MQTT – Message Queue Telemetry Protocol

MQTT – Network Level Transmission

MQTT uses TCP as the basic protocol and thus works according to the client/server principle.

The broker is the server that can accept connections on TCP port 1883 by default. The MQTT users act as TCP clients and connect to the broker when required.

MQTT – Protocol level data exchange

With MQTT there are two roles that a client can assume.

Publisher
As a publisher, the client sends data to the broker. This can be measured values, switching statuses or any other process data. In addition to readable text content, MQTT also allows binary data. Whether the publisher transmits its data to the broker when there is a change or cyclically depends on the application.

Subscriber
The subscriber accepts data from the broker. In the role of the subscriber, the client tells the broker which data it would like to receive or subscribe to after the connection has been established.

Each MQTT client can be a publisher, subscriber or both. So there is no mandatory master/slave principle as with other industrial protocols. All MQTT end devices or clients are initially equal at the MQTT level. Who supplies data and who receives it is determined exclusively by the application through the publisher/subscriber assignment.

Topic
The MQTT broker manages the data to be exchanged according to data endpoints. The data endpoints are named using topics. These are strings, i.e. character chains, which can be structured similar to the URL when the website is called up.

Example:

A W&T web thermo-hygrobarometer measures temperature, humidity and air pressure in the server room of the W&T company building. Each of the three values ​​represents a data endpoint.

The topics for this could look like this:

wut/serverraum/temperatur
wut/serverraum/luftfeuchte
wut/serverraum/luftdruck

The web thermohygrobarometer transfers the measured values ​​as a payload, i.e. as data content, under these topics to the MQTT broker via MQTT publish.

Any MQTT client can now send a Subscribe specifying the desired topic to the MQTT broker. Here the MQTT client sends a Subscribe wut/serverraum/temperaturand thus subscribes to the temperature value of the web thermohygrobarometer.

As long as the PC is connected to the broker as an MQTT client, it automatically receives the value sent by the web thermohygrobarometer via publish.

The subscriber can work with wildcards “#” when specifying the topics.

Beispiel: wut/serverraum/# abonniert Temperatur, Luftfeuchte und Luftdruck für den Serverraum.

In addition to “#”, there is also “+” as a wildcard. If you enter “+”, only end topics of the corresponding level are subscribed to – topics from subsequent levels are ignored.

Each subscribed value has its own data broadcast.

The transmission of the data is byte-oriented and therefore binary-transparent – so any content can be transmitted. For all text content, UTF8 is specified as the format for standardization.