OPC UA vs MQTT: How to Choose for Industry? A Comparison of IoT Protocols, We Want Both!

First, understand what OPC UA and MQTT are, as well as their differences and similarities.‌OPC UA and MQTT are two common industrial IoT communication protocols suitable for different scenarios‌.

OPC UA‌

Supports complex data structures and high security, suitable for industrial scenarios that require precise control and data integrity (such as digital twins and smart manufacturing).

Device status monitoring, production data collection (such as bidirectional communication with PLCs in digital twin systems). ‌

MQTT‌

A lightweight protocol suitable for resource-constrained devices and low-bandwidth environments, ideal for large-scale device interconnectivity and simple data transmission.

Sensor data reporting, remote control, and other scenarios.

In the field of industrial automation control, the choice of communication protocol directly affects the overall system’s efficiency, performance, and stability. From sensors to enterprise systems, from data exchange between multiple devices to real-time communication for platform monitoring… developers need to spend a lot of time handling various protocols and making trade-offs between powerful protocols and lightweight, efficient ones.

It is well known that powerful protocols and lightweight protocols each have their own characteristics, advantages, and disadvantages.

• Powerful protocols (such as Modbus TCP,OPC UA, etc.)

• Complex message routing, high reliability

• Uses point-to-point mode, messages are routed to queues via switches

• Financial systems (high reliability of AMQP), autonomous driving (low latency of DDS)

• High overhead (AMQP requires a stable network), complex configuration. ‌

• Lightweight and efficient protocols (such as MQTT)

• Optimized for resource-constrained devices, suitable for low-bandwidth, unstable networks

• MQTT is based on publish/subscribe, with the Broker managing message distribution, using UDP to reduce overhead

• MQTT’s low power consumption, smart home, IoT devices.

• Low overhead (MQTT header is only 2 bytes), supports asynchronous communication.

With technological advancements, both OPC UA and MQTT provide a solution for industrial IoT that does not require trade-offs, allowing for a “both-and” approach. This combination offers developers more possibilities, integrating complex industrial scenarios with distributed systems to form a high-performance communication architecture.

Basic Information

1. OPC UA (OPC Unified Architecture)

OPC UA is a data exchange standard aimed at the industrial automation field, designed to achieve interconnectivity between devices and systems, overcoming the past information silos caused by fragmented device protocols.

As a platform-independent architecture, OPC UA supports data communication from low-level sensors to high-level enterprise medical platforms, while providing security, scalability, and multifunctionality. It ensures data security through encrypted communication and user authentication mechanisms; its services include data reading, writing, monitoring, and event handling. Additionally, OPC UA has good scalability, allowing it to run on different operating systems and hardware architectures. This standard is particularly important in industrial IoT and digital transformation, enhancing overall operational efficiency through efficient collaboration between production, management, and service systems.

2. MQTT (Message Queuing Telemetry Transport)

MQTT is a lightweight messaging protocol designed for low-bandwidth, unstable network environments. This protocol is known for its simplicity and efficiency, often used for communication between IoT devices.

MQTT is based on a publish/subscribe model, where clients can choose to subscribe to certain topics and send and receive data through a message broker (Broker), which can be understood as a relay station. Unlike traditional protocols, MQTT is suitable for resource-constrained devices, such as battery-powered sensors or embedded devices. Its advantages include support for low-power communication and real-time data transmission, providing efficient information exchange while minimizing network load.

It is worth noting that it also supports QoS (Quality of Service) levels to ensure reliable message delivery, meeting the reliability requirements of different application scenarios. In smart cities, remote monitoring, and home automation, MQTT has become one of the core communication protocols due to its simplicity and practicality.

Protocol Comparison

OPC UA and MQTT focus on slightly different usage scenarios:

OPC UA excels in the industrial automation field, commonly used for data exchange between complex production equipment and management systems, such as monitoring device status, handling alarm events, and transmitting more complex data types.

On the other hand, MQTT is emphasized in smart cities, home automation, and other scenarios due to its lightweight design, such as real-time sensor data publishing, remote monitoring, and low-power device communication.

Publish-Subscribe (Pub/Sub) Model

The publish/subscribe (Pub/Sub) model is a loosely coupled messaging mechanism that connects message publishers and subscribers through a message broker (Broker).

Publishers send data sets to the message middleware, and subscribers receive data by subscribing to the corresponding topics. The data set writer generates messages and sends them to the middleware via the network layer, while the data set reader is responsible for receiving messages from the middleware. This architecture allows the system to scale efficiently, supporting a large number of subscribers and providing flexible data delivery solutions. This model performs excellently in IoT and industrial control scenarios, ensuring the efficiency and reliability of complex data distribution. Both OPC UA and MQTT support the publish/subscribe model.

