Table of Contents What is the MQTT Protocol? 1.1 MQTT's Design Philosophy & Key Features 1.2 Why is MQTT Indispensable in IoT/IIoT? Basic Principles and Workflow of MQTT 2.1 Publish/Subscribe Pattern: Evolution from IBM MQ Series 2.2 Detailed Explanation of MQTT Packet Structure 2.3 MQTT 3.1, 3.1.1, 5.0: Version Differences & Migration Guide 2.4 MQTT Application Scenarios in IoT (Modbus/MQTT/OPC UA Integration Explained) Detailed Understanding of MQTT Protocol Architecture 3.