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There are 22 sets of OptiX RTN 980 for 27 microwave links at this moment. All are configured to have 256QAM (256QAM maximum modulation, QPSK minimum modulation) and AM feature is enabled. There were alarm AM_DOWNSHIFT, MW_BER_SD and MW_LOF appeared. After a checking with the physical hardware and Line of Sight, it was found that the LOS was blocked by a passing cruiseship. The RSL was dropped and the AM Function is working with the alarm AM_DOWNSHIFT appeared. However after the cruise ship already passed by, the Tx modulation of the link cannot resumed to its 256QAM (maximum) over almost 50 hours. Finally, there was a need to disable the AM function first so that it will go back to original modulation of 256QAM, and then enabled the AM function again.In 24 hours observation, the link stayed at 256QAM modulation.
Whenever the AM Function is enabled for a link, and the RSL is dropped due to fading or blockage, this alarm will appear. It is not an ABNORMAL alarm. It is alarm that shown the AM Function is in action. If the RSL drop, there is a need for the link to automatically increase the ODU TSL level so that the link will remain stable. But when the ODU TSL level is increased, the link capacity will decreased. In this case, 256QAM will decreased to the Guaranteed modulation/Minimum Modulation which was configured in the AM Function Configuration. When this happened, the AM_DOWNSHIFT alarm will appear. So, when the RSL became normalised to the original level, the alarm AM_DOWNSHIFT will cleared. If it does not clear, just acknowledge the alarm and it will be cleared.
However,if the AM downshifting failed to back to normal AM after RSL is recovered, there are many factors that contributing to this problem. There is a need to go onsite to check these factors. It probably due to internal/external frequency interference, waveguide damage or ODU degrading. In the alarm screenshot, it can be seen that thereare few MINOR alarms appeared together with the alarm AM_DOWNSHIFT such as MW_BER_SD and MW_RDI. So a physical checking need to be performed onsite for any physical abnormalities. Thorough troubleshooting is recommended to find the abnormality as for microwave link it is very hard cannot pinpoint where is the fault. The fault can be everywhere whether software or hardware.
After a checking with the physical hardware and Line of Sight, the root cause of the problem is the Line of Sight was partially blocked by a passing cruiseship. The RSL was dropped and the AM Function is working with the alarm AM_DOWNSHIFT appeared
The solution to the problem isto make sure that the Line of Sight is cleared from any obstacles either fully or partially. If the Line of Sight is compromised by any blocking, the alarm will keep appearing. Also try to clear the accompany alarms such as MW_BER_SD and MW_LOF. These alarms are appearing for a reason. There is need to have thorough check of the physical connection of the link from the antenna, waveguide, coupler, ODU and IF cables.