Fault symptom
At a network site in China, the SA network's 5G-to-4G handover success rate averaged about 95% per day. The relatively high handover failure rate degraded users' voice call and data experience.
Fault analysis
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Using AMF counters, failures were found to originate mainly from the MME.
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Using EMS+ signaling statistics, the number of 5G-to-4G failures was counted from 08:00 to 10:00 on November 22, 2021. Handover failure in target 5GC/NG-RAN node or target system accounted for 81.64%, and Unknown Target ID accounted for 18.16%, as shown below.
a. Handover failure in target 5GC/NG-RAN node or target system
Within this category, HO Wait Forward Relocation Response failure accounted for 99.92%. This step is where the MME returns a failure during the 5G-to-4G handover, corresponding to the MME cause in AMF counters, as shown below.
b. Unknown Target ID
For this cause, HO trans fail accounted for 59.55% and Trans aborted accounted for 33.83%. These failures were mainly due to 5G base station configuration issues or link breaks, as shown below.
Troubleshooting
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gNB-related: RAN should check the 5G base station neighbor configuration and verify whether there are link breaks or similar issues.
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MME-related: By capturing signaling for users with frequent handover failures, two main problems were identified.
Problem 1: Forward Relocation Response (reject)
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Signaling traces showed the same user attempting multiple handovers. The Forward Relocation Response failures all occurred on the same MME.
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If the MME itself were faulty, the scope of impact would be larger, so MME-side faults were not initially assumed.
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The handover process involves AMF, MME, and eNB. During handover, AMF forwards TA and eNB parameters to the MME, so these parameters were investigated first.
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When comparing the Forward Relocation Request messages, the TA and eNB information were identical for both failed and successful handovers.
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Checking the eNB revealed that the failing MME could not query that eNB ID. In other words, the eNB was not coupled with that MME, causing handover failure.
Problem 2: 4G base station returns HANDOVER FAILURE
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Signaling traces for the same user showed the MME directly returning HANDOVER FAILURE to the AMF.
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Tracing the HANDOVER FAILURE signaling, normally after MME -> eNB sends a handover request, the eNB -> MME should send a handover request acknowledge.
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In the failed signaling, the MME returned a failure directly to the AMF. This could indicate an MME error or that the MME was simply passing through information from the eNB.
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Failure analysis showed that the MME was passing through the eNB's information, meaning the handover failure was returned by the eNB.
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After RAN investigated the specific causes and addressed the configuration/link issues, the problem was resolved and the handover metric returned to normal.
