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Possible 5G NSA abnormal release root causes

Mohamed ELAdawi

Mohamed ELAdawi

— 2 min read
Possible 5G NSA abnormal release root causes

This post summarizes possible 5G NSA abnormal release root causes:

  • The detection of NR Radio Link Failure (RLF) can be carried out by either the UE(MN) or the gNodeB (SN).
  • A 5G Call Drop (RLF) is identified and declared through various factors, including, and not limited to the following:

    The following summarizes most of the possible root causes of 5G NSA retainability issues:

    1- 4G RF issues     (RRC Reestablishment): This is mainly RF issues in the 4G Leg. For example RRC Reestablishment failure.

    2 - Core Related After the SgNB Addition was completed, the eNodeB (MN) initiated an e-RAB Modification indication to the MME to change the traffic path from SGW to MN to SGW to SN. During the path switch exchange, there might be scenarios where the MME does not reply to the E-RAB Modification indication, and this will be counted as a 5G NSA Drop.

    3 - Transmission Fault: This might be caused by an X2 interface fault during transmission over X2-U. For example, in the scenario assumed below, there was an X2 Link fault during the SN change procedures, which might lead to a drop.

    4- 5G MAX DL RTX Reached: Once the maximum number of DL RLC Retransmissions reaches the configured threshold, the g-NB (SN) will initiate an SgNB Release Required message, which causes radio-connection-with-UE-Lost.

    5- Random Access Problems: In 5G NSA, RACH procedures towards the 5G Leg start after the SgNB addition procedure is completed. Accordingly, any failures in the RA Process will lead to a 5G NSA Drop. As shown in the figure below, once the RA Process fails, the UE initiates an SCG Failure information with the cause randomAccessProblem to the eNodeB (MN). Then, the MN initiates an SgNB Release request due to RAProblem to the SN.

    6- MAX UL RLC RTX Reached: During 5G NSA UL Data transfer, the UE might face a UL Interference or Poor UL Coverage/Quality which might lead into problems in UL Packets delivery. As shown in the figure below, once the maximum number of UL RLC Retransmissions reaches the configured threshold, the UE reported a SCG Failure information with cause rlc-MaxNumRetx then the e-NB (SN) will initiate an SgNB Release request message, which includes the same cause.

    7- T310Expiry
    UE will report SCG Failure information with cause When Radio Link Failure is detected is detected due to T310 expires.

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