5.15.11.2.2 Hierarchical NSAC architecture

The main differences between the NSACFs deployed in a non-Hierarchical architecture and NSACFs deployed in a Hierarchical architecture is as follows:

• The NSCAF is enhanced to support PDU session admission quota-based control.
• When the local maximum number of PDU sessions is reached, the NSACF interacts with the Primary NSACF to handle the request. The Primary NSACF either return an increased local maximum number to the NSACF, or reject the local maximum number value update request if all the global maximum number are consumed based on the status of established PDU sessions to the network slice.

NOTE: For Hierarchical NSAC architecture global maximum number used within this specification is synonymous with the maximum number of allowed registered UEs or established PDU Sessions for an S-NSSAI subject to Network Slice Admission Control (NSAC) for the entire PLMN and outbound roamers.

• Based on the response from Primary NSACF, the NSACF updates the local maximum number if updated value is received from the Primary NSACF. The NSACF updates local maximum number value (if received) and determines whether to accept or reject the NSAC request for PDU session establishment based on the local maximum number value.
• The update of local maximum number value by the Primary NSACF can also happen at any time through the Nnsacf_NSAC_LocalNumberUpdate service operation as described in clause 4.2.11.6 TS 23.502 [3]. The updated values provided from the Primary NSACF to NSACFs may directly apply to current NSAC pending request in NSACF and are used for all future requests.
• The Primary NSACF subscribes with all NSACFs to obtain the number of currently established PDU sessions at all NSACFs. Based on the obtained information, the Primary NSACF can update the NSACF with local maximum number of established PDU sessions.