5G System Learning - Downlink NAS Transport (2)

I. Downlink NAS Transport Scenario In the 5G system, the downlink NAS transport process is used when the AMF needs to transparently send NAS messages to the UE via the NG-RAN node and there is a logical NG connection associated with the UE, or the AMF has received the RAN UE NGAP ID IE in the INITIAL UE MESSAGE message, or the NG-RAN node has sent the INITIAL UE MESSAGE message through another NG interface instance to start a logical NG connection associated with the UE.

II. NAS Transport Content Processing In addition to the content in 5G System Learning - Downlink NAS Transport, other downlink NAS content is processed as follows;

• If the DOWNLINK NAS TRANSPORT message contains the UE Radio Capability ID IE, the NG-RAN node shall (if supported) use it as specified in TS 23.501 and TS 23.502.
• If the DOWNLINK NAS TRANSPORT message contains the Target NSSAI Information IE, the NG-RAN node may use this information as specified in TS 23.501.
• If the DOWNLINK NAS TRANSPORT message contains a Partially Allowed NSSAI IE, the NG-RAN node (if supported) shall infer the UE's partially allowed network slice from it, store and replace any previously received Partially Allowed NSSAI, and use it as specified in TS 23.50.
• If the DOWNLINK NAS TRANSPORT message contains a Masked IMEISV IE, the NG-RAN node (if supported) shall use it to determine the UE's characteristics for subsequent processing.
• If the Downlink NAS Transport message contains a Mobile IAB Authorization IE, the NG-RAN node (if supported) shall store the received Mobile IAB Authorization status in the UE context. If the Mobile IAB Authorization IE is set to "Unauthorized" for Mobile IAB-MT, the NG-RAN node (if supported) shall ensure that no UE is served by this Mobile IAB node.

3. During the Initial UE Message Interaction Procedure, even if the RAN UE NGAP ID IE has been assigned in the "Initial UE Message" message sent via another NG interface instance, the NG-RAN node SHOULD use the "AMF UE NGAP ID IE" and "RAN UE NGAP ID IE" received in the "Downlink NAS Transport" message as the logical connection identifier.

4. During the UE Radio Capability Information Indication Procedure, if the Downlink NAS Transport message contains a UE Capability Information Request IE set to "requested" and UE capability-related information has been successfully retrieved from the UE, the NG-RAN node SHOULD trigger the UE Radio Capability Information Indication Procedure.

5. Downlink NAS Transport Abnormal Scenarios:

• If a Partially Allowed NSSAI IE is received in a DOWNLINK NAS TRANSPORT message, and the total number of S-NSSAIs contained in the Allowed NSSAI and Partially Allowed NSSAI exceeds 8, the NG-RAN node SHOULD consider the procedure to have failed.
• If any S-NSSAI present in the Partially Allowed NSSAI IE is also present in the Allowed NSSAI IE, the NG-RAN node shall consider the procedure failed.