The optical transport network (OTN) supports the operation and management aspects of optical networks of various architectures@ e.g.@ point-to-point@ ring and mesh architectures. This Recommendation defines the interfaces of the OTN to be used within and between subnetworks of the optical network@ in terms of: ?C OTN hierarchy; ?C functionality of the overhead in support of multi-wavelength optical networks; ?C frame structures; ?C bit rates; ?C formats for mapping client signals. The interfaces defined in this Recommendation can be applied at user-to-network interfaces (UNI) and network node interfaces (NNI) of the OTN.