5G NR NSA Network Access Issues and Possible Root Cause

Many of the operator across the global has started providing 5G services using NSA mode with Option 3.x . In NSA mode the network topology consist of a LTE cell, NR cell both connected to 4G Core Network. To get access to the 5G services UE has to first connect to 4G LTE cell as LTE is the master cell and then later based on the measurement a suitable 5G NR cell is added to UE.

In this post, we have listed some of the most common field issue seen when a 5G UE want access to the 5G NR cell access. At high level this issues can be classified in following categories / issues:

  • Issue #0 UEs fail to access the LTE Cell
  • Issue #1 : B1 measurement is not delivered after the UE accesses the LTE network
  • Issue #2: UE does not report the B1 measurement report
  • Issue #3: Upon Receiving the B1 event, LTE cell does not initiate the SgNB ADD Request
  • Issue #4 SgNB ADD Request is rejected by 5G NR Cell
  • Issue #5 LTE network does not reply with a SgNB Reconfiguration Complete message
  • Issue #6 UE does not initiate RACH over the air interface
  • Issue #7 MSG 2 (RAR Message) over the air interface times out
  • Issue #8 Failed to access the MSG3 over the air interface
  • Issue #9 Call is released immediately after the access is successful

For more deatils, please check:

Can any one brothers elaborate issue #9 and give more insight about it?

(Issue #9 Call is released immediately after the access is successful)

Cause 9 refers that LTE bearer is released immediately after sg is added.
Check for “S1AP ERAB Modification Configuration message contains the Cause IE=radionetworkunspecified” and then it is a core network issue.

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For the Cause IE=radio network unspecified"…
We faced same behavior with MME-CMM (nokia) and LTE NETWORK was multiple of (Nokia. Ericsson, zte) and the reason behind was belong to the profile sctp parameter not well configured from MME.

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So S1/SCTP not properly configured for LTE that is still an MME issue.

This kind of issue cannot be detected in drivetest. It needs complete OSS traces Uu, S1 and Xn traces to locate the fault in MME related to SCTP.

Those kind of events you will not see them in other drive tests logs unless you dig heavily inside message browser or you use a strong post processing tool.

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This issue can be directly related with erab failure rate.

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QXDM has same level debugging capabilities.
One need to enable debug logs in qxdm settings.

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