4.6.2.5 Mobility management-based network slice admission control

A serving PLMN or SNPN can perform network slice admission control for the S-NSSAI(s) subject to NSAC to monitor and control the number of registered UEs per network slice. The timing of the network slice admission control is managed by the EAC mode per network slice, which can be either activated or deactivated for the network performing network slice admission control. The EAC mode is activated when the number of UEs associated with the S-NSSAI reaches a certain threshold (see 3GPP TS 23.502 [9])

If the EAC mode is activated for an S-NSSAI, the AMF performs network slice admission control before the S-NSSAI subject to NSAC is included in the allowed NSSAI or the partially allowed NSSAI sent to the UE. During a registration procedure (including initial registration or mobility registration updating from another AMF), if the AMF determines that the maximum number of UEs has been reached for:

• one or more S-NSSAIs but not all S-NSSAIs in the requested NSSAI, then the AMF includes the allowed NSSAI or the partially allowed NSSAI and the rejected NSSAI accordingly in the REGISTRATION ACCEPT message as specified in the clauses 5.5.1.2.4 and 5.5.1.3.4;
• all S-NSSAIs in the requested NSSAI but there are one or more default S-NSSAIs which can be allowed to the UE, then the AMF includes the allowed NSSAI or the partially allowed NSSAI containing these default S-NSSAIs and the rejected NSSAI accordingly in the REGISTRATION ACCEPT message as specified in the clauses 5.5.1.2.4 and 5.5.1.3.4; or
• all S-NSSAIs in the requested NSSAI and there are no default S-NSSAIs which can be allowed to the UE, then the AMF includes the rejected NSSAI accordingly in the REGISTRATION REJECT message as specified in the clauses 5.5.1.2.5 and 5.5.1.3.5.

If the EAC mode is deactivated for an S-NSSAI, the AMF performs network slice admission control after the S-NSSAI subject to NSAC is included in the allowed NSSAI or the partially allowed NSSAI sent to the UE. While the AMF is waiting for response from the NSACF for the S-NSSAI, the AMF processes the NAS signalling message related to the S-NSSAI as usual i.e. like S-NSSAI in the allowed NSSAI or the partially allowed NSSAI. After the network performs the network slice admission control, if the AMF determines that the maximum number of UEs has been reached for:

• one or more S-NSSAIs but not all S-NSSAIs in the allowed NSSAI or the partially allowed NSSAI, then the AMF updates the allowed NSSAI or the partially allowed NSSAI and the rejected NSSAI accordingly using the generic UE configuration update procedure as specified in the clause 5.4.4;
• for all S-NSSAIs in the allowed NSSAI or the partially allowed NSSAI but there are one or more default S-NSSAIs which can be allowed to the UE, then the AMF updates the allowed NSSAI or the partially allowed NSSAI containing these default S-NSSAIs and the rejected NSSAI accordingly using the generic UE configuration update procedure as specified in the clause 5.4.4; or
• for all S-NSSAIs in the allowed NSSAI or the partially allowed NSSAI and there are no default S-NSSAIs which can be allowed to the UE, then the AMF performs the network-initiated de-registration procedure and includes the rejected NSSAI in the DEREGISTRATION REQUEST message as specified in the clause 5.5.2.3 except when the UE has an emergency PDU session established or the UE is establishing an emergency PDU session.

When the UE has an emergency PDU session established or the UE is establishing an emergency PDU session, the AMF updates the rejected NSSAI using the generic UE configuration update procedure as specified in the clause 5.4.4 and informs the SMF to release all PDU sessions associated with the S-NSSAI. During the generic UE configuration update procedure, the AMF includes the 5GS registration result IE in the CONFIGURATION UPDATE COMMAND message and sets the Emergency registered bit of the 5GS registration result IE to "Registered for emergency services". After the emergency PDU session is released, the AMF performs the network-initiated de-registration procedure as specified in the clause 5.5.2.3.

Based on operator policy, the mobility management based network slice admission control is not applicable for the S-NSSAI used for emergency services, or the mobility management based network slice admission control result is ignored for the S-NSSAI used for emergency services.

Based on operator policy, the mobility management based network slice admission control is not applicable for the UEs configured for priority services, or the mobility management based network slice admission control result is ignored for the UEs configured for priority services.

NOTE: A UE configured for priority services can be identified based on the RRC establishment cause received from the NG-RAN or based on the MPS priority information in the user's subscription context obtained from the UDM.

The mobility management based network slice admission control is not applicable to a UE that is registering or registered for onboarding services in SNPN.