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 5.0 of this Recommendation includes the text of Amendments 2, 3, 4, Corrigendum 2 and Erratum 1 to Edition 4.0 of this Recommendation, support for frequency and time synchronisation, addition of Beyond 100 Gbit/s OTU, ODU and OPU frame formats, overhead, ODU multiplexing, client mappings and optical layer terminology updates. Edition 5.0 furthermore deleted the OPUk concatenation specifications and the ATM mapping into OPUk specification and changed the TCM ACT and FTFL overhead bytes into EXP overhead bytes.