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 6.1 of this Recommendation includes the addition of tables with GCC bit rates, enhancements to the description of OTUCn-M behaviour, addition of an appendix that describes the implications on fault management for the case that the OTSiG (de)modulator process and associated OTSiG-O|OCh-O_TT function are located in adjacent equipment, and several typographical/editorial corrections.