5.15.11.1.2 Hierarchical NSAC architecture

In the Hierarchical NSAC architecture, the NSACFs deployed in the NSAC service areas interacts with the Primary NSACF when needed and as explained below.

The main differences between the non-Hierarchical architecture and the Hierarchical architecture is that the AMFs and the NSACFs deployed in the