OPC UA PubSub

Leverages User Datagram Protocol (UDP) multicasting on the local substation LAN to distribute data without a central message broker. This eliminates a single point of failure and minimizes latency for high-speed, time-critical data like GOOSE or Sampled Values mapped to PubSub.

• Zero-broker architecture: Publishers send directly to multicast groups.
• Sub-microsecond synchronization: Integrates with Precision Time Protocol (PTP) for deterministic delivery.
• Use case: Replacing traditional IEC 61850 publisher-subscriber mechanisms on the process bus with a unified, OPC UA-encoded data stream.

Maps OPC UA PubSub datasets onto standard messaging protocols like Advanced Message Queuing Protocol (AMQP) and MQ Telemetry Transport (MQTT) for cloud integration. A central broker handles store-and-forward, topic routing, and subscriber management.

• Protocol bridging: Translates IEC 61850 Logical Node data into JSON or binary payloads for enterprise brokers.
• Decoupled consumers: Enables multiple cloud applications to subscribe to a single Merging Unit data stream without direct IED connections.
• Use case: Streaming substation Disturbance Recorder files in real-time to a central analytics platform via an MQTT broker.

Defines a structured collection of OPC UA Variable values published as a single atomic message. The DataSetMetaData describes the structure, data types, and semantics of the payload, enabling subscribers to interpret the data without prior out-of-band configuration.

• Self-describing payloads: Each message carries its own schema, ensuring interoperability.
• Field mapping: Directly maps attributes from Logical Nodes like XCBR (circuit breaker) position or MMXU (measurement) current.
• Version control: DataSetMetaData includes a configuration version to manage schema evolution across publishers and subscribers.

The core architectural components that replace the rigid client-server relationship. A Publisher defines and emits DataSets, while Subscribers dynamically discover and consume them. This decoupling allows for flexible, many-to-many communication patterns.

• Dynamic discovery: Subscribers can locate publishers using standard OPC UA discovery services.
• Loose coupling: Publishers have no knowledge of subscribers, enabling independent scaling.
• Use case: A single Phasor Measurement Unit (PMU) publisher streams synchrophasor data to multiple subscribers: a local Wide-Area Monitoring System, a cloud historian, and a real-time oscillation detection application.

Implements message-level security for PubSub data flows, independent of the transport protocol. Publishers sign and encrypt individual messages using symmetric keys distributed via a Security Key Service (SKS).

• Group security model: A shared symmetric key secures a multicast group, ensuring only authorized subscribers can decrypt data.
• Compliance: Aligns with IEC 62351 security requirements for power system communications.
• Use case: Encrypting Sampled Values traffic on a process bus to prevent unauthorized injection of malicious current measurements into protection relays.

Supports two primary encoding formats for PubSub messages. JSON mapping provides human-readable, web-friendly payloads for enterprise integration, while UADP (UA Binary) offers compact, high-performance encoding for resource-constrained IEDs and low-bandwidth links.

• JSON: Ideal for cloud ingestion and integration with SCADA analytics platforms.
• UADP: Optimized for Process Bus applications requiring minimal overhead and deterministic parsing.
• Use case: An Intelligent Electronic Device (IED) publishes UADP-encoded GOOSE tripping signals on the local LAN while simultaneously publishing JSON-encoded status updates to a cloud broker.

OPC UA PubSub often maps Logical Nodes (LNs) and Common Data Classes (CDCs) from IEC 61850 into its information model. This allows GOOSE and Sampled Values semantics to be preserved when bridging process bus data to enterprise systems. Key mappings include:

• XCBR (Circuit Breaker) status
• MMXU (Measurement) for three-phase power
• PDIS (Distance Protection) start and trip signals

OPC UA PubSub decouples data distribution from the client-server connection. It supports two primary transports:

• UDP Multicast: Enables one-to-many, low-latency distribution on a local network, ideal for controller-to-controller communication
• AMQP/MQTT: Brokered messaging for cloud integration, allowing substation data to traverse firewalls and reach SCADA or analytics platforms

PubSub supports two wire encodings to balance interoperability and efficiency:

• JSON: Human-readable, ideal for cloud ingestion and debugging, but higher bandwidth
• UADP (UA Binary): Compact, high-performance encoding optimized for TSN (Time-Sensitive Networking) and deterministic local networks Selecting the right encoding depends on whether the consumer is a cloud function or a real-time IED.

For deterministic publisher-subscriber communication, OPC UA PubSub can leverage IEEE 802.1 TSN standards. This provides:

• Time-synchronized frame delivery with bounded low latency
• Traffic shaping to prevent interference between critical protection data and background monitoring traffic
• Essential for converging Sampled Values and GOOSE traffic onto a unified substation network

When bridging substation OT data to enterprise IT via PubSub, IEC 62351 security controls apply:

• Authentication of publishers to prevent data injection
• Integrity protection using digital signatures on published datasets
• Role-Based Access Control (RBAC) to restrict which subscribers can receive sensitive operational data This ensures the pub-sub channel does not become a vector for false data injection attacks.

OPC UA PubSub supports two fundamental distribution patterns:

• Brokerless (UDP): Publishers send directly to network addresses; no intermediary. Used for local interlocking and protection where every millisecond counts
• Brokered (MQTT/AMQP): A central message broker manages subscriptions and queues. Ideal for cloud analytics, digital twin synchronization, and wide-area demand response orchestration