OPC UA Architecture: Information Model, Security, Sessions and Transport

IEC 62541 standard structure

IEC 62541 consists of 13 numbered parts. Parts 1–7 define the core architecture; parts 8–14 cover mappings, security, and profiles. For building automation integrators, the most relevant parts are 1 (Concepts), 3 (Address Space Model), 4 (Services), 6 (Mappings — encoding and transport), 7 (Profiles), and the companion specification IEC 62541-100 (OPC UA for Buildings).

Information model: NodeClass types and References

Every piece of data in an OPC UA server is represented as a Node in the address space. Nodes have a NodeClass attribute that defines their role. Nodes are linked to each other by typed References — for example, HasComponent, HasProperty, Organizes, HasSubtype. This creates a directed graph that clients can browse to discover server content without prior knowledge of the data structure.

Address space: namespaces and browsable tree

The address space is structured as a browsable tree. Every node is identified by a NodeId consisting of a namespace index and an identifier (numeric, string, or GUID). Namespace 0 (ns=0) is always the OPC UA standard namespace containing built-in types and standard nodes. Namespace 1+ are assigned by the server for vendor or application-specific content. Clients discover the namespace table via the GetNamespaceArray service call.

Objects (ns=0;i=85)  ← OPC UA standard root
├── Server (ns=0;i=2253)  ← always present, server diagnostics
└── Building_01 (ns=1;s=Building_01)  ← application namespace
    ├── Floor_02 (ns=1;s=Building_01/Floor_02)
    │   ├── AHU_01 (ns=1;s=AHU_01)  [Object]
    │   │   ├── SupplyAirTemperature (ns=1;s=AHU_01.SAT)  [Variable, Float]
    │   │   │   ├── EngineeringUnits  [Property: "°C"]
    │   │   │   ├── EURange  [Property: Low=0, High=60]
    │   │   │   └── StatusCode  [Property: Good/Bad/Uncertain]
    │   │   ├── FanSpeed (ns=1;s=AHU_01.FanRPM)  [Variable, UInt16]
    │   │   ├── DamperPosition (ns=1;s=AHU_01.Damper)  [Variable, Byte, 0–100%]
    │   │   └── ResetAlarm (ns=1;s=AHU_01.ResetAlarm)  [Method]
    │   └── VAV_Zone_2A (ns=1;s=VAV_2A)  [Object]
    │       ├── RoomTemperature (ns=1;s=VAV_2A.RoomTemp)  [Variable, Float]
    │       └── Setpoint (ns=1;s=VAV_2A.SP)  [Variable, Float, writable]
    └── Meters (ns=1;s=Meters)
        └── Main_kWh (ns=1;s=Meter.kWh)  [Variable, Double]

Clients navigate from the Objects root using the Browse service, following HierarchicalReferences (HasComponent, Organizes, HasProperty) to discover nodes. The TranslateBrowsePathsToNodeIds service allows clients to jump directly to a known path without traversing the entire tree — useful for performance-sensitive polling of known variables.

Transport options

OPC UA defines three transport bindings. The choice affects latency, firewall traversal, and protocol overhead. OPC UA Binary over TCP is the default for plant-floor BMS integration; HTTPS and WebSockets are used for cloud or browser-accessible interfaces.

Security modes and security policies

OPC UA security is negotiated at the Secure Channel level before any application messages are exchanged. The server advertises its supported EndpointDescriptions (combinations of MessageSecurityMode and SecurityPolicyUri). The client selects one when opening a connection. Never use None in a production environment — it transmits all data in plaintext with no integrity protection.

The SecurityPolicyUri selects the cryptographic algorithms. Current recommended policies (as of IEC 62541-7:2022) are:

Authentication methods

Authentication is performed at the Session layer (above the Secure Channel). OPC UA defines three ActivateSession identity token types. Certificate-based authentication is the most secure and is required for industrial security standards such as IEC 62443-3-3 Security Level 2.

Sessions and subscriptions

A Session is established on top of the Secure Channel after authentication. Sessions have a RequestedSessionTimeout (typically 60–3600 seconds); if the client does not send a keep-alive before timeout, the server deletes the session and all its subscriptions. Clients should handle reconnection with session re-establishment or transfer.

Subscriptions are the primary mechanism for efficient data monitoring. Instead of polling variables repeatedly (which generates unnecessary traffic), clients create a subscription with a PublishingInterval and add MonitoredItems to it. The server samples each MonitoredItem at the SamplingInterval and queues value changes; when the PublishingInterval fires, all queued notifications are sent in a single PublishResponse message.

CreateSubscription request:
  RequestedPublishingInterval: 1000  // ms — how often server sends Publish
  RequestedMaxKeepAliveCount: 10     // Publish cycles before keep-alive sent
  RequestedLifetimeCount: 30         // LifetimeCount × PublishingInterval = session timeout
  MaxNotificationsPerPublish: 1000   // cap notifications per PublishResponse
  PublishingEnabled: true

MonitoredItem parameters:
  SamplingInterval: 500              // ms — server samples item at this rate
                                     // must be ≥ server MinSupportedSampleRate
  QueueSize: 10                      // buffer for overrun; 1 = last value only
  DiscardOldest: true                // discard oldest on queue overflow
  DeadbandType: None / Absolute / Percent
  DeadbandValue: 0.5                 // Absolute: only report if change > 0.5 units
                                     // Percent: only report if change > 0.5% of EURange

OPC UA companion specifications

Companion specifications extend the base OPC UA standard with domain-specific information models. They define standard ObjectTypes, VariableTypes, and namespace URIs so that devices from different vendors expose data in a consistent, interoperable structure. Building automation engineers should be aware of the following companion specifications: