) – Profiles
Part 7 specifies the Profiles that are available for OPC UA Applications. These Profiles
provide groupings of functionality that can be used for conformance level certification. OPC
UA Applications will be tested against the Profiles.
– Part 8 (IEC 62541-8) – Data Access
Part 8 specifies the use of OPC UA for data access.
– Part 9 (IEC 62541-9) – Alarms and Conditions
Part 9 specifies use of OPC UA support for access to Alarms and Conditions. The base
system includes support for simple Events; this specification extends that support to include
support for Alarms and Conditions.
– Part 10 (IEC 62541-10) – Programs
Part 10 specifies OPC UA support for access to Programs.
– Part 11 (IEC 62541-11) – Historical Access
Part 11 specifies use of OPC UA for historical access. This access includes both historical
data and historical Events.
– Part 12 (IEC 62541-12) – Discovery and Global Services
Part 12 specifies how Discovery Servers operate in different scenarios and describes how
UA Clients and Servers should interact with them. It also defines Information Models for
Certificate management, key credential management and authorization services.
– Part 13 (IEC 62541-13) – Aggregates
Part 13 specifies how to compute and return aggregates like minimum, maximum, average
etc. Aggregates can be used with current and historical data.
– Part 14 (IEC 62541-14) – PubSub
Part 14 specifies the OPC Unified Architecture (OPC UA) PubSub communication model.
The PubSub communication model defines an OPC UA publish subscribe pattern in addition
to the Client Server pattern defined by the Services in IEC 62541-4.
– Part 15 (IEC 62541-15) – Safety
Part 15 extends OPC UA to fulfil the requirements of functional safety as defined in the
IEC 61508 series of standards and in IEC 61784-3.
– Part 16 (IEC 62541-16) – State Machines
Part 16 specifies the basic infrastructure to model state machines.
– Part 17 (IEC 62541-17) – Alias Names
Part 17 specifies a manner of configuring and exposing an alternate well-defined name for
any OPC UA Node in a Server or system.
– Part 18 (IEC 62541-18) – Role-Based Security
Part 18 specifies the basic infrastructure to model role-based access control (RBAC)
– Part 19 (IEC 62541-19) – Dictionary References
Part 19 specifies the basic infrastructure to reference from an OPC UA Information Model
to external dictionaries like IEC Common Data Dictionary or ECLASS.
– Part 20 (IEC 62541-20) – File Transfer
Part 20 specifies the basic infrastructure to model file transfers and file systems.
– Part 21 (IEC 62541-21) – Device Onboarding
Part 21 specifies the life cycle of Devices and Composites and mechanisms to verify their
authenticity, set up their security and maintain their configuration.
– Part 22 (IEC 62541-22) – Base Network Model
Part 22 specifies an OPC UA Information Model for a basic set of network related
components to be used in more specific Information Models.
IEC 62541-1:2025 © IEC 2025
– Part 23 (IEC 62541-23) – Common ReferenceTypes
Part 23 specifies common types of references between Nodes.
– Part 24 (IEC 62541-24) – Scheduler
Part 24 specifies an OPC UA information model to expose and configure the dates and times
specific actions are executed by the OPC UA Server.
5 Overview
5.1 Scope
The IEC 62541 series is applicable to components in all industrial domains, such as industrial
sensors and actuators, control systems, Manufacturing Execution Systems and Enterprise
Resource Planning Systems, including the Industrial Internet of Things (IIoT), Machine To
Machine (M2M) and others. These systems are intended to exchange information and to use
command and control for industrial processes. The IEC 62541 series defines a common
infrastructure model to facilitate this information exchange. The IEC 62541 series specifies the
following:
• the information model to represent structure, behaviour and semantics;
• the message model to interact between applications;
• the communication model to transfer the data between end-points;
• the conformance model to guarantee interoperability between systems.
5.2 General
OPC UA is a platform-independent standard through which various kinds of systems and
devices can communicate by sending request and response Messages between Clients and
Servers or NetworkMessages between Publishers and Subscribers over various types of
networks. It supports robust, secure communication that assures the identity of OPC UA
Applications and resists attacks.
In the Client Server model, OPC UA defines sets of Services that Servers can provide, and
individual Servers specify to Clients what Service sets they support. Information is conveyed
using OPC UA-defined and vendor-defined data types, and Servers define object models that
Clients can dynamically discover. Servers can provide access to both current and historical
data, as well as Alarms and Events to notify Clients of important changes. OPC UA can be
mapped onto a variety of communication protocols and data can be encoded in various ways to
trade off portability and efficiency.
In addition to the Client Server model, the IEC 62541 series defines a mechanism for Publishers
to transfer the information to Subscribers using the PubSub model.
5.3 Design goals
OPC UA provides a consistent, integrated AddressSpace and service model. This allows a
single Server to integrate data, Alarms and Events, and history into its AddressSpace, and to
provide access to them using an integrated set of Services. These Services also include an
integrated security model.
