What is architecture?
Architecture is the coherent set of principles, models, agreements and guidelines that an organisation uses to design, configure and continuously steer its processes, information, applications, infrastructure and industrial systems. It records how the parts relate to each other and which choices apply, so that change does not lead to fragmentation. In the context of OT convergence, architecture connects the office world of IT with the production world of OT: from ERP down to the PLC.
🧠 Why is architecture important?
Without architecture, projects make decisions in isolation. The result is a patchwork of interfaces, duplicated data and systems that nobody fully understands any more. Architecture delivers:
| Benefit | Explanation |
|---|---|
| Coherence | Structure across processes, applications and infrastructure |
| Standardisation | Promotes reuse, simplicity and interchangeability |
| Future-proofing | Supports scalability and technological evolution |
| Governance | Helps with steering, decision-making and compliance |
| Interoperability | Essential for collaboration between supply-chain partners and domains, such as IT and OT |
| Security | Makes zones, interfaces and dependencies visible, the basis of Security by Design |
Architecture prevents chaos and fragmentation: it creates oversight, coherence and decision-making information.
🧩 Which layers make up an enterprise architecture?
A sound enterprise architecture describes the relationships between four classic layers, extended with an OT layer in industrial organisations:
- Business architecture — processes, goals, roles and capabilities
- Information architecture — data flows, terminology and information models
- Application architecture — systems, integrations and interfaces
- Technical architecture — infrastructure, networks, platforms and protocols
- OT architecture — Embedded systems, sensors, field equipment, controllers and Real-time networks
The first four layers align with the architecture domains of TOGAF (business, data, application and technology), which the method works out in phases B, C and D. In modern government organisations and industrial environments these layers are inseparably linked: a change to a production process almost always affects data, applications and the network as well.
🛠️ Which frameworks and modelling languages are used?
Architects rarely start from scratch. They use a method to develop architecture and a language to record it:
| Framework | Type | Essence |
|---|---|---|
| TOGAF | Method and framework (The Open Group) | Architecture Development Method (ADM): Preliminary Phase, phases A to H and continuous requirements management; the current version is the 10th Edition from April 2022 |
| ArchiMate | Modelling language (The Open Group) | Visual notation for strategy, business, application, technology and physical elements; version 3.2 from October 2022 |
| COBIT | Governance framework (ISACA) | Translates business goals through a goals cascade into objectives for IT governance and management |
| NORA | Government reference architecture | Shared principles for the entire Dutch public sector |
ArchiMate has Dutch roots: the language was developed between 2002 and 2004 by a consortium led by the Telematica Instituut and handed over to The Open Group in 2008. As a result, ArchiMate is the dominant notation in the Netherlands, especially in the public sector.
🏛️ Which Dutch reference architectures exist?
| Architecture | Scope | Maintained by |
|---|---|---|
| NORA | Dutch Government Reference Architecture, for the entire public sector | ICTU |
| GEMMA | Municipal Model Architecture, for municipalities | VNG Realisatie |
| WILMA | Water Board Information and Logical Model Architecture, for the water authorities | Het Waterschapshuis |
NORA was developed in 2005 and its first version appeared in September 2006. It is the umbrella framework from which the domain architectures GEMMA and WILMA are derived. These reference architectures provide models, standards and design principles for information flows, data exchange, security and management, and they refer to control frameworks such as the BIO. For water authorities and municipalities that operate pumping stations, treatment plants or traffic installations, they touch OT systems directly.
🏭 What does architecture look like in the OT world?
Formal reference models have existed in OT for a long time, but they were often used separately from enterprise architecture. With the rise of Smart Industry, IIoT and the integration of SCADA with office systems, both worlds need to come together in a single architecture picture. The main OT reference architectures are:
| Model | Origin | Purpose |
|---|---|---|
| Purdue Model | Theodore J. Williams and a consortium around Purdue University, published by ISA in 1992 | Hierarchical levels from field (0) to enterprise; basis for segmentation |
| ISA-95 | ISA, internationally IEC 62264 | Integration of ERP and MES with production through shared models and terminology |
| RAMI 4.0 | Plattform Industrie 4.0, DIN SPEC 91345 (2016) and IEC PAS 63088 (2017) | Three-dimensional model with six layers: asset, integration, communication, information, functional and business |
| NAMUR Open Architecture | NAMUR, process industry | Second data channel alongside core process control, in principle one-way for monitoring and optimisation |
| IT OT Security Reference Architecture | Various frameworks and vendors | Blueprint for zones, IDMZ, access and monitoring between IT and OT |
Within these models, architecture plays a concrete role in security and operations:
| Aspect | Architecture role in OT |
|---|---|
| Security by Design | Embed OT security from the earliest design choices |
| Network segmentation | Define logical separation between zones and layers, following the zones and conduits model |
| Lifecycle Management | Structure the management of embedded devices, firmware and versions over lifetimes that often reach twenty years or more |
| OT–IT integration | Describe and manage interfaces between production and office environments, for example through a Unified Namespace |
| Standardisation | Use ISA-95 for integration and IEC 62443 (originally developed by the ISA-99 committee) for security |
Applying architecture principles in OT makes it possible to integrate IT and OT responsibly, while preserving safety, availability and control.
