5.15.3 Subscription aspects

The Subscription Information shall contain one or more S-NSSAIs i.e. Subscribed S-NSSAIs. The subscription information shall include at least one default S-NSSAI. The UDM sends at the most 16 Subscribed S-NSSAIs to AMF, i.e. the number that can fit in a Configured NSSAI. The subscription information the UDM sends to the AMF shall include at least one default S-NSSAI.

If an S-NSSAI is marked as default, then the network is expected to serve the UE with a related applicable Network Slice instance when the UE does not send any permitted S-NSSAI to the network in a Registration Request message as part of the Requested NSSAI.

The Subscription Information for each S-NSSAI may contain:

• a Subscribed DNN list and one default DNN; and
• the indication whether the S-NSSAI is marked as default Subscribed S-NSSAI; and
• the indication whether the S-NSSAI is subject to Network Slice-Specific Authentication and Authorization; and
• Network Slice Simultaneous Usage Group (NSSRG) information (see clause 5.15.12).

The network verifies the Requested NSSAI the UE provides in the Registration Request against the Subscription Information. For the S-NSSAIs subject to Network Slice-Specific Authentication and Authorization the clause 5.15.10 applies.

NOTE 1: It is recommended that at least one of the Subscribed S-NSSAIs marked as default S-NSSAI is not subject to Network Slice-specific Authentication and Authorization, in order to ensure access to services even when Network Slice-specific Authentication and Authorization fails.

NOTE 2: It is recommended to minimize the number of Subscribed S-NSSAIs in subscriptions for NB-IoT or NR RedCap capable UEs to minimize overhead for signalling a large number of S-NSSAIs in Requested NSSAI in RRC and NAS via NB-IoT or NR RedCap.

In roaming case, the UDM shall provide to the VPLMN only the S-NSSAIs from the Subscribed S-NSSAIs the HPLMN allows for the UE in the VPLMN. If the UE is subject to restrictions of simultaneous registration of network slices (i.e. if the Subscription Information for the S-NSSAIs contains NSSRG information), the UDM provides to the VPLMN a subscribed S-NSSAIs and, if applicable, NSSRG information, as described in clause 5.15.12.

NOTE 3: Network slice instances supporting an S-NSSAI subject to Network Slice-Specific Authentication and Authorization need to be deployed with AMFs supporting Network Slice-Specific Authentication and Authorization, otherwise S-NSSAIs requiring Network Slice-Specific Authentication and Authorization would be incorrectly allowed without execution of Network Slice-Specific Authentication and Authorization.

NOTE 4: Network slice instances supporting an S-NSSAI subject to Network Slice Admission Control (NSAC) for number of registered UE per network slice need to be deployed with AMFs supporting NSAC, otherwise S-NSSAIs requiring NSAC would be incorrectly allowed without execution of NSAC.

When the UDM updates the Subscribed S-NSSAI(s) to the serving AMF, based on configuration in this AMF, the AMF itself or the NSSF determines the mapping of the Configured NSSAI for the Serving PLMN and/or Allowed NSSAI to the Subscribed S-NSSAI(s). The serving AMF then updates the UE with the above information as described in clause 5.15.4.