5.15.11.0 General

The Network Slice Admission Control Function (NSACF) monitors and controls the number of registered UEs per network slice and/or the number of PDU Sessions per network slice for the network slices that are subject to Network Slice Admission Control (NSAC). The NSACF is configured with the maximum number of UEs and/or the maximum number of PDU Sessions allowed to be served per S-NSSAI subject to NSAC. The NSACF is also configured with information indicating applicable access type(s) for the S-NSSAI (i.e. 3GPP Access Type, Non-3GPP Access Type, or both).

The NSACF also provides event-based Network Slice status notifications and reports to the consumer NFs (e.g. AF).

A NSACF can be configured with the NSAC Service Area(s) it serves. The NSAC Service Area Identifier is a unique identifier in a PLMN or SNPN. The consumer NFs which use the NSAC services are configured with a single and network slice independent value of the NSAC Service Area Identifier so they can discover the correct NSACF (see clause 6.3.22).

The NSACF may be responsible for one or more S-NSSAIs. For one S-NSSAI there may be one or multiple NSACFs deployed in a network (a PLMN or a SNPN) as follows:

• If a PLMN or SNPN is configured with a single NSAC service area, there is a single NSACF configured with the maximum number of UEs per network slice and/or the maximum number of PDU Sessions per network slice, which are valid in the network. In this case there is no need to provision any NSAC Service Area Identifier value in the PLMN or SNPN.
• If a PLMN or SNPN is configured with multiple NSAC service areas, an NSACF may be deployed on a NSAC service area basis, which can be one NSACF instance or one NSACF Set. There are three NSAC architecture options:
• Option 1: non-Hierarchical NSAC architecture. In this architecture, independent NSACFs are deployed in every NSAC service area. There is no interaction between the NSACFs deployed in different NSAC service areas. Each NSACF is configured with the maximum number of UEs per network slice and/or the maximum number of PDU Sessions which are valid in the NSAC service area (see clauses 5.15.11.1.1 and 5.15.11.2.1 for more details).
• Option 2: Centralized NSAC architecture. In this architecture, a single centralized NSACF is deployed in the network to handle admissions in all NSAC service areas. The centralized NSACF is configured with the total number of UEs per network slice and the maximum number of PDU Sessions for the entire PLMN. NSAC Requests from AMF or SMF to the single centralized NSCAF in this case includes the NSAC service area of the NF consumer if multiple NSAC service areas are deployed in PLMN.

NOTE 1: It is possible to configure in the centralized architecture the maximum number of registered UEs and/or the maximum number of PDU sessions per NSAC service area if required by the operator. In this case, NSAC can be performed on a per NSAC service area.

• Option 3: Hierarchical NSAC architecture is deployed in the network. There are two roles of NSACF and interaction between them may be required (see clauses 5.15.11.1.2 and 5.15.11.2.2 for more details):
• Primary NSACF, controls and distributes of the maximum number of UEs and/or the maximum number of PDU Sessions for other NSACF(s) deployed in different NSAC service Area. The Primary NSACF handles overall NSAC for an S-NSSAI at the global level (i.e. it is ultimately responsible for the NSAC for an S-NSSAI).

‐ NSACF is responsible for one or multiple NSAC service Area. And one NSAC service area is only associated with one NSACF instance or one NSACF Set.

NOTE 2: When multiple NSACFs are deployed, how the maximum number of UEs per network slice and the maximum number of PDU Sessions per network slice is distributed (by OAM for Option 1 and by the primary NSACF for Option 3) among multiple NSACFs, i.e. the algorithm of the maximum number distribution, is implementation specific.

NOTE 3: When multiple NSACFs are deployed based on option 1, the UE moves to new NSAC service area with a different NSACF and if the number of UE or PDU Sessions in the target NSACF has reached the maximum number, whether the session continuity can be guaranteed is left to implementation.

NOTE 4: When multiple NSACFs are deployed based on Hierarchical NSAC architecture, it is possible that the role of Primary NSACF and the role of NSACF are co-located at the same NSACF instance.

Subject to operator policy and national/regional regulations, the AMF may exempt UEs and the SMF may exempt PDU sessions from NSAC when the UE and/or PDU Session is used for Emergency service or for Critical and Priority services (e.g. MCX, MPS).

When the AMF receives a Registration Request for an Emergency Registration, or with a Registration Request with an Establishment Cause indicating a priority service (e.g. MCX, MPS) or when the AMF determines that there is a priority subscription (e.g. MPS, MCX) in the UDM, the AMF may accept the registration request without applying NSAC, i.e. the AMF triggers the NSAC procedure, but the response from the NSACF is ignored at the AMF.

When the SMF receives a PDU Session Establishment Request for an emergency PDU Session or a PDU Session Establishment Request with a priority header, the SMF may accept the PDU Session Establishment Request without applying NSAC, i.e. the SMF triggers the NSAC procedure, but the response from the NSACF is ignored at the SMF.

Alternatively, when NSAC is exempted for the UE and/or PDU Session, the AMF and the SMF skip the corresponding NSAC procedure, i.e. this UE (respectively PDU Session) is not counted towards the maximum number of UEs (respectively PDU Sessions).

The support of NSAC for the S-NSSAI used for onboarding as described in clause 5.30.2.10 is optional and subject to Onboarding Network operator policies. However, NSAC for S-NSSAI used for onboarding is not applicable to UEs that registered in ON-SNPN with Registration Type "SNPN Onboarding".

In the case of NSAC for maximum number of PDU Sessions, when the NSACF rejects the request from the SMF to increase the number of PDU Sessions, the SMF may provide to the UE a back-off timer associated with reject cause set to 'Maximum number of PDU Sessions per S-NSSAI reached' for an Access Type as described in clause 4.2.11.4 of TS 23.502 [3]. If the UE receives from the SMF a back-off timer associated with the reject cause set to 'Maximum number of PDU Sessions per S-NSSAI reached' for an Access Type, the UE shall not request the establishment of a PDU Session for this S-NSSAI on the indicated Access Type until the back-off timer expires.