🎯 What are architecture principles?
Architecture principles are durable starting points that guide design decisions. TOGAF describes every principle with four parts: a name, a statement, a rationale (why) and implications (what it means for the organisation and technology). NORA contains an extensive set of such principles for government. Examples that are common in IT/OT environments:
- Availability and safety come first — changes in OT must never put the physical process at risk
- Capture data once, use it many times — one source per data item, shared through standard interfaces
- No direct connections between the office network and control systems — traffic always passes through an IDMZ
- Open standards over vendor-specific solutions — for example OPC UA for machine integration
- Security is part of the design — in line with Security by Design and Zero Trust
🧭 How do you approach architecture in an IT/OT organisation?
- Define scope and purpose — the whole enterprise, a single site or one domain? Record which decisions the architecture needs to support.
- Map the current state — processes, applications, networks and OT assets, preferably in ArchiMate and structured according to the Purdue Model.
- Formulate principles — a few dozen principles with rationale and implications, agreed with business, IT, OT and security.
- Design the target architecture — including zones and conduits based on a risk assessment in line with IEC 62443-3-2.
- Draw up a roadmap — migration steps that take planned shutdowns and the long lifetime of OT installations into account.
- Embed governance — an architecture board checks projects against the principles, so the architecture does not end up gathering dust.
For organisations that fall under NIS2, this approach also helps you demonstrate that your security measures are appropriate and coherent.
👷 What does an architect do?
An architect translates strategy into coherent design choices and safeguards them during delivery. There are several roles: the enterprise architect works at organisational level, the domain or solution architect at the level of a programme or system, and the OT architect focuses on control systems, industrial networks and their connection to IT. That last role is scarce, because it combines knowledge of process automation, cybersecurity and enterprise architecture.
❓ Frequently asked questions
What is the difference between architecture and design?
Architecture describes the coherence, principles and frameworks for an entire organisation or domain. A design works out one concrete solution within those frameworks, such as a network or an application interface. In short, architecture sets the rules of the game and design plays it.
What is enterprise architecture?
Enterprise architecture is architecture at the level of the whole organisation, describing business, information, applications and technology in relation to each other. Frameworks such as TOGAF and the ArchiMate modelling language are the most widely used tools for it. In industrial companies, OT architecture is also part of the enterprise architecture.
What is a reference architecture?
A reference architecture is a reusable model with principles, building blocks and standards for an entire sector or type of environment. Examples include NORA for the Dutch government, GEMMA for municipalities, the Purdue Model for industrial control and RAMI 4.0 for Industry 4.0. Organisations derive their own architecture from such a reference architecture.
Is the Purdue Model still useful with cloud and IIoT?
The Purdue Model remains useful as a frame of reference for functions and trust levels in industrial environments, even though cloud and IIoT data flows increasingly cross several levels at once. Many organisations therefore combine Purdue with the zones and conduits model of IEC 62443. Architecture records which exceptions to the strict hierarchy are allowed.
What role does ArchiMate play in OT architecture?
Since version 3, ArchiMate includes a physical layer with elements such as equipment, facility and distribution network, so OT assets and physical installations can be modelled too. This lets you describe IT and OT architecture in a single, coherent ArchiMate model. It makes dependencies between office systems and production visible.
Why is architecture important for OT security?
Architecture shows which systems exist, how they are connected and which zones need which security level. Without that overview, network segmentation in line with IEC 62443 cannot be designed properly. Architecture is therefore the foundation of security by design in OT environments.
📌 In summary
Architecture is the backbone of manageable, secure and coherent information services and process automation. Whether for municipal IT, public-sector standards such as NORA or industrial OT networks built on the Purdue Model and IEC 62443, architecture uses principles, models and governance to provide direction, alignment and durability.
See also TOGAF, ArchiMate, OT Architecture and IT OT Security Reference Architecture.
See also OTArchitect.nl