"The purpose of this Recommendation is to provide optical interface specifications towards the realization of transversely compatible bidirectional dense wavelength division multiplexing (DWDM) systems@ primarily intended for metro applications. The tail-end equipment (TEE) transmitters have the capability to automatically adapt their DWDM channel frequency to the optical demultiplexer/optical multiplexer (OD/OM) or optical add/drop multiplexer (OADM) port they are connected to using feedback from the head-end equipment (HEE) via the head-to-tail message channel (HTMC). This Recommendation defines and provides values for optical interface parameters of point-to-point DWDM applications on single-mode optical fibres through the use of the ""black link"" approach with both of the propagation directions sharing the same optical fibre end-to-end. For the applications in this version of the Recommendation@ the black-link does not contain optical amplifiers. This Recommendation describes bidirectional DWDM systems that include the following features: ?C channel frequency spacing: 50 GHz and 100 GHz; ?C bit rate of signal channel: Up to 10 Gbit/s; ?C transmission distance: Up to 20 km; ?C capacity: Up to 40 bidirectional channels. Specifications are organized according to application codes"