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 6.3 of this Recommendation adds specifications for OTUk, k=0, references to specific bit patterns that may appear in Status and Payload Type overhead fields during the presence of the FlexO Squelch text pattern [ITU-T G.709.1], a clarification of the ODU Locked maintenance signal and an enhancement of the introductory text in annex L.