Recommendation ITU-T G.698.1 provides optical parameter values for physical layer interfaces of dense wavelength division multiplexing (DWDM) systems primarily intended for metro applications. Applications are defined using optical interface parameters at the single-channel connection points between optical transmitters and the optical multiplexer, as well as between optical receivers and the optical demultiplexer in the DWDM system. This Recommendation uses a methodology which fixes the maximum attenuation of the multiplexer/demultiplexer and fibre together and, therefore, does not specify the maximum fibre-link length explicitly. This Recommendation includes unidirectional DWDM applications at 2.5 and 10 Gbit/s with 100 GHz channel frequency spacing, as well as applications at 10 Gbit/s with 50 GHz channel frequency spacing. This latest revision of Recommendation ITU-T G.698.1 includes DWDM applications at 25 Gbit/s with 100 GHz channel frequency spacing.