5.3.4.3.3 Redirection to dedicated frequency band(s) for an S-NSSAI

If a Network Slice, S-NSSAI, is configured to be available only in TAs covering specific dedicated frequency band(s), then there may be a need to redirect the UE to the dedicated frequency band(s) when such S-NSSAI is requested. If the Requested NSSAI contains S-NSSAI(s) that are not available in the UE's current TA, see clause 5.15.8, the AMF itself or by interacting with the NSSF as described in clause 5.15.5.2.1 may determine a Target NSSAI to be used by the NG-RAN, in addition to the information the AMF receives, such as the Allowed NSSAI and the RFSP for the Allowed NSSAI, to attempt to redirect the UE to a cell and TA in another frequency band and TA that supports the S-NSSAIs in the Target NSSAI. The Target NSSAI includes at least one S-NSSAI from the Requested NSSAI not available in the current TA, but available in another TA in different frequency band possibly overlapping with the current TA and optionally additional S-NSSAIs from the Requested NSSAI that are configured to be available within the same TAs as the S-NSSAIs not available in the current TA. If the serving PLMN supports the subscription-based restrictions to simultaneous registration of network slices (see clause 5.15.12) and if the UE has NSSRG as part of the subscription information received from the HPLMN, the Target NSSAI includes only S-NSSAIs sharing at least one NSSRG.

The Target NSSAI may be excluding some of the S-NSSAIs in the Allowed NSSAI and include some of the rejected S-NSSAIs due to lack of support in the TA where the UE is located based on network policies that are in line with customer and operator agreements.

The Target NSSAI shall only include S-NSSAIs that can be provided in an Allowed NSSAI, or in an Allowed NSSAI and Partially Allowed NSSAI, for the UE. The Target NSSAI includes at least one Rejected S-NSSAI and may include e.g.:

• all or a subset of the Rejected S-NSSAIs for RA, all or a subset of the S-NSSAIs rejected partially in the RA, all or a subset of Partially Allowed NSSAI when none of the S-NSSAIs in the Requested S-NSSAI were available in the TA where the UE is;
• all the S-NSSAIs of the Allowed NSSAI, all the S-NSSAIs of the Partially Allowed NSSAI and all or a subset of the Rejected S-NSSAIs for the RA and all or subset of S-NSSAIs rejected partially in the RA;
• a subset of the S-NSSAIs in the Allowed NSSAI, a subset of the S-NSSAIs in the Partially Allowed NSSAI and all or a subset of the Rejected S-NSSAIs for the RA and all or subset of S-NSSAIs rejected partially in the RA, if the operator policy is to prefer this Target S-NSSAI to the Allowed NSSAI.

The AMF should retrieve an RFSP Index suitable for the Target NSSAI and includes the RFSP Index in the information sent to the NG-RAN. The AMF retrieves the RFSP Index from the PCF or, in case PCF is not deployed the AMF determines the RFSP Index according to local configuration. The RFSP index associated to the Target NSSAI is considered if the NG-RAN succeeds to redirect the UE to a new TA where the Target NSSAI, or some S-NSSAIs of the Target NSSAI are supported, otherwise the RFSP index of the Allowed NSSAI is considered.

If the Requested NSSAI contains S-NSSAI(s) which map to S-NSSAI(s) of the HPLMN subject to Network Slice-Specific Authentication and Authorization that are not available in the UE's current TA, the AMF shall proceed with the Network Slice-Specific Authentication and Authorization procedure as described in clause 4.2.9 of TS 23.502 [3]. If the AMF determines a new Allowed NSSAI and/or Partially Allowed NSSAI at the end of Network Slice-Specific Authentication and Authorization steps and some S-NSSAI is not available in the UE's current TA, a Target NSSAI and corresponding RFSP index may be determined and provided to NG-RAN during UE Configuration Update procedure as described in clause 4.2.4.2 of TS 23.502 [3].

The NG-RAN shall attempt to find cells of TAs that can support all the S-NSSAIs in the Target S-NSSAIs and if no such cell of a TA is available the RAN can attempt to select cells of TAs that best match the Target S-NSSAI. The NG-RAN shall attempt to ensure continuity of the PDU Sessions with activated User Plane associated with the S-NSSAIs in the Allowed NSSAI and/or Partially Allowed NSSAI which are in the Target NSSAI. Also, the NG-RAN should attempt to ensure continuity of service for the S-NSSAIs of the Allowed NSSAI and/or Partially Allowed NSSAI also available in the Target NSSAI, before prioritizing cells that are not supporting one or more of the S-NSSAI of the Allowed NSSAI and/or Partially Allowed NSSAI also available in the Target NSSAI.

The NG-RAN attempts to determine target cell(s) supporting the Target NSSAI considering the UE Radio Capabilities (i.e. the AMF (if available in the UE context) shall provide the NG-RAN with the current UE Radio Capability Information or the RACS UE Radio Capability ID when a Target NSSAI is provided, if the NG-RAN had not yet received any of them, or, if the AMF cannot provide any of these, the UE Radio Capability Information may be retrieved by the NG-RAN from the UE).

Once the target cells are determined, the NG-RAN initiates RRC redirection procedure towards the target cells, or the NG-RAN initiates handover for the UE with active PDU Sessions associated with the S-NSSAIs which are in the Target NSSAI, if possible.

After a successful redirection or handover of the UE to a new TA inside the current RA, the UE may request a PDU Session and activate UP resources for a PDU Session for S-NSSAIs of Partially Allowed NSSAI that are supported in the new TA and the UE may request to register S-NSSAIs rejected partially in the RA that are not rejected in the new TA, as described in clause 5.15.17.

After a successful redirection or handover of the UE to a new TA outside the current RA, the UE shall perform a Mobility Registration Update procedure and the S-NSSAIs that the new TA supports can be allowed if the UE requests them. In order to ensure that the UE is redirected to a TA outside the current RA when there are S-NSSAIs Rejected for the RA, thus triggering a Mobility Registration Update procedure enabling the UE to request the S-NSSAI(s) that were rejected for the RA, the AMF shall set the RA so that the RA does not include TAs supporting the S-NSSAIs rejected for the RA included in the Target NSSAI when the AMF provides a Target NSSAI to the RAN.