External transceivers and modems are available for Wi-Fi, cellular, Ethernet, Bluetooth, and any other protocol you can think of.
The following photo shows an industrial IoT device that uses a cellular modem:
In the preceding diagram, the lower circuit board implements the functionality of the device (monitoring cryogenic liquids, in this case). On this board, the power supply system, sensor electronics, and a low-cost, 16-bit microcontroller are found. A cellular modem is located on the top circuit board, which plugs into a socket on the lower board.
External modems often communicate with their host (microcontroller) through asynchronous serial. The protocols used vary considerably among systems.
One common protocol family for communicating with modems is the Hayes command set also called AT commands. These commands consist of short text-based...
