"Industrial Load Management System Using MQTT Protocol for Real-Time Energy Optimization ", Thesis of Industrial management

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MQTT Client MQTT Broker Connect Connect Acknowledge Subscribe ee ee, Subscribe Acknowledge ew ag, = =, Fublish Acknowledge —S— = = ee Fig. 4.1 MQTT Client To Broker As an example we will look at the variable length header in a connection packet. MQTT packet =control + length + protocol name + Protocol Level +Connect Flags + keep alive +Payload. It is interesting that the client ID field is sent as the first part of the payload, and not as part of the header. 4.1.3 MQTT Basics Clients do not have addresses like in email systems, and messages are not sent to clients: Instead messages are published to broker on a topic. The job of an MQTT broker is to filter messages based on topic and then distribute them to subscribers.A client can receive these messages by subscribing to that topic on the same broker.In this model there isno direct connection between a publisher and subscriber. The model is similar to broadcast radio and TV were a TV ar radio station broadcasts on a channel and listeners/viewers tune in to that channel. However unlike the broadcast TV/radio model in MQTT all clients can publish (broadcast) and subscribe (receive).MQTT uses a publish subscnbe model which requires the use of a centralBroker as shown in the Fig. 4.2. 33 Broker Message sent to subscribers Message Publishe Fig. 4.2 Publisher — Subscriber Model 4.1.4 Topics — MQTT Addressing These are like channels in the TV radio model. Topics connect the publisher and subscriber. In MQTT there is no formal structure and a publisher is free to choose its own topic names and structure. The following rules apply to Topic Names and Topic Filters: e All Topic Names and Topic Filters are case sensitive ¢ Topic Names and Topic Filters can include the space character Topic Names and Topic Filters can include the space character A leading or trailing */ creates a distinct Topic Name or Topic Filter A topic Name or Topic Filter consisting only of the */" character is valid Topic Names and Topic Filters MUST NOT include the null character Topic Names and Topic Filters are UTF-8 encoded strings. they MUST NOT encode to more than 65535 bytes 34