MQTT Protocol tutorial using Mosquitto and CloudMQTT

The MQTT protocol only defines the format of the data being transmitted and provides a pub-sub kind of communication interface between multiple devices. 
There are three main entities involved in MQTT,

1) Publisher : The device which publishes messages on a particular topic.

2) Subscriber : The device which receives messages by subscribing to a particular topic. 

3) Broker : The server which takes care of forwarding the messages sent by the publishers to all the subscribers. 


Concept : 
Its similar to you subscribing to newsletters on websites or news magazines,

– You subscribe to a topic of your interest, like Politics. 

– Now whenever there is any news on Politics, you will be sent a mail published by the magazine server. 

– Similarly that mail will be sent to all users whoever is subscribed to the topic Politics. 

Its not a perfect example but that should give you an idea.


Demonstration :

Now in today’s example we will see how we can publish a message to a given topic and also we will try to receive a message by subscribing to the same topic. 

To do this, we will need to understand the format of the packets involved in the process of connecting with the broker and sending publish and subscribe packets. 

There are many open source and free brokers available on the cloud, The open source MQTT software site mosquitto.org also has a free MQTT broker running at test.mosquitto.org which you can use as a free broker. There are other servers like CloudMQTT.com and HiveMQ.

In this example we will only concentrate on Mosquitto and CloudMQTT. 

Procedure for publishing a message :

1) Send a Connect packet to the broker (With Authentication if needed)

2) Wait for Connect acknowledgement packet from the broker

3) and when Connect acknowledgement is received , send the publish message packet with appropriate topic name

4) Wait for the Publish acknowledgement packet

Procedure for subscribing to a topic :

1) Send a Connect packet to the broker (With Authentication if needed)

2) Wait for Connect acknowledgement packet from the broker

3) and when Connect acknowledgement is received , send the subscribe packet with appropriate topic name

4) Wait for the subscribe acknowledgement packet


MQTT packets when sending to Mosquitto broker  without Authentication: 

 

 The above image shows the format of the important packets used in MQTT communication with Mosquitto broker, all numbers shown above are in hexadecimal format. 


MQTT packets when sending to CloudMQTT broker with Authentication: 

The above image shows the format of the important packets used in MQTT communication with CloudMQTT.com broker, all numbers shown above are in hexadecimal format.


To understand these packets in a better way i recommend to watch the video below which i made to show a live demo of how we can form these packets for different MQTT brokers. 


Download link to Excel file used in my video : Click here

Also see : 

 

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