Abstract: An optical line termination (OLT) device (12) generates a plurality of optical signals having different respective wavelengths (&lgr;1, &lgr;2), each optical signal carrying data, and wavelength-division-multiplexes the optical signals. A plurality of optical network units (ONUs 141-145) are connected to the OLT device (12) by way of a passive optical network (6) so as to receive the wavelength-division-multiplexed optical signals. Each ONU (14) has a wavelength selection unit operable in dependence upon control information sent from the OLT (12) to the ONU (14) concerned by way of the passive optical network (6) to select one of the optical signals of the plurality, and also has a detector for processing the selected optical signal to derive therefrom the data carried thereby.