Abstract: A signal received in a wavelength division multiplexing system is delayed and an exclusive OR logical operation is performed on the received signal and the delayed received signal. Then, the direct current voltage of a selection signal which is a result of the operation is measured to identify the bit-rate of the received signal. When a mixture of various signals having different bit-rates are used over a network in the wavelength division multiplexing system, a receiving terminal can automatically recognize information on the bit-rate of a received optical signal and extract a reference clock signal from the received signal, thereby reproducing the received optical signal without distortion, using the clock signal.