This Recommendation applies to the functional characteristics of interoffice and long-haul line systems using Optical Amplifiers (OAs), including optical multiplexing. Existing Recommendations such as G.958, and the G.780-Series have not considered these new technologies. As such this Recommendation is the first attempt to characterize the functions that OAs and optical multiplexing require to describe these capabilities. Its present structure and contents describe the systems where optical interfaces for single-channel systems with optical amplifiers and optical interfaces for multichannel systems with optical amplifiers are implemented.