Symptom
The G network is built with OptiX BWS 1600G. At site B, the LWFS board for receiving services from site L are provisioned with three wavelengths. When the pre-BER is 0, uncorrected bit errors are generated occasionally on the LWFS board, thus affecting the services on the connected SDH equipment at site B. As a result, the SDH equipment reports bit errors.
There is one span between sites B and L. The attenuation of this span is relatively high and the network topology is given in Figure 1.
Cause analysis
When the pre-FEC BER meets the requirement but bit errors are generated randomly, it is recommended to check the involved board and then clean the fibers on the optical path. If the problem persists, coordinate the customer to test the fibers by using an OTDR. In general, it can be determined that random bit errors result from high fiber reflection if the involved board is normal and the fibers on the optical path are clean.
During fault identification, it is found that bit errors are generated because of fiber reflection.
Procedure
- Ensure that the error correction function of the LWFS board is enabled.
- Considering that bit errors are generated on three wavelengths, clean the fiber jumpers used on the main optical path. The fault persists.
- Apply a fiber loopback on the WDM side of the LWFS board. Observe the 24-hour performance of the board and find no bit error is generated. Thus, determine that the LWFS board is normal.
- Rule out that the fault results from high fiber PMD because the pre-FEC BER is stable.
- Coordinate the customer to test the fibers by using an OTDR, and find that the reflection of a fiber splicing point is high. Then, replace the faulty fiber and test the service performance for 48 hours. No bit error is generated during the testing and the problem is resolved.
