Transport network interfaces operate to transport for Optical Transport Unit frames over an Optical Transport Network. Besides FEC bits for the Optical Transport Unit frames, the transmitting transport network interface provides sequences of error-determining bits for the Optical Transport Unit frames sent on working and protection communications channels. There is at least one sequence for each Optical Transport Unit frame, the number of bits in the at least one sequence much smaller than the number of bits in the Optical Transport Unit frame. The receiving transport network interface determines the bit error rates for the working and protection channels from the sequences of error-determining bits without decoding said Forward Error Correction bits and can select the working and protection channels accordingly.