5.15.5.2.2 Modification of the Set of Network Slice(s) for a UE

The set of Network Slices for a UE can be changed at any time while the UE is registered with a network and may be initiated by the network, or by the UE, under certain conditions as described below.

The network, based on local policies, subscription changes and/or UE mobility and/or UE Dispersion data classification, operational reasons (e.g. a Network Slice instance is no longer available or load level information or service experience for a Network Slice or network slice instance provided by the NWDAF), may change the set of Network Slice(s) to which the UE is registered and provide the UE with a new Registration Area and/or Allowed NSSAI and the mapping of this Allowed NSSAI to HPLMN S-NSSAIs, for each Access Type over which the UE is registered. In addition, the network may provide the Configured NSSAI for the Serving PLMN, the associated mapping information and the rejected S-NSSAIs. The network may perform such a change over each Access Type during a Registration procedure or trigger a notification towards the UE of the change of the Network Slices using a UE Configuration Update procedure as specified in clause 4.2.4 of TS 23.502 [3]. The new Allowed NSSAI(s) and the mapping to HPLMN S-NSSAIs are determined as described in clause 5.15.5.2.1 (an AMF Re-allocation may be needed). The AMF provides the UE with:

• an indication that the acknowledgement from UE is required;
• Configured NSSAI for the Serving PLMN (if required), rejected S-NSSAI(s) (if required) and TAI list and
• the new Allowed NSSAI with the associated mapping of Allowed NSSAI for each Access Type (as applicable) unless the AMF cannot determine the new Allowed NSSAI (e.g. all S-NSSAIs in the old Allowed NSSAI have been removed from the Subscribed S-NSSAIs).

Furthermore:

• If the changes to the Allowed NSSAI require the UE to perform immediately a Registration procedure because they affect the existing connectivity to AMF (e.g. the new S-NSSAIs require a separate AMF that cannot be determined by the current serving AMF, or the AMF cannot determine the Allowed NSSAI) or due to AMF local policies also when the changes does not affect the existing connectivity to AMF:
• The serving AMF indicates to the UE the need for the UE to perform a Registration procedure without including the GUAMI or 5G-S-TMSI in the access stratum signalling after entering CM-IDLE state. The AMF shall release the NAS signalling connection to the UE to allow to enter CM-IDLE after receiving the acknowledgement from UE.
• When the UE receives indications to perform a Registration procedure without including the GUAMI or 5G-S-TMSI in the access stratum signalling after entering CM-IDLE state, then:
• The UE deletes any stored (old) Allowed NSSAI and associated mapping as well as any (old) rejected S-NSSAI.
• The UE shall initiate a Registration procedure with the registration type Mobility Registration Update after the UE enters CM-IDLE state as specified in as described in step 4 of clause 4.2.4.2 of TS 23.502 [3]. The UE shall include a Requested NSSAI (as described in clause 5.15.5.2.1) with the associated mapping of Requested NSSAI in the Registration Request message. Also, the UE shall include, subject to the conditions set out in clause 5.15.9, a Requested NSSAI in access stratum signalling but no GUAMI.
• If the AMF determines that the S-NSSAI in the Allowed NSSAI is replaced with Alternative S-NSSAI over an Access Type, the AMF provides the mapping of old S-NSSAI to the Alternative S-NSSAI to the UE over the same Access Type (as described in clause 5.15.19).

If there is an established PDU Session associated with emergency services, then the serving AMF indicates to the UE the need for the UE to perform a Registration procedure but does not release the NAS signalling connection to the UE. The UE performs the Registration procedure only after the release of the PDU Session used for the emergency services.

In addition to sending the new Allowed NSSAI to the UE, when a Network Slice used for a one or multiple PDU Sessions is no longer available for a UE, the following applies:

• If the Network Slice becomes no longer available under the same AMF and the Network Slice Replacement is not used (e.g. due to UE subscription change), the AMF indicates to the SMF(s) which PDU Session ID(s) corresponding to the relevant S-NSSAI shall be released. SMF releases the PDU Session according to clause 4.3.4.2 of TS 23.502 [3]. If the Network Slice Replacement is used, the AMF performs Network Slice Replacement as described in clause 5.15.19.
• If the Network Slice becomes no longer available upon a change of AMF (e.g. due to Registration Area change), the new AMF indicates to the old AMF that the PDU Session(s) corresponding to the relevant S-NSSAI shall be released. The old AMF informs the corresponding SMF(s) to release the indicated PDU Session(s). The SMF(s) release the PDU Session(s) as described in clause 4.3.4 of TS 23.502 [3]. Then the new AMF modifies the PDU Session Status correspondingly. The PDU Session(s) context is locally released in the UE after receiving the PDU Session Status in the Registration Accept message.

The UE uses either the URSP rules (which includes the NSSP) or the UE Local Configuration as defined in clause 6.1.2.2.1 of TS 23.503 [45] to determine whether ongoing traffic can be routed over existing PDU Sessions belonging to other Network Slices or establish new PDU Session(s) associated with same/other Network Slice.

In order to change the set of S-NSSAIs the UE is registered to over an Access Type, the UE shall initiate a Registration procedure over this Access Type as specified in clause 5.15.5.2.1.

If, for an established PDU Session:

• none of the values of the S-NSSAIs of the HPLMN in the mapping of the Requested NSSAI to S-NSSAIs of the HPLMN included in the Registration Request matches the S-NSSAI of the HPLMN associated with the PDU Session; or
• none of the values of the S-NSSAIs in the Requested NSSAI matches the value of the S-NSSAI of HPLMN associated with the PDU Session and the mapping of the Requested NSSAI to S-NSSAIs of the HPLMN is not included in the Registration Request,

the network shall release this PDU Session as follows.

• the AMF informs the corresponding SMF(s) to release the indicated PDU Session(s). The SMF(s) release the PDU Session(s) as described in clause 4.3.4 of TS 23.502 [3]. Then the AMF modifies the PDU Session Status correspondingly. The PDU Session(s) context is locally released in the UE after receiving the PDU Session Status from the AMF.

A change of the set of S-NSSAIs (whether UE or Network initiated) to which the UE is registered may, subject to operator policy, lead to AMF change, as described in clause 5.15.5.2.1.

If the AMF supports the Network Slice Replacement feature and is configured to use the NSSF to trigger the Network Slice Replacement, the AMF subscribes with the NSSF for notifications when any of the S-NSSAIs served by the AMF (e.g. the S-NSSAI in the Serving PLMN and the HPLMN S-NSSAI in roaming case) has to be replaced as described in clause 5.15.19.

If the AMF supports the Network Slice Instance Replacement and configured to use Network Slice Instance Replacement, the AMF subscribes with the NSSF for notifications when a Network Slice instances served by the AMF is congested or no longer available as described in clause 5.15.20.

The AMF may perform Network Slice Replacement for the PDU Session as described in clause 5.15.19.