MQTT vs. OPC UA in Modern IIoT: Which Protocol Should You Choose?

If you are an Enterprise Architect or IT/OT Manager tasked with designing the backbone of a new digital factory, you will inevitably hit the most debated crossroads in industrial automation: Should we build our infrastructure on MQTT or OPC UA?

For years, the industry has aggressively pitted these two protocols against each other. Vendors fiercely defend their chosen silo. However, reducing this to a simple "A vs. B" cage match is a costly architectural mistake.

In this deep dive, we will strip away the marketing fluff. We will analyze how MQTT and OPC UA mechanically operate under the hood, where each excels, where they catastrophically fail, and why modern platforms like Proxus use both to create an unstoppable Unified Namespace.

1. OPC UA: The Heavyweight Champion of the Local Floor

OPC Unified Architecture (OPC UA) was released in 2008 as the successor to the notorious Windows-COM-based OPC DA. It was designed by the OPC Foundation with one massive goal: to be the ultimate, secure, vendor-agnostic standard for industrial data exchange.

How it Works (Client/Server Polling)

OPC UA operates primarily on a Client/Server architecture. An OPC UA Server sits on a piece of equipment (like a Siemens S7-1500 PLC). An OPC UA Client (like a SCADA system) connects to the server and constantly asks: "What is the temperature? What is the temperature? What is the temperature?"

Where OPC UA Excels

• Complex Information Modeling: OPC UA is not just a transport protocol; it is a massive structural dictionary. It natively understands complex object types (e.g., "This isn't just a float; it's a Fanuc Robot Arm with 6 axes, and here are the strict relationships between them").
• Local Network Speed: On a stable, hardwired, Gigabit local area network (LAN), OPC UA provides phenomenal, deterministic speed for machine-to-machine (M2M) synchronization.
• Deep Penetration: Every major automation vendor (Siemens, Rockwell, Beckhoff) supports it natively in their high-end hardware.

The OPC UA Bottleneck (Why it fails at Cloud Scale)

OPC UA is a "heavy" protocol. Because it relies heavily on polling, it is incredibly chatty. Pushing OPC UA traffic across a cellular network (LTE/5G) or a weak Wi-Fi connection to a cloud dashboard creates massive bandwidth saturation.

Furthermore, you must tightly configure point-to-point connections. If you have 50 PLCs and 5 client applications (ERP, MES, SCADA, AI), you are suddenly managing 250 distinct, brittle connections. This completely breaks the agility required for a Unified Namespace.

2. MQTT: The Agile, Lightweight Disruptor

Message Queuing Telemetry Transport (MQTT) was invented in 1999 specifically to monitor oil pipelines over incredibly expensive, awful satellite connections.

How it Works (Publish/Subscribe)

MQTT utterly destroys the Client/Server polling model. It uses a Pub/Sub architecture centered around a "Broker". Instead of asking for data, edge devices (Publishers) simply push data to the Broker only when a value actually changes (Report-by-Exception). Consumers (Subscribers) tell the Broker, "Hey, let me know if the temperature changes on Machine 1."

Where MQTT Dominates

• Bandwidth Efficiency: Because it only reports exceptions, MQTT can reduce industrial network traffic by 80-90% compared to OPC UA polling. It thrives on terrible, intermittent Wi-Fi or cellular networks.
• Massive Scalability: Adding a new AI Analytics app? It simply subscribes to the Broker. The PLC doesn't even know the AI app exists. You can scale to millions of concurrent connections seamlessly.
• State Awareness: With features like "Last Will and Testament" (LWT), if a machine loses power, the Broker instantly notifies the entire factory that the data is dead.

The Problem with "Naked" MQTT

MQTT is pure transport. It is a shipping envelope that does not care what is inside. You could publish a beautifully structured JSON string, or you could publish a garbled binary mess. This lack of standardization created chaos in early IIoT adoption, forcing engineers into painful data-mapping exercises.

3. The Great Equalizer: MQTT + Sparkplug B

To solve the "Wild West" payload problem of naked MQTT, the Eclipse Foundation introduced Sparkplug B.

Sparkplug B is a specification built on top of MQTT. It enforces a rigorous, standardized payload format using Protocol Buffers (ProtoBuf) and dictates strict rules for session state management (Birth and Death certificates).

With Sparkplug B, MQTT suddenly gains the structural predictability of OPC UA, but keeps the massive scalability and low overhead of Pub/Sub. This combination is the undisputed backbone of the modern Unified Namespace architecture.

4. The Verdict: When to Use Which?

The reality is that you do not have to choose. A master-class IT/OT architecture uses both protocols exactly where they belong.

Scenario A: Strict Machine-to-Machine Control (Winner: OPC UA)

If a robotic welding arm needs to talk to a safety PLC in 5 milliseconds to prevent a physical collision on the local factory floor, use OPC UA (or PROFINET/EtherCAT). It is deterministic, heavy, and built for local closed-loop control.

Scenario B: Enterprise Data Collection & Cloud AI (Winner: MQTT Sparkplug B)

If you want to pull data from 300 machines across 4 different factories into a central data lake for OEE calculation and predictive maintenance, use MQTT with Sparkplug B. OPC UA will collapse under the network weight and connection complexity at this scale.

5. Bridging the Gap with Proxus

The hardest part of Industry 4.0 is getting your legacy OPC UA (and Modbus/Siemens S7) equipment to speak MQTT Sparkplug B to the enterprise.

This is exactly what the Proxus Edge Gateway was engineered to solve.

Stop fighting protocol wars and start moving data. Want to see how Proxus can bridge your OPC UA equipment to an MQTT Unified Namespace in minutes, without writing a single line of custom code?

Talk to a Solutions Architect →