The possible causes are as follows:
1. The fiber is cut or the fiber is improperly connected.
2. The line bit error is too large. The adjacent NEs report the transient alarms R_LOF and R_LOS.
3. The optical board of the transmitting end is faulty.
4. The environmental temperature is too high. The NE abnormality alarm occurs.
5. It is caused by the clock board or cross-connect board problem.