OPC Unified Architecture

OPC Unified Architecture (OPC UA) is a machine to machine communication protocol for industrial automation developed by the OPC Foundation. Distinguishing characteristics are:

  • Focus on communicating with industrial equipment and systems for data collection and control
  • Open - freely available and implementable under GPL 2.0 license [1]
  • Cross-platform - not tied to one operating system or programming language
  • Service-oriented architecture (SOA)
  • Inherent complexity - the specification consists of 1250 pages in 14 documents
  • Robust security
  • Integral information model, which is the foundation of the infrastructure necessary for information integration where vendors and organizations can model their complex data into an OPC UA namespace to take advantage of the rich service-oriented architecture of OPC UA. There are over 35 collaborations with the OPC Foundation currently. Key industries include pharmaceutical, oil and gas, building automation, industrial robotics, security, manufacturing and process control.

History

Although developed by the same organization, OPC UA differs significantly from its predecessor, Open Platform Communications (OPC). The Foundation's goal for OPC UA was to provide a path forward from the original OPC communications model (namely the Microsoft Windows-only process exchange COM/DCOM) that would better meet the emerging needs of industrial automation.[2]

After more than three years of specification work and another year for a prototype implementation, the first version of the Unified Architecture was released in 2006.

The current version of the specification is on 1.04 (22 Nov 2017[3]). The new version of OPC UA now has added publish/subscribe in addition to the client/server communications infrastructure.

Innovations

Although the original binding to COM/DCOM helped OPC to distribute well, it had several drawbacks:

  • Frequent configuration issues with DCOM;
  • No configurable time-outs;
  • Microsoft Windows only;
  • Lower security;
  • No control over DCOM (COM/DCOM is kind of a black box, developers have no access to sources and therefore have to deal with bugs or insufficient implementations).

These drawbacks along with a number of other considerations pushed the decision to develop a new and independent stack for OPC UA, which replaces COM/DCOM. The main characteristics of this communication stack were:

  • Multi-platform implementation, including portable ANSI C, Java and .NET implementations;
  • Scalability: from smart sensors and smart actuators to mainframes;
  • Multi-threaded, as well as single-threaded/single-task operation—necessary for porting the stack to embedded devices;
  • Security, based on new standards;
  • Configurable time-outs for each service;
  • Chunking of big datagrams.

This communication stack reflects the beginning of various innovations. The OPC UA architecture is a service-oriented architecture (SOA) and is based on different logical levels.

OPC Base Services are abstract method descriptions, which are protocol independent and provide the basis for OPC UA functionality. The transport layer puts these methods into a protocol, which means it serializes/deserializes the data and transmits it over the network. Two protocols are specified for this purpose. One is a binary TCP protocol, optimized for high performance and the second is Web service-oriented.

The OPC information model is a so-called Full Mesh Network based on nodes. These nodes can include any kind of meta information, and are similar to the objects of object-oriented programming (OOP). A node can have attributes for read access (DA, HDA), methods that can be called (Commands), and triggered events that can be transmitted (AE, DataAccess, DataChange). Nodes hold process data as well all other types of metadata. The OPC namespace contains the type model.

Client software can verify what Profiles a server supports. This is necessary to obtain information, if a server only supports DA functionality or additionally AE, HDA, etc. Additionally, information can be obtained about whether a server supports a given profile. New and important features of OPC UA are:

  • Redundancy support
  • Heartbeat for connections in both directions (to indicate whether the other end is "alive"). This means that both server and client recognize interrupts.
  • Buffering of data and acknowledgements of transmitted data. Lost connections don't lead to lost data anymore. Lost datagrams can be refetched.

At the OPC UA DevCon in October 2006 in Munich the first prototypes were presented live. Various UA Servers have been shown on a Beckhoff programmable logic controller and an embedded test board from Euros. The Beckhoff PLC is based on Windows XP Embedded and the embedded controller is based on the real-time operating system Euros. The company Embedded Labs Ltd demonstrated an OPC-UA Server based on their own C++ UA Stack executing on a single chip ARM microcontroller with 64kB RAM. In October 2012 the German Fraunhofer-Application Center IOSB-INA and the Institute for industrial Information Technologies (inIT) shows, that an OPC-UA server is scalable downto 15 kB RAM and 10 kB ROM and therefore usable at chip level.[4]

Protocols

OPC UA supports two protocols.[5] This is visible to application programmers only via changes to the URL. The binary protocol is opc.tcp://Server and http://Server is for Web Service. Otherwise OPC UA works completely transparent to the API.

