A method is disclosed for monitoring a communication link between a first apparatus and a second apparatus, the method comprising receiving concurrently from the first apparatus (e.g., a central office) at the second apparatus (e.g., an optical demarcation point at a customer premises) a communication signal at a communication wavelength, .lamda..sub.1 and a test signal at a test wavelength .lamda..sub.t, permanently separating the test signal from the communication signal at the second apparatus and sending to the first apparatus on a return path between the first apparatus and the second apparatus a signal indicating whether the test signal was received at the second apparatus. An apparatus is disclosed including an interface for receiving concurrently the communication signal and the test signal, a filter for permanently separating the test signal from the communication signal and a coupler for permanently placing a signal on a return path.