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 6.6 (Amendment 4) of this Recommendation adds parallel logic equations and related text for fgTSOH CRC-4 and fgTSMxOH CRC-6 implementation, multiplexing up to 4 E1 CBR clients to fgOPUflex(1), enhancements to the description of mapping of VC-n client signals into fgOPUflex and fgGMP processes, and some editorial corrections.