Recommendation ITU-T G.709/Y.1331 defines the requirements for the optical transport network (OTN) interface signals of the optical transport network, in terms of: – OTN hierarchy – functionality of the overhead in support of multi-wavelength optical networks – frame structures – bit rates – formats for mapping client signals. Edition 6.5 (Amendment 3) of this Recommendation adds mapping of 800GBASE-R clients, clarifications to the description of GMP, and a new fine-grained path layer and tributary slot structure.