Bit error alarm (BEFFEC_EXC) is generated when optical power gets close to the threshold.
Symptom
Station A and B form an OTM with station C functioning as an OLA in between. The BWS 1600G is used at each station and the LWF is used as the OTU board.
The LWF at station A in slot 3 keeps reports the BEFFEC_EXC alarm transiently. Query the WDM performance of the board. It is found that there are a lot of FEC correction events (the number of corrected bytes is larger than 100 thousand).
Cause Analysis
- The input optical power of the OTU board is extremely high or low.
- The settings of the FEC mode of the interconnected OTUs are different.
- The fiber connector is dirty or is connected improperly.
- An inappropriate DCM is installed.
- The board at the opposite station is faulty.
- The dispersion is too high, or signal-to-noise ratio is too low, the fiber is faulty.
Procedure
- The board reports the alarm indicating that excessive bit errors are detected. Query the optical power of the board. The input optical power on the WDM side is -6.9 dBm, which is within the normal range.
- Perform the following steps to rectify the possible fiber faults. Check the fiber jumpers and clean the fiber connectors at stations A and B. There is no obvious change in the performance and the alarm persists.
- Query the performance of other LWF boards at station A. There are FEC correction events in the three LWF boards in slots 4-6 and the BER before FEC correction is very high.
- Query the optical power of all boards at station A. The input optical power on the WDM side of every LWF board is about -7 dBm (slightly higher than -7 dBm). Query the threshold of the input optical power at the IN interface of the board by using the T2000. The threshold is -2.0 dBm. According to the optical power adjustment principle (plus 3 minus 5), the input optical power must be 5 dBm smaller than the receiver overload. The input optical power of the OTU at station A is close to the threshold but not reach the value that triggers an optical power alarm. The bit errors may be generated because the optical power is slightly high.
- The optical power of every board is slightly high. In this case, you can adjust the optical power by decreasing the gain of the OAU board. Query the gain of the OAU at station A. The gain is close to the lower limit. Decrease the gain of the OAU at station C by using the T2000. Then, query the optical power of every LWF at station A. The input optical power at the IN interface is about -9 dBm. Observe the performance. The number of error correction events decrease dramatically and the LWF in slot 3 does not report the alarm any more.
Reference Information
The OAU supports the gain adjustment by software, which can be set by using the T2000. The operation is very easy.
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