How it Works
This section describes how this feature works.
During the PCF-initiated modification, when the Xn-handover is in progress, the following scenarios are noted:
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The AN gets released or the UE is in RRC inactive state and not reachable.
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The AMF relays the received unsuccessful transfer radio network cause code, in the N2 content of the SmContextUpdate message to SMF.
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These cause codes could be standard radio network causes or there could be some customized radio network cause codes being sent from the gNB.
Previously, these cause codes were rejected by the SMF and the PCF was attempting multiple times the same PCF modifications. Now, the SMF doesn't reject immediately, and behaves differently for different cause codes, based on the new N2 trigger configuration to avoid multiple reattempts from the PCF.
The following scenarios are supported in the SMF for the PDU Modify procedure, based on the received N2 cause code:
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When the cause code indicates that the Xn-handover is in progress or the AN gets released, then the following activities occur:
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The SMF suspends the ongoing PDU session modification.
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It resumes back after the Xn-handover or the AN Release.
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When the cause code indicates that the UE is RRC inactive and not reachable, then the following activities occur:
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The SMF rejects the PDU session modification.
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It reports the rule failure to the PCF.
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By default, this feature gets activated for a few standard RRC inactive cause codes with default guard timeout and zero max-retry.
For the following cause codes, the SMF suspends session modification, and resumes only after the Xn-handover activity gets over:
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_RadioNetwork_NG_intra_system_handover_triggered
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_RadioNetwork_NG_inter_system_handover_triggered
For the following cause codes, the SMF rejects the session modification:
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_RadioNetwork_UE_in_RRC_INACTIVE_state_not_reachable
Along with the standard cause codes, a new N2 trigger CLI is introduced to configure the different customized radio network cause codes, and the corresponding SMF actions.
Note | The non-roaming PCF-initiated modification scenarios are supported as a part of this feature. |