ModemManager provides a unified high level API for communicating with (mobile broadband) modems. While the basic commands are standardized, the more advanced operations (like signal quality monitoring while connected) varies a lot.

Using.  ModemManager is a system daemon and is not meant to be used directly from the command line. However, a command line client (mmcli) is provided, which may be used to test the different functionality provided during plugin development.

Implementation.  ModemManager is a DBus system bus activated service (meaning it's started automatically when a request arrives). It is written in C. The devices are queried from udev and automatically updated based on hardware events. There are DBus-interface specific GInterfaces, which should be implemented by any device specific implementation. There is a generic MMBroadbandModem implementation that provides a generic implementation of the most common operations in both GSM and CDMA modems.

Plugins.  Plugins are loaded on startup, and must implement the MMPlugin interface. It consists of a couple of methods which tell the daemon whether the plugin supports a port and to create custom modem implementations. It most likely makes sense to derive custom modem implementations from one of the generic classes and just add (or override) operations which are not standard. Writing new plugins is highly encouraged!