Determination of the location of an error condition or a failure includes receiving at a network interface a first framed digital signal from customer premises equipment, and determining whether the first framed digital signal indicates a failure. Overhead bits are modified in the first framed digital signal to generate a second framed digital signal, such that the modification is equivalent to insertion of errors into the first framed digital signal at a bit error ratio (BER) of not greater than a predetermined ratio, if the first framed digital signal indicates a failure. The second framed digital signal is then sent from the network interface in place of the first framed digital signal to indicate that the failure reported by the first framed digital signal is located in the customer premises equipment. Otherwise, if no failure is indicated, the first framed digital signal is transmitted without any modifications.