5.15.5.3 Establishing a PDU Session in a Network Slice

The PDU Session Establishment in a Network Slice instance to a DN allows data transmission in a Network Slice instance. A PDU Session is associated to an S-NSSAI and a DNN. A UE that is registered in a PLMN over an Access Type and has obtained a corresponding Allowed NSSAI, shall indicate in the PDU Session Establishment procedure the S-NSSAI according to the NSSP in the URSP rules or according to the UE Local Configuration as defined in clause 6.1.2.2.1 of TS 23.503 [45] and, if available, the DNN the PDU Session is related to. The UE includes the appropriate S-NSSAI from this Allowed NSSAI and, if mapping of the Allowed NSSAI to HPLMN S-NSSAIs was provided, an S-NSSAI with the corresponding value from this mapping.

If the UE cannot determine any S-NSSAI after performing the association of the application to a PDU Session according to clause 6.1.2.2.1 of TS 23.503 [45], the UE shall not indicate any S-NSSAI in the PDU Session Establishment procedure.

The network (HPLMN) may provision the UE with Network Slice selection policy (NSSP) as part of the URSP rules, see clause 6.6.2 of TS 23.503 [45]. When the Subscription Information contains more than one S-NSSAI and the network wants to control/modify the UE usage of those S-NSSAIs, then the network provisions/updates the UE with NSSP as part of the URSP rules. When the Subscription Information contains only one S-NSSAI, the network needs not provision the UE with NSSP as part of the URSP rules. The NSSP rules associate an application with one or more HPLMN S-NSSAIs. A default rule which matches all applications to a HPLMN S-NSSAI may also be included.

The UE shall store and use the URSP rules, including the NSSP, as described in TS 23.503 [45]. When a UE application associated with a specific S-NSSAI requests data transmission:

• if the UE has one or more PDU Sessions established corresponding to the specific S-NSSAI, the UE routes the user data of this application in one of these PDU Sessions, unless other conditions in the UE prohibit the use of these PDU Sessions. If the application provides a DNN, then the UE considers also this DNN to determine which PDU Session to use. This is further described in clause 6.6.2 of TS 23.503 [45].
• If the UE does not have a PDU Session established with this specific S-NSSAI, the UE requests a new PDU Session corresponding to this S-NSSAI and with the DNN that may be provided by the application. In order for the RAN to select a proper resource for supporting network slicing in the RAN, RAN needs to be aware of the Network Slices used by the UE. This is further described in clause 6.6.2 of TS 23.503 [45].

If the AMF is not able to determine the appropriate NRF to query for the S-NSSAI provided by the UE, the AMF may query the NSSF with this specific S-NSSAI, location information, PLMN ID of the SUPI. The NSSF determines and returns the appropriate NRF to be used to select NFs/services within the selected Network Slice instance. The NSSF may also return an NSI ID to be used to select NFs within the selected Network Slice instance to use for this S-NSSAI.

The AMF or NSSF may select an S-NSSAI (if the UE does not provide an S-NSSAI for the PDU session establishment) and a Network Slice instance, based on load level and/or Observe Service Experience and/or Dispersion analytics from NWDAF, as described in TS 23.288 [86].

The discovery and selection of the NSSF is defined in clause 6.3.28.

SMF discovery and selection within the selected Network Slice instance is initiated by the AMF when a SM message to establish a PDU Session is received from the UE. The appropriate NRF is used to assist the discovery and selection tasks of the required network functions for the selected Network Slice instance.

The AMF queries the appropriate NRF to select an SMF in a Network Slice instance based on S-NSSAI, DNN, NSI-ID (if available) and other information e.g. UE subscription and local operator policies, when the UE triggers PDU Session Establishment. The AMF may select the SMF among the set of the SMF instance(s) returned by the NRF or locally configured in the AMF, based on network data analytics (NF load, etc.) from the NWDAF as described in TS 23.288 [86]. The selected SMF establishes a PDU Session based on S-NSSAI and DNN.

When the AMF belongs to multiple Network Slice instances, based on configuration, the AMF may use an NRF at the appropriate level for the SMF selection.

For further details on the SMF selection, refer to clause 4.3.2.2.3 of TS 23.502 [3].

When a PDU Session for a given S-NSSAI is established using a specific Network Slice instance, the CN provides to the (R)AN the S-NSSAI corresponding to this Network Slice instance to enable the RAN to perform access specific functions.

The UE shall not perform PDU Session handover from one Access Type to another if the S-NSSAI of the PDU Session is not included in the Allowed NSSAI of the target Access Type.

In non-roaming case, if the Serving PLMN ID is not the same as the PLMN ID of the SUPI and a mapped S-NSSAI value of the HPLMN is present in the Allowed NSSAI, the procedures as defined in clause 4.23.5 of TS 23.502 [3] is used for PDU Session Management, with the following differences:

• In PDU Session Establishment procedure the AMF selects the SMF according to the S-NSSAI of the mapped S-NSSAI value of the HPLMN and selects I-SMF according to the S-NSSAI of the S-NSSAI value of the Serving PLMN and provides the S-NSSAI of the Serving PLMN and the mapped S-NSSAI of the HPLMN as received from the UE to the selected (I-)SMF.