In industrial applications, OPC UA’s publish/subscribe mechanism particularly emphasizes support for complex data and the transmission of semantic information. However, in scenarios requiring low-power communication or lightweight data transmission, MQTT’s advantages become more pronounced. This advantage encourages developers to explore combining the semantic capabilities of OPC UA with the efficient transmission characteristics of MQTT, forming a new communication method: OPC UA over MQTT. This approach utilizes the MQTT protocol as the transport layer for the OPC UA publish/subscribe architecture, achieving efficient collaboration between protocols and further expanding the application scenarios of OPC UA.

Unified Automation’s SDK provides support for MQTT integration in the OPC UA publish/subscribe model. This support is achieved through flexible interface design, allowing developers to customize message delivery and processing methods according to specific needs. This enables the complex data model of OPC UA to leverage the transmission characteristics of MQTT, achieving efficient data distribution while ensuring the integrity of semantic information. This is especially important for IoT scenarios that require support for a large number of subscribers.

Unified Automation’s SDK achieves efficient collaboration between OPC UA and MQTT by providing extension interfaces and flexible configuration tools for MQTT integration. Developers can find a balance between complex data semantics and lightweight transmission characteristics, designing high-performance solutions for edge computing, distributed industrial systems, and other scenarios. This design takes into account both the technological advancement and practicality, providing new possibilities for IoT communication architecture.

The combination of OPC UA and MQTT not only overlays their respective advantages but also deeply matches actual needs. For example, through Unified Automation’s SDK, developers can utilize the rich semantic capabilities of OPC UA to handle complex industrial data while leveraging the lightweight and efficient characteristics of MQTT to achieve real-time distributed communication. Faced with increasingly complex industrial IoT scenarios, we do not need to choose between the two; instead, we can have both, integrating their strengths to explore more innovations and possibilities for industrial communication architecture.

MQTT publish/subscribe model has the following advantages over the classic OPC UA client-server model:

• Scalability: The publish/subscribe model can effectively handle a large number of devices and systems, making it very suitable for industrial automation and IoT applications.
• Real-time data exchange: The publish/subscribe model is specifically designed for real-time data exchange, allowing devices and systems to perceive and adapt to environmental changes promptly.
• Reduced network traffic: The publish/subscribe model effectively reduces network traffic by only transmitting the data that devices and systems need, rather than all data.

The OPC Foundation has released the OPC UA publish/subscribe specification, which establishes a publish/subscribe communication model for OPC UA that can use the MQTT protocol as a transport method. The OPC UA publish/subscribe model is very powerful, bringing significant benefits to industrial automation and IoT applications.

Comparison of MQTT Sparkplug and OPC UA

MQTT Sparkplug and OPC UA each have their strengths and weaknesses, so in different usage scenarios, one protocol may be more suitable than the other. Some of the main differences between these two protocols and their derivatives are as follows:

Metric	MQTT	MQTT Sparkplug	OPC UA	OPC UA over MQTT
Message Transmission Model	Publish/Subscribe	Publish/Subscribe	Client/Server	Publish/Subscribe
Bandwidth Usage	Lowest overhead, low bandwidth and low power	Lowest overhead, low bandwidth and low power	High code usage, high bandwidth	High code usage, high bandwidth
Message Payload	Undefined	Lightweight messages, usually smaller than OPC UA	Complex data types, with message sizes potentially much larger than MQTT Sparkplug	Complex data types, with message sizes potentially much larger than MQTT Sparkplug
Interoperability	No interoperability	Interoperable (18 data types)	Highly interoperable (60 data types)	Highly interoperable (60 data types)
Scalability	Highly scalable	Highly scalable, capable of handling millions of messages per second	Good scalability, but requires a more complex architecture to handle large amounts of data	Better scalability than the OPC UA client/server model
Integration Convenience	Simple to use, minimal configuration requirements	Simple to use, minimal configuration requirements	Requires more installation and configuration	Requires more installation and configuration
Quality of Service	QoS 0 (at most once), QoS 1 (at least once), and QoS 2 (exactly once)	QoS 0 (at most once), QoS 1 (at least once), and QoS 2 (exactly once)	Provides a reliable transport layer, ensuring messages are delivered in order and not lost	Provides a reliable transport layer, ensuring messages are delivered in order and not lost
Status Awareness	No	Yes	Yes	Yes
Auto Discovery	No	No	Yes	Yes
Application Domains	IoT, home automation, and M2M applications	Industrial IoT and M2M applications	Industrial automation	Industrial automation
Real-time Performance	Yes	Yes	Yes	Yes
Security	Security is not as strong as OPC-UA	Security is not as strong as OPC-UA	Digital certificates, digital signatures, data encryption, and security authentication	Digital certificates, digital signatures, data encryption, and security authentication
Information Model	No built-in information modeling support	Supports complex information modeling, but not as much as OPC UA	Supports complex information modeling systems, allowing the creation of complex data structures and models	Supports complex information modeling systems, allowing the creation of complex data structures and models

In summary, OPC UA is an open standard that includes a clearly defined set of data type specifications. MQTT Sparkplug is also an open standard, but it does not do as well in standardizing data types. Therefore, during data transmission, the protocol overhead generated by MQTT Sparkplug is smaller.