5G System Learning - Downlink NAS Transport

1. Downlink NAS: The Downlink NAS Transfer procedure is used when the AMF only needs to transparently send NAS messages to the UE via the NG-RAN node and a logical NG connection associated with the UE exists, or the AMF has received the RAN UE NGAP ID IE in the INITIAL UE MESSAGE message, or the NG-RAN node has initiated a logical NG connection associated with the UE by sending an INITIAL UE MESSAGE message via another NG interface instance.

2. Downlink NAS Transfer is shown in Figure 8.6.2.2-1 below, where:

The AMF initiates this procedure by sending a DOWNLINK NAS TRANSPORT message to the NG-RAN node. If a logical NG connection associated with the UE is not established, the AMF shall assign a unique AMF UE NGAP ID to the UE and include it in the DOWNLINK NAS TRANSPORT message. Upon receiving the AMF-UE NGAP ID IE in the DOWNLINK NAS TRANSPORT message, the NG-RAN node establishes a logical NG connection associated with the UE.

• If the DOWNLINK NAS TRANSPORT message includes the RAN Paging Priority IE, the NG-RAN node may use it to determine the priority for paging the UE in the RRC_INACTIVE state. The NAS-PDU IE contains an AMF-UE message, which is forwarded uninterpreted within the NG-RAN node.

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• If the Mobility Restriction List IE is included in the Downlink NAS Transport message, the NG-RAN node shall overwrite any previously stored mobility restriction information in the UE context.If the downlink NAS transport message contains information in the Mobility Restriction List IE, the NG-RAN node shall use this information to: determine the target of subsequent mobility operations, and the NG-RAN node shall provide information about the target of the mobility operation to the UE;

3. Select the appropriate SCG during dual connectivity operation; assign the appropriate RNA to the UE when moving it to the RRC_INACTIVE state; and, in addition:

• If the downlink NAS transport message does not contain the Mobility Restriction List IE and no mobility restriction information has been previously stored, the NG-RAN node shall assume that the UE is not subject to roaming and access restrictions, except for PNI-NPN mobility as described in TS 23.501.

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• The NG-RAN node shall assume that roaming or access to a CAG cell is permitted only if the downlink NAS transport message contains the Allowed PNI-NPN List IE, as described in TS 23.501.

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• If the downlink NAS transport message contains the RAT/Frequency Selection Priority Index IE, the NG-RAN node shall (if supported) use it as defined in TS 23.501.

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• If the AMF has not previously sent the UE Aggregate Maximum Bit Rate IE, it shall be sent to the NG-RAN node. If included in a downlink NAS transport message, the NG-RAN node shall store the UE aggregate maximum bit rate in the UE context and use the received UE aggregate maximum bit rate for all non-GBR QoS flows of the associated UE as defined in TS 23.501.

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• If the Legacy AMF IE is included in a downlink NAS transport message, the NG-RAN node shall consider the logical NG connection associated with this UE to have been redirected from another AMF identified by the Legacy AMF IE to this AMF. If the Extended Legacy AMF IE is included in a downlink NAS transport message, the NG-RAN node shall (if supported) consider the logical NG connection associated with this UE to have been redirected from another AMF identified by the Extended Legacy AMF IE to this AMF.

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• If the SRVCC Operation Possible IE is included in the DOWNLINK NAS TRANSPORT message, the NG-RAN node shall (if supported) store the contents of the received SRVCC Operation Possible IE in the UE context and use it as defined in TS 23.216. If the Extended Connected Time IE is included in the DOWNLINK NAS TRANSPORT message, the NG-RAN node shall (if supported) use it as defined in TS 23.501.

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• If the Enhanced Coverage Restriction IE is included in the DOWNLINK NAS TRANSPORT message, the NG-RAN node shall (if supported) store this information in the UE context and use it as defined in TS 23.501.

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• If the UE Differentiation Information IE is included in the DOWNLINK NAS TRANSPORT message, the NG-RAN node shall (if supported) store this information in the UE context for further use in accordance with TS 23.50.

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• If the CE-mode-B Restriction IE is included in the downlink NAS transport message, and the Enhanced Coverage Restriction IE is not set to "Restricted", and the Enhanced Coverage Restriction information stored in the UE context is not set to "Restricted", the NG-RAN node shall (if supported) store this information in the UE context and use it as defined in TS 23.501.

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• If the UE Radio Capabilities IE is included in the downlink NAS transport message, the NG-RAN node shall store this information in the UE context and use it as defined in TS 38.300.

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• If the End Indication IE is included in the downlink NAS transport message and is set to "No Additional Data", the NG-RAN node shall consider the UE Radio Capabilities IE to be in addition to the included NAS PDU.