The purpose of this example is to toggle the LED on the subscriber board by publishing LED_ON and LED_OFF commands from the publisher board. Since MQTT-SN network topology demands a gateway, which serves as a translator between MQTT-SN clients and an MQTT broker, a third-party MQTT-SN gateway has been used. It has been taken from Eclipse Paho project. For additional information on the gateway used, see Eclipse Paho MQTT-SN C/C++ client for Embedded platforms.

As the MQTT-SN network in this example runs on Thread, the MQTT-SN Gateway plays the role of a Thread Border Router, which means it is a gateway between the Internet and the Thread Network. It can be run on a Raspberry Pi with Raspbian-based firmware. To connect the MQTT-SN Gateway to the Thread network, you need Nordic's nRF52840 or nRF52833 Development Kit running the Network Co-Processor (NCP). For details on how to set up a Thread Border Router and run MQTT-SN Gateway, refer to the Thread Border Router documentation.

Both client nodes indicate their Thread network state on LED1. When the device is not connected to the network, the LED starts blinking. Once the device connects to a network, either as a Child or as a Router, the LED stops blinking and remains turned on. On startup, nodes automatically enter the Thread network with default parameters. The first device that is turned on becomes a Leader of the network, while the rest of the nodes become Children and Routers.

Setup

You can find the source code and the makefiles in the following folders:

<InstallFolder>\examples\thread\mqttsn_client_publisher
<InstallFolder>\examples\thread\mqttsn_client_subscriber

Subscriber

The subscriber node is controlled by specific actions triggered by pressing the buttons. The effect of pressing the buttons is reflected on the LED state.

Subscriber button assignments

Button 2: Search for a gateway.
- Pressing sends a multicast SEARCH GATEWAY message, which launches the gateway discovery procedure. It is necessary to connect to the gateway.
Button 3: Connect or disconnect from the MQTT-SN network.
- Pressing sends the following unicast messages:
  - If the node is not connected to the gateway: a CONNECT message to the gateway, which connects the node to the MQTT-SN network. Once the connection is established, a topic is registered with a unicast REGISTER message.
  - If the node is connected to the MQTT-SN network: a DISCONNECT message to the gateway, which disconnects the node from the MQTT-SN network.
Button 4: Send a SUBSCRIBE/UNSUBSCRIBE message.
- Pressing sends the following unicast messages:
  - When the client is not subscribing: a unicast SUBSCRIBE message to the gateway.
  - When the client is subscribing: a unicast UNSUBSCRIBE message to the gateway.
- If the node is not connected to the network, pressing the button has no effect.

When a MQTT-SN client has subscribed to a topic, every message published by other nodes on that topic is sent to the subscribing client. Unsubscribing from a topic cancels the subscription, so that no messages on that topic are forwarded to the client.

Subscriber LED assignments

The Subscriber uses LED assignments as described in Thread BSP LED and button reference for the following LEDs:

LED1 – Used to indicate the network connection state.

The following LED reserved for user application purposes are assigned in this example on subscriber:

LED3: Toggled by Publisher (see Publisher button assignments).
LED4: MQTT-SN network state:
- Off - Disconnected.
- On - Connected.

Publisher

The subscriber node is controlled by specific actions triggered by pressing the buttons. The effect of pressing the buttons is reflected on the LED state.

Publisher button assignments

The button assignments are identical to Subscriber button assignments, with the exception of the following button:

Button 4: Send a PUBLISH message.
- Pressing sends a unicast PUBLISH message to the gateway.
- If the node is not connected to the network, pressing the button has no effect.

Note: The Publisher node sends LED_ON and LED_OFF commands alternately, regardless of the state of LED3 on the subscriber board. It might happen that the state of LED4 on the subscriber board and the published command are the same. In such a situation, pressing Button 4 on the publisher board will have no effect on the state of LED3 on the subscriber board, despite fully functional and working network. Pressing Button 4 on the publisher board again will change the command and the publisher will toggle the LED3 state.

Publisher LED assignments

The publisher uses LED assignments as described in Thread BSP LED and button reference for the following LEDs:

LED1 – Used to indicate the network connection state.

The following LED reserved for user application purposes are assigned in this example on publisher:

LED4: MQTT-SN network state:
- Off - Disconnected.
- On - Connected.

Testing

Build the example according to the instructions in Building examples.
Turn the MQTT-SN gateway on according to the instructions in Thread Border Router.
Program a development board with the MQTT-SN Subscriber application.
Program another development board with the MQTT-SN Publisher application.
Let the clients find the MQTT-SN gateway by pressing Button 2 on both boards.
Connect the clients to the gateway by pressing Button 3 on both boards.
Subscribe by pressing Button 4 on the subscriber client board.
Try toggling LED3 on the subscriber by pressing Button 4 on the publisher board.
Unsubscribe by pressing Button 4 on the subscriber board again. As a result, pressing Button 4 on the publisher board will have no effect on the state of LED3 on the subscriber board.

Note: The majority of the messages is sent through a broker in the cloud. It might be necessary to wait a few seconds for all messages to be delivered.

Table of Contents