[Case Show] Many Users Under the Same PON Port Have Dialing Error 678 Because Op

Procedure

1. Because all ONUs under the PON port have this fault intermittently. Therefore, it can be determined that the fault is not caused by a single ONU.
2. Replace the optical splitter with a new one. It is found that the fault persists. Therefore, it can be determined that the fault is not caused by the optical splitter.
3. Connect the ONU to the PON port on another board of the OLT. It is found that the fault persists. Therefore, it can be determined that the fault is not caused by the PON port of the OLT.
4. Check the optical path. The distance from an ONU to the OLT is about 1700 m. The measured Rx optical power on the primary PON port of the optical splitter is about 2 dB; the measured Rx optical power on each ONU port is about -6.3 dB. As indicated in documentation, the Rx optical power of the PON port on the ONU should be from -8 dB to -24 dB. However, the Rx optical power of any of these ONUs exceeds -8 dB. Therefore, the optical power may be too strong.
5. Add a 5 dB optical attenuator on the primary PON port of the OLT. Then, the measured Rx optical power of the ONU becomes about -12 dB. Observation for about a week shows that none user under the PON port reports dialing error 678 again. That is, this fault is rectified.