4.6.2.10 Mobility management aspects of handling network slices with NS-AoS not matching deployed tracking areas

An operator can choose to let the NS-AoS of an S-NSSAI not match the existing tracking area boundaries (see clause 5.15.18 of 3GPP TS 23.501 [8]). In order to support this deployment option, the operator has to ensure that an AMF covering the NS-AoS operates as described below.

The support for S-NSSAI location validity information by the UE and the network, respectively, is optional. If a UE supports S-NSSAI location validity information, the UE indicates that it supports S-NSSAI location validity information during the registration procedure (see clause 5.5.1). The AMF can provide a UE which has indicated that it supports S-NSSAI location validity information with S-NSSAI location validity information (see clauses 5.4.4 and 5.5.1). The S-NSSAI location validity information consists of, for each of the applicable S-NSSAI(s) in the configured NSSAI:

• an S-NSSAI; and
• a list of cell identities of TA(s) belonging to the registration area where the related S-NSSAI(s) is available in some cells but not all cells of one or more TAs, which represents the NS-AoS of the S-NSSAI.

The UE shall consider itself to be inside the NS-AoS of the S-NSSAI if the cell identity of the current serving cell matches any of the identities in the S-NSSAI location validity information. Otherwise, the UE shall consider itself to be outside the NS-AoS of the S-NSSAI.

NOTE 1: The cell identity of the current serving cell is received from the lower layers.

For an S-NSSAI in the S-NSSAI location validity information, even if the S-NSSAI is included in the rejected NSSAI with a rejection cause value set to "S-NSSAI not available in the current registration area" or is included in the partially rejected NSSAI, the UE is allowed to request the S-NSSAI if the UE determines that it is inside the NS-AoS of the S-NSSAI.

NOTE 2: If the service area list for the service area restrictions is available in the UE, the service area list is evaluated before the S-NSSAI location validity information.

For an S-NSSAI limited by NS-AoS, if the UE in 5GMM-CONNECTED mode does not support S-NSSAI location validity information and the AMF determines that:

• the UE is outside the NS-AoS, then the AMF may:
• provide the UE with an allowed NSSAI or a partially allowed NSSAI excluding the S-NSSAI, and optionally a rejected NSSAI for the current registration area including the S-NSSAI; and

NOTE 3: If excluding the S-NSSAI limited by NS-AoS results in an empty allowed NSSAI or partially allowed NSSAI, the AMF includes one or more default S-NSSAIs in the provided allowed NSSAI or partially allowed NSSAI.

• indicate to the SMF to release all PDU sessions associated with the S-NSSAI; or
• the UE is in the NS-AoS, then the AMF may update the allowed NSSAI or partially allowed NSSAI to include the S-NSSAI in the allowed NSSAI or partially allowed NSSAI.

If the UE that does not support S-NSSAI location validity information requests a PDU session establishment for an S-NSSAI limited by NS-AoS and the AMF determines that the UE is outside the NS-AoS of the S-NSSAI, the AMF may perform S-NSSAI based congestion control for the S-NSSAI as specified in clauses 5.3.11 and 5.4.5.

The S-NSSAI location validity information is only applicable to 3GPP access.