5.3.4.3.4 Network Slice based cell reselection and Random Access

When one or more S-NSSAI(s) are associated with NSAG(s), the UE may perform Network Slice based cell reselection and Random Access as described in TS 38.300 [27], TS 38.304 [50], TS 38.331 [28], TS 38.321 [143] and TS 24.501 [47].

When providing NSAG Information to the UE, the AMF shall also provide the NSAG priority information for the NSAGs provided in the NSAG Information. The AMF determines the NSAG priority information based on configured local policy of the serving PLMN or SNPN.

NOTE 1: How the AMF assigns the NSAG priority information per UE is not specified but AMF can take into account information like e.g. UE MM capabilities, Subscribed S-NSSAIs and HPLMN.

NOTE 2: The AMF can assign same priority value for NSAGs provided in the NSAG Information.

If the UE has received NSAG Information from the AMF, the UE shall use the NSAG Information provided by the AMF for cell reselection and Random Access as described below. If the UE has not received any NSAG Information from the AMF, the UE shall not use Network Slice based cell reselection and Random Access at all.

The UE NAS provides to the UE AS the NSAG Information as received from the AMF and the S-NSSAIs in the Allowed NSSAI and any Partially Allowed NSSAI as input to cell reselection, except when the UE intends to register with a new (including any S-NSSAIs rejected partially in the RA) set of S-NSSAIs with a Requested NSSAI different from the current Allowed NSSAI and any Partially Allowed NSSAI, in which case the UE NAS provides to the UE AS layer the NSAG Information as received from the AMF and the S-NSSAIs in the Requested NSSAI and this may trigger a cell reselection, before sending the Registration Request including the new Requested NSSAI.

For Network Slice based Random Access, different Random Access resources may be assigned to different NSAG(s). The UE determines Random Access configuration among NSAGs that are published in SIB for Random Access and that are associated to the S-NSSAIs triggering the access. If the signalling transaction triggering the access attempt is related to more than one network slice and the S-NSSAIs of these network slices are associated with more than one NSAG for Random Access, the NSAG with the highest priority is selected.

NOTE 3: How the UE NAS provides the NSAGs priorities to UE AS is based internal UE interface and not specified.

When a S-NSSAI is replaced by an Alternative S-NSSAI for a UE supporting the Network Slice Replacement feature (see clause 5.15.19) then the AMF provides updated NSAG information also including the Alternative S-NSSAI to the UE when the Alternative S-NSSAI was not part of the UE Configured NSSAI and is Added to the UE configured NSSAI. The NSAG priority may take into account the Alternative S-NSSAI is a replacement of the replaced NSSAI if the Alternative S-NSSAI is not part of the UE subscription.