Recommendation ITU-T G.698.5 provides optical parameter values for physical layer interfaces of dense wavelength division multiplexing (DWDM) systems primarily intended for mobile fronthaul and metro applications in the O-band, optimized for 10-km and 20-km transmission distances. Applications are defined using optical interface parameters and values for single-channel interfaces of multichannel wavelength division multiplexing (WDM) optical systems in point-to-point applications. This Recommendation uses a system architecture comprising a head-end equipment (HEE) connecting to the tail-end equipment (TEE) through a black link. For mobile fronthaul applications, the HEE is in a central office while the TEE is in a remote antenna site. A single bidirectional transmission fibre is used in the black link to connect the HEE to the TEE. This version of the Recommendation includes bidirectional single-fibre WDM applications at 25 Gbit/s per channel with a nominal optical channel frequency spacing of 800 GHz.