The binary protocol offers the best performance/least overhead, takes minimum resources (no XML Parser, SOAP and HTTP required, which is important for embedded devices), offers best interoperability (binary is explicitly specified and allows fewer degrees of freedom during implementation) and uses a single arbitrarily choosable TCP port for communication easing tunneling or easy enablement through a firewall.

The Web Service (SOAP) protocol is best supported from available tools, e.g., from Java or .NET environments, and is firewall-friendly, using standard HTTP(S) ports.

Binary is supported by all implementations, while only .NET implementation supports SOAP.

Specifications

The OPC UA specification is a multi-part specification and consists of the following parts:

  1. Concepts
  2. Security Model
  3. Address Space Model
  4. Services
  5. Information Model
  6. Mappings
  7. Profiles
  8. Data Access
  9. Alarms and Conditions
  10. Programs
  11. Historical Access
  12. Discovery
  13. Aggregates
  14. PubSub

In contrast to the COM-based specifications, the UA specifications are not pure application specifications. They describe typically UA internal mechanisms, which get handled through the communication stack and are normally only of interest for those that port a stack to a specific target or those that want to implement their own UA stack.

The OPC UA application developers code against the OPC UA API and therefore mainly use API documentation. Nevertheless, part 3, 4, and 5 may be of interest for application developers.[6]

Discussion

The OPC-UA protocol specification consists of 14 documents for a total of 1250 pages. Due to this complexity, existing implementations are usually incomplete. In addition, the existence of several serialization formats, as well as the possibility of selectively implementing certain services such as PubSub, eventually lead to a great heterogeneity of the OPC-UA connection points. Under these conditions, it is finally difficult to develop client applications that are independent of the specific implementation of each server. In this sense, OPC-UA does not achieve its promise of ensuring good interoperability of systems. This can be seen typically in factory and infrastructure projects integrating various PLC technologies, each delivered with a different and limited implementation of the OPC-UA protocol.

The specification is still evolving, the last specification document volume 14 is dated February 6, 2018, while the first publication of the standard OPC-UA dates from 2006.

As a result, despite considerable marketing efforts to support its adoption, OPC-UA may be considered at this stage as a standardization attempt rather than an established standard.

UA communication stack

The architecture of a UA application, independent of whether it is the server or client part, is structured into levels.

Some parts equalize to the former COM Proxy/Stubs and get provided by the OPC Foundation. The portability level is new; it simplifies porting the UA ANSI C stack also to other target platforms. A port layer for Windows and Linux gets also provided by the OPC Foundation.

UA security

UA Security consists of authentication and authorization, encryption and data integrity via signatures. For Web Services the WS-SecureConversation gets used and is therefore compatible to .NET and other SOAP implementations. For the binary variant, the algorithms of WS-SecureConversation have been followed and also converted to a binary equivalent. This is named as UA Secure Conversation.

There is also a mixed version where the code is binary, but the transport layer is SOAP. This is a compromise between efficient binary coding and firewall-friendly transmission. Binary coding always requires UA Secure Conversation. The authentication uses X.509 certificates exclusively. It relies on the application developer to choose which certificate store the UA application gets bound to. For instance, it is possible to use the public key infrastructure (PKI) of an Active Directory.

OPC UA APIs

UA APIs are available in several programming languages. Commercial SDK are available for C, C++, Java, and .NET. Open-source stacks are available at least for C, C++, Java, Javascript(node) and Python .

C++ Implementation

  • The open62541 project provides an Open Source implementation for OPC UA server and clients and is licensed under the Mozilla Public License v2.0. Besides Linux and Windows, it also supports OS X, QNX and different embedded systems as compilation target.
  • The ASNeG project provides a C++ open source (Apache License 2.0) OPC UA Application Server and an OPC UA Web Server (beta state, currently only base functions).[7]
  • The FreeOpcUa project provides an open source (LGPL) server and client implementation in C++.
  • The UAF project offers an open source (LGPL) C++/Python implementation .

