1. Check the fiber connection. No problem is found. Use the optical power meter to detect the OUT port of station B. There is no light.
2. Replace the FIU. The problem is solved, and the alarm reported by station A is cleared.
3. Check the original FIU carefully. The check result shows that the fiber connector is not inserted into the internal OUT port securely.
4. After the fiber connector is inserted securely, replace the FIU again. The problem is solved.