Wavelength division multiplexed channel tagging

The present invention relates to wavelength division multiplexed channel tagging and more particularly to a method of and apparatus for tagging channels in return to zero systems such as soliton systems. A method of tagging individual wavelength division multiplexed channels within a plurality of said channels. The method comprises the step of creating a tagging tone in the form of an analogue spectral line tone, by providing a series of regularly spaced time slots substantially throughout each of said tagged channels for receipt of binary tagging pulses. The binary tagging pulses in each tagged channel are arranged in a different manner to provide a characteristic tag for each tagged channel.