SYSTEM FOR SUPERVISING REPEATER

PURPOSE:To locate a repeater causing an error by allowing a repeater to interrupt a detection signal into a main line signal based on a control signal inherent in each repeater transmitted sequentially by a transmission terminal station in an optical cable relay system. CONSTITUTION:Each repeater 3 is provided with an error measuring circuit 7 checking a parity code inserted to the main line signal and supervising a code error rate. When the error measuring circuit 7 detects a line error, the circuit 7 inserts an identification signal 8 for a prescribed time to the main line signal. This signal is detected by a landing station. The operating command to the error measuring circuit 7 is performed by a control signal 5 inserted to the main line signal from the landing station 2. The identification signal 8 does not normally exert any effect on the main wire signal, and the main line signal is interrupted by the insertion of the control signal, but the effect is negligibly small.