A supervisory channel is provided on an optical path (31) between nodes of an optical communication network. The nodes are arranged to use a set of wavelengths allocated for carrying traffic channels. An optical signal (16) which carries a supervisory channel is generated at a supervisory channel transmitter (15) and added (12) to the optical path (31) downstream of an optical amplifier (11). The optical signal (16) has a wavelength which is one of the set of wavelengths allocated for carrying traffic. The method is performed at a time when the wavelength is not being used to carry traffic. An impairment parameter of the received optical signal is measured at a supervisory channel receiver (15). The receiver is a coherent receiver and the impairment parameter is chromatic dispersion or polarization mode dispersion.