4.23.3 Registration procedure

23.5023GPPProcedures for the 5G System (5GS)Release 18TS

The following impacts are applicable to clause 4.2.2.2 (Registration procedure) when the UE has established PDU Session(s):

– Step 5: The UE context transferred from old AMF includes SMF information. If I-SMF is available for the PDU Session(s), the received SMF information includes I-SMF information and SMF information.

– Step 10: The (target) AMF determines whether I-SMF insertion/change/removal is needed. If the (target) AMF does not have the service area of the SMF(s), the (target) AMF queries the NRF to get service area information of the received SMF(s). The (target) AMF determines on a per PDU Session basis whether a (new) I-SMF needs to be selected based on UE location and service area of the received SMF information. It includes the following cases:

a. if the received SMF information includes only SMF information and service area of SMF includes the area where the UE camps, new I-SMF selection is not needed; or

b. if the received SMF information includes both I-SMF information and SMF information and service area of I-SMF includes the area where the UE camps, the I-SMF can be reused; or

c. if the received SMF information includes both I-SMF information and SMF information and the UE moves into the service area of SMF, the I-SMF removal process is triggered; or

d. if the received SMF information includes only SMF information and the service area of SMF does not include the area where the UE camps, the (target) AMF selects an I-SMF. The I-SMF insertion process is triggered; or

e. if the received SMF information includes both I-SMF and SMF information and the service area of SMF and I-SMF does not include the area where the UE camps, the (target) AMF selects a new I-SMF. The change of I-SMF process is triggered.

– For each PDU Session, if the UE context retrieved from the old AMF includes an I-SMF and the (target) AMF determines the I-SMF needs to be changed or removed, the (target) AMF includes a corresponding indication in Namf_Communication_RegistrationStatusUpdate sent to old AMF

– Step 17: the (target) AMF contacts the SMF/I-SMF ("cases" below are same as for step 10).

For case a), no additional change to step 17 of clause 4.2.2.2.2 is needed for the update of the PDU Session.

For case b), the SMF in step 17 of clause 4.2.2.2.2 is changed to I-SMF and in addition, the reference clause 4.2.3.2 is changed to clause 4.23.4.2. If the AMF has changed, the new AMF invokes Nsmf_PDUSession_UpdateSMContext (SM Context ID at SMF) towards the I-SMF.

For case c) i.e. for I-SMF removal, the (target) AMF invokes Nsmf_PDUSession_CreateSMContext (SM Context ID at SMF) towards the SMF. Steps from step 10 onwards as described in clause 4.23.4.3 are executed.

For cases d) and e), i.e. for I-SMF insertion or change, the (target) AMF invokes Nsmf_PDUSession_CreateSMContext (PDU Session ID, SM Context ID at SMF) towards the new I-SMF. Steps from step 3 onwards as described in clause 4.23.4.3 are executed with the following enhancements:.

– Step 9 (for cases d and e): The N2 SM information is only provided by the I-SMF to the AMF when N3 tunnel needs to be established (i.e. due to buffered DL data in old I-SMF/old-I-UPF or AMF has indicated to I-SMF to active user plane for the PDU session based on List of PDU Sessions To Be Activated received from the UE).

– Step 16 (i.e. case c): The N2 SM information is only provided by the SMF to the AMF when N3 tunnel need to be established (i.e. due to buffered DL data in old I-SMF/old-I-UPF or the AMF has indicated to the SMF to active user plane for the PDU session based on List of PDU Sessions To Be Activated received from the UE).

– Step 17 is executed when N3 tunnel needs to be established. The NAS message Service Accept is replaced with Registration Accept (i.e. step 21 in clause 4.2.2.2).

– Step 17a and 17b is triggered by old AMF towards the old I-SMF based on the I-SMF change or removal indication received from target AMF, when the timer (i.e. started in step 5 of clause 4.2.2.2) in old AMF expires.

NOTE: Step 17a is executed by old AMF together with steps 14d and 14e in clause 4.2.2.2.

– Steps 18 to 21 (i.e. cases d and e) and steps 22 to 25 (i.e. case c): These steps are executed if N2 SM information is provided by the I-SMF/SMF in step 9 or step 16 above.

– Step 21: This step is omitted if step 17 of clause 4.23.4.3 is executed as described above.