This Recommendation provides physical layer inter-domain interface (IrDI) specifications for optical networks that may employ wavelength division multiplexing (WDM). These interfaces may also be employed, where appropriate, as intra-domain interfaces (IaDI). The physical layer specifications are valid for non-optical transport network (non-OTN) IrDIs, while also allowing application on OTN IrDIs according to [ITU-T G.709]. In the case of a non-OTN IrDI, OTN management capabilities are not required. The IrDIs within the optical transport network (OTN) are provided by unidirectional, point-to-point, single and multichannel line systems. Their primary purpose is to enable transversely compatible interfaces to span the boundary between two administrative domains. The IrDI specifications include intra-office, short-haul and long-haul applications, without line amplifiers. NOTE – For the purposes of this Recommendation, the term "administrative domain" is understood to mean the extent of resources that belong to a single player, such as a network operator, a service provider or an enduser. Administrative domains of different players do not overlap among themselves.

The specifications are organized according to application codes that take into account the many possible combinations of channel counts, optical tributary signal types, span distances, fibre types and system configurations. The reference configuration and application codes form the foundation for specifying the optical networking physical layer parameters

In this Recommendation, the use of an optical supervisory channel has not been considered (except for the discussion in Appendix IV). Future versions and other new ITU-T Recommendations will further address this aspect of the OTN, possibly involving an arrangement of optical network elements on either side of an optical subnetwork interface that is more complex than point-to-point. For these applications, different parameters beyond those specified for a point-to-point configuration may be required.

This Recommendation presumes that the optical tributary signals transported within network media channels are digital rather than analogue. Specifications for systems enabling transport of analogue optical tributary signals are for further study.