Symptom
The network topology of network F is shown in Figure 1. On NE 21 at site K, the OTU board connecting site E and site K works normally. Some day Multiple TN11NS3 boards report IN_PWR_LOW alarms on the WDM side.
The OTU board on NE21 of site K is connected to the OTU board on NE10 of site E with OLA sites in between.
Cause analysis
The OTU board reports the IN_PWR_LOW alarm. This indicates that the input optical power of the WDM side is lower than the lower threshold and higher than the module sensitivity. If this problem is not taken in time, the input optical power continues to drop and there is a risk of service interruption.
The possible causes of the fault are as follows:
- Fiber connectors are dirty or damaged. Fiber jumpers are over-bent, damaged or aged.
- An attenuator of excessively high attenuation is attached to the optical port on the board that reports the alarm.
- On the board at the opposite site, an attenuator of excessively high attenuation is attached to the transmit optical port or the optical transmitter module is faulty.
- Optical signals are over-attenuated during transmission but not amplified sufficiently.
- The board that reports the alarm is faulty.
Procedure
- Multiple boards report IN_PWR_LOW alarms in a period. The fourth cause (that is, optical signals are over-attenuated during transmission but not amplified sufficiently) is with greatest possibility.
- Query the 24-hour performance value of each board in the reverse direction of the receive link of the OTU board and compare changes of optical power. It is found that the optical power on the IN port of the M40v board decreases largely. Continue to query the performance value in the reverse direction. It is found that the optical power of all the boards on the link (all the way to the FIU) decreases largely. Thus, the fault is located on the intermediate link.
- Query every OLA site on the link by using the same method. It is found that the optical power on the boards at every OLA site decreases except NE KA where optical power does not change.
- Properly decrease the attenuation value of the pre-amplified VA4 at NE KA and increase the optical power. Then, the downstream optical power increases. Eventually, the IN_PWR_LOW alarm reported by the OTU board at site K is cleared and the problem is resolved.
- The cause of this problem is traced back to the road building between site KA and site OI. There are five fiber cuts at site KA in three months. After the cut fibers are spliced, their attenuation increases, affecting the entire WDM system under certain conditions.
Reference Information
Conclusions and suggestions for this case are as follows:
- Critical alarms must be cleared in time. Some alarms may not result in service interruption. However, if not cleared in time, those alarms may interrupt services at any time and much reference information is lost. For example, the 24-hour performance can be saved for less than a week. If so, the problem will be resolved later with more difficulty.
- During the routine network maintenance, information about boards' input/output optical power, gain values and attenuation values queried during the normal operation of the network should be saved. The information can be referred to in case of a line fault.
- Alarms on the entire network are cleared.
