Each of these example projects use an mBed microcontroller and SIM5230 3G modem.
This project requires a remote device send data from a single sensor over GPRS/3G. Data should be uploaded each minute to a remote server that does light processing on the data and displays it to the user.
This project is similar to Project A except it requires bi-directional communication. The settings on the device are periodically updated by administrators. Remote control of a relay is also required.
MQTT plays nicely in to this scenario because of its publish/subscribe architecture. Remote devices can initiate a persistent connection with a long keep-alive delay and periodically monitor a topic for messages from the server.
This method uses a smaller amount of bandwidth compared to traditional polling mechanisms as the keep-alive messages are a couple of bytes, and the device is not constantly checking for updates where none exist.
Having a persistent connection means that, unlike HTTPS, the TLS handshake does not happen each time a request is made as the connection is not being rebuilt each time.
Unlike the previous projects this project triggers a camera when a sensor’s threshold exceeds a given value. The image is then sent back to the remote server. This project is not suited to a pure MQTT implementation due to the requirement of HD images.