OPC UA also allows Servers to provide Clients with type definitions for the Objects accessed
from the AddressSpace. This allows Information Models to be used to describe the contents of
the AddressSpace. OPC UA allows data to be exposed in many different formats, including
binary structures and XML or JSON documents. The format of the data can be defined by OPC,
other standard organizations or vendors. Through the AddressSpace, Clients can query the
Server for the metadata that describes the format for the data. In many cases, Clients with no
pre-programmed knowledge of the data formats will be able to determine the formats at runtime
and properly utilize the data.
IEC 62541-1:2025 © IEC 2025
OPC UA adds support for many relationships between Nodes instead of being limited to just a
single hierarchy. In this way, a Server can present data in a variety of hierarchies tailored to
the way a set of Clients would typically like to view the data. This flexibility, combined with
support for type definitions, makes OPC UA applicable to a wide array of problem domains. As
illustrated in Figure 1, OPC UA is not targeted at just the SCADA, PLC and DCS interface, but
also as a way to provide greater interoperability between higher level functions.

Figure 1 – OPC UA target applications
OPC UA is designed to provide robustness of published data. A major feature of all OPC servers
is the ability to publish data and Event Notifications. OPC UA provides mechanisms for Clients
to quickly detect and recover from communication failures associated with these transfers
without having to wait for long timeouts provided by the underlying protocols.
OPC UA is designed to support a wide range of Servers, from plant floor PLCs to enterprise
Servers. These Servers are characterized by a broad scope of size, performance, execution
platforms and functional capabilities. Therefore, OPC UA defines a comprehensive set of
capabilities, and Servers can implement a subset of these capabilities. To promote
interoperability, OPC UA defines subsets, referred to as Profiles, to which Servers can claim
conformance. Clients can then discover the Profiles of a Server, and tailor their interactions
with that Server based on the Profiles. Profiles are defined in IEC 62541-7.
The OPC UA is layered to isolate the core design from the underlying computing technology
and network transport. This allows OPC UA to be mapped to future technologies as necessary,
without negating the basic design. Mappings and data encodings are described in IEC 62541-6.
Several data encodings are defined:
• XML/text,
• UA Binary,
• JSON.
IEC 62541-1:2025 © IEC 2025
In addition, several protocols are defined:
• OPC UA TCP,
• HTTPS,
• WebSockets.
OPC UA Applications that support multiple transports and encodings will allow the end users to
make decisions about trade-offs between performance and compatibility at the time of
deployment, rather than having these trade-offs determined by the OPC vendor at the time of
product definition.
OPC UA is designed as the migration path for OPC clients and servers that are based on
Microsoft COM technology (OPC Classic). Care has been taken in the design of OPC UA so
that existing data exposed by OPC COM servers (DA, HDA and A&E) can easily be mapped
and exposed via OPC UA. Vendors can choose migrating their products natively to OPC UA or
use external wrappers to convert from OPC COM to OPC UA and vice-versa. Each of the
specifications of OPC Classic developed by OPC Foundation defined its own address space
model and its own set of Services. OPC UA unifies the previous models into a single integrated
address space with a single set of Services.
OPC UA PubSub opens new application fields for OPC UA. The following are some example
uses for PubSub:
• Configurable peer to peer communication between controllers and between controllers and
HMIs. The peers are not directly connected and do not even need to know about the
existence of each other. The data exchange often requires a fixed time-window; it can be
point-to-point or a multi-point connection.
• Asynchronous workflows. For example, an order processing application can place an order
on a message queue or an enterprise service bus. From there it can be processed by one
or more workers.
• Logging to multiple systems; for example, sensors or actuators can write logs to a monitoring
system, an HMI, an archive application for later querying, and so on.
• Servers representing services or devices can stream data to applications hosted in the
cloud. For example, backend servers, big data analytics for system optimization and
predictive maintenance.
PubSub is not bound to a particular messaging system. Rather it can be mapped to various
different systems as illustrated with two examples:
• PubSub with UDP is well-suited in production environments for frequent transmissions of
small amounts of data. It also allows data exchange in one-to-one and one-to-many
configurations.
• The use of established messaging protocols (e.g. the ISO/IEC AMQP 1.0 protocol or the
MQTT 5.0 protocol) with JSON data encoding supports the cloud integration path and readily
allows handling of the information in modern stream and batch analytics systems.
5.4 Integrated models and services
5.4.1 Security model
5.4.1.1 General
OPC UA security is concerned with the authentication of Clients and Servers, the authentication
of users, the integrity and confidentiality of their communications, and the verifiability of claims
of functionality. It does not specify the circumstances under which various security mechanisms
are required. That specification is crucial, but it is made by the designers of the system at a
given site and can be specified by other standards.
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