.NET implementation

The .NET implementation uses ANSI C for the lower levels and implements the rest natively in .NET. That means only the handling of the socket and the Message-Chunking gets integrated from the ANSI C stack. De-serialization takes place directly in .NET and therefore gets converted directly into .NET structures and objects. This provides better performance than de-serializing into a C structure first and then copying the data to a .NET structure afterwards.

Java implementation

Various stacks for Java were being developed. Similar to .NET, there are principally three variants:

  1. Encapsulate the complete ANSI C stack via JNI, which complicates portability. Although the stack can be ported to different operating systems, it needs to get compiled for those individually. Also, the data needs to get copied to the JNI boundary, but benefits from the performance of C during de-serialization.
  2. Code directly on the network layer (similar to the current .Net implementation) and de-serialize in Java. This saves one data copy execution, but still depends on the C stack.
  3. Write a native Java OPC UA stack. This was observed to be the most portable, but estimated to take the most engineering effort to implement. The Eclipse Milo project provides a pure-Java, open-source, implementation of the UA 1.03 client and server specification.[8]

Alternatively, there is the simple variant to only support the WebService protocol. For that, a SOAP Toolkit that supports WS-Security is needed.

Python implementation

The FreeOpcUa project provides an implementation entirely in the Python programming language (compatible with Python 2, 3 and pypy) and provides high-level abstractions of an OPC-UA client and server which can be used as is or readily extended for custom applications.

IEC 62541

IEC 62541[9] is a standard for OPC Unified Architecture.

IEC 62541 Overview
ID release date title
IEC/TR 62541-1 2016 OPC Unified Architecture - Part 1: Overview and Concepts
IEC/TR 62541-2 2016 OPC Unified Architecture - Part 2: Security Model
IEC 62541-3 2015 OPC Unified Architecture - Part 3: Address Space Model
IEC 62541-4 2015 OPC Unified Architecture - Part 4: Services
IEC 62541-5 2015 OPC Unified Architecture - Part 5: Information Model
IEC 62541-6 2015 OPC Unified Architecture - Part 6: Mappings
IEC 62541-7 2015 OPC Unified Architecture - Part 7: Profiles
IEC 62541-8 2015 OPC Unified Architecture - Part 8: Data Access
IEC 62541-9 2015 OPC Unified Architecture - Part 9: Alarms and Conditions
IEC 62541-10 2015 OPC Unified Architecture - Part 10: Programs
IEC 62541-11 2015 OPC Unified Architecture - Part 11: Historical Access
IEC 62541-13 2015 OPC Unified Architecture - Part 13: Aggregates
IEC 62541-100 2015 OPC Unified Architecture - Part 100: Device Interface

See also

References

  1. ↑ https://opcfoundation.org/license/gpl.html
  2. ↑ Mahnke, Wolfgang; Leitner, Stefan-Helmut https://library.e.abb.com/public/75d70c47268d78bfc125762d00481f78/56-61%203M903_ENG72dpi.pdf OPC Unified Architecture - The future standard for communication and information modeling in automation], 3/2009 ABB Review 3/2009, page 56-61
  3. ↑ https://opcfoundation.org/developer-tools/specifications-unified-architecture
  4. ↑ The world's smallest OPC-UA server comes from Germany
  5. ↑ Leitner, Stefan-Helmut; Mahnke, Wolfgang OPC UA – Service-oriented Architecture for Industrial Applications, 11/2006 Softwaretechnik-Trends ISSN 0720-8928
  6. ↑ Massaro, Simone What is OPC UA and how does it affect your world?, 5/15/2008 planetengineering.com
  7. ↑ "ASNeG – open source OPC UA Application Server". Retrieved 2015-09-11.
  8. ↑ "OPC Unified Architecture (UA) client and/or server functionality in any JVM-based project". Retrieved 22 Aug 2016.
  9. ↑ "IEC Webstore for IEC 62541". Retrieved 1 June 2018.

Literature

  • Wolfgang Mahnke, Stefan-Helmut Leitner, Matthias Damm: OPC Unified Architecture. Springer Verlag 2009; ISBN 978-3-540-68898-3
  • Lange, J., Iwanitz, F., Burke, T. OPC From Data Access to Unified Architecture 2010; ISBN 978-3-8007-3242-5
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