Abstract: A modem unit, typically a wireless modem unit, has a bidirectional digital data port, a modem, a transceiver, and a microcontroller programmed to control the unit according to a control program. The modem unit has a command mode, and the microcontroller is programmed to recognize certain commends by means of a prefix as remote commands and retransmit them to a remote modem unit rather than acting on them locally. The microcontroller is also programmed to monitor operating parameters of the modem unit and optionally to insert data relating to these parameters into a prefix to data strings transmitted by the modem unit and strip such prefixes from data strings received by the modem unit. The microcontroller controls switches which determine the routing of data between the port, the modem and the microcontroller so that the above functions may be implemented.