5.15.18.3 Network based monitoring and enforcement of Network Slice Area of Service not matching deployed Tracking Areas

OAM may configure RRM policies for S-NSSAIs on a per cell basis as defined in TS 28.541 [149], i.e. cells outside the Network Slice Area of Service while in a TA supporting the S-NSSAI are allocated with no RRM resources for the S-NSSAI.

The network may enforce the NS-AoS for an S-NSSAI as follows:

1. The network may monitor the validity of the S-NSSAI for UE in CM-CONNECTED state, i.e. the AMF subscribes to the AoI using the Location information of the S-NSSAI location availability information as described in TS 38.413 [34].

2. If the non-supporting UE makes a PDU Session establishment request with an S-NSSAI that is not valid as per the S-NSSAI location availability information, the AMF may reject the NAS Transport message with a back-off timer using S-NSSAI based congestion control as described in clause 5.19.7.4.

3. If the AMF determines that the UE in CM-CONNECTED has moved outside the NS-AoS, the AMF performs the following logic:

• If the non-supporting UE has other S-NSSAI(s) in the Allowed NSSAI, then the AMF may update the UE with a UE Configuration Update by removing the S-NSSAI from the Allowed NSSAI and optionally indicating the S-NSSAI as rejected in the Registration Area and then, the AMF requests the SMF to release any PDU Sessions with that S-NSSAI as per step 1f in clause 4.2.3.4 in TS 23.502 [3].
• If the non-supporting UE does not have any other S-NSSAI in the Allowed NSSAI, then the AMF may update the UE with a UE Configuration Update by removing the S-NSSAI from the Allowed NSSAI and optionally which optionally also signals to the UE that the S-NSSAI is rejected in the Registration Area and adding a default S-NSSAI to the Allowed NSSAI (if one exists that is not the same as the S-NSSAI in the rejected S-NSSAI) and then, the AMF requests the SMF to release any PDU Sessions with the removed S-NSSAI as per step 1f in clause 4.3.4.2 in TS 23.502 [3].

NOTE 1: When the AMF enforces the NS-AoS, it is up to AMF configuration whether the AMF removes the S-NSSAI only from Allowed NSSAI, or also indicates the S-NSSAI as rejected S-NSSAI in the RA, or only releases the associated PDU Sessions (as for the case in step c here below).

• For a non-supporting UE that does not have any other S-NSSAI which could be included in the Allowed NSSAI, then the AMF indicates to the SMF to release the PDU Session.

4. If the AMF determines that the S-NSSAI becomes valid e.g. the UE has moved into the NS-AoS, the AMF may update the UE with a UCU e.g. including the S-NSSAI in the Allowed NSSAI.

5. When the AMF determines that the S-NSSAI of a PDU Session is restricted to an NS-AoS in the PDU session, the AMF indicates to the SMF that the PDU Session is subject to area restriction for the S-NSSAI. As a result, the SMF subscribes to "UE mobility event notification" event for reporting UE presence in Area of Interest by providing S-NSSAI to the AMF as described in clauses 5.6.11 and 5.3.4.4 if this event has not been subscribed before. The supporting AMF shall provide this indication in all the subsequent PDU Session update message to the SMF as long as the S-NSSAI of the PDU Session is subject to area restriction. The AMF determines the UE presence in Area of Interest as described in Annex D, clauses D.1 and D.2 of TS 23.502 [3] and notifies the result to the SMF. When SMF is notified that the UE location is outside of Area of Interest, SMF shall not send user data as payload of NAS message (see clause 5.31.4.1) in downlink directions and disable data notification. When the AMF does not indicate to the SMF that the PDU Session is subject to area restriction for the S-NSSAI, based on local configuration, the SMF may unsubscribe the "UE mobility event notification" event in AMF if this event has been subscribed before.

6. When the SMF is notified by the AMF that the UE location is UNKNOWN as defined in Annex D, clauses D.1 and D.2 of TS 23.502 [3], then based on operator policy SMF may enable downlink data notification and trigger the Network triggered Service Request procedure to active the UP connection or send user data as payload of a NAS message (see clause 5.31.4.1) when the SMF receives downlink data or Data Notification from UPF.

NOTE 2: When I-SMF/V-SMF is inserted for the PDU session it is the I-SMF/V-SMF to handle the area restrictions for the S-NSSAI. The SMF/H-SMF is not involved in this procedure.