In optical channel transport unit overhead (OTU OH), what are general communication channels 1 and 2(GCC1/GCC2) used for?()
A、for trail trace identification
B、as the backward defect indicator
C、to transmit information between OTU termination points
D、to extend command and management functions over several frames
E、General communication channels 1 and 2 (GCC1/GCC2) do not belong to OTU OH