Abstract: The disclosed full-duplex communication system includes a pair of full-duplex modems, one at each end of a two wire circuit. Each of the modems includes a transmitter for transmitting over the two wire circuit a stream of synchronous data at a first carrier frequency, and a receiver for receiving over the two wire circuit a stream of synchronous data at a second carrier frequency. The receiver includes means for coherently demodulating the stream of synchronous data. In a preferred embodiment, an asynchronous-to-synchronous converter converts asynchronous input data into a stream of synchronous data for transmission by the transmitter. Also, the preferred embodiment transmits the data and receives the data as double side band suppressed carrier quadrature amplitude modulation. The system includes a remote test capability for automatically looping back test data for comparison with the transmitted test data.