The dongle allows the installer to connect a wired ethernet cable to the communicator. This provides a dual-path configuration, allowing both IP and Cellular communication to be used. In the event that the original ethernet dongle is lost or damaged, this is the replacement. Alarm.com dual-path communication works a bit differently to some other services. When dual-paths are available, Alarm.com communication modules send all signals via both paths simultaneously. Whichever signal reaches the Alarm.com server first, that signal is processed by the server. The signal received second is discarded. The server can tell when a signal is a duplicate of a signal already received, or a brand new signal. By handling communication this way, Alarm.com ensures that the fastest available communication path is always used. When a system is installed in a location with an internet connection available, it is always best practice to use that connection whenever possible. Read more: https://www.alarmgrid.com/products/alarm-com-adc-200m-ev-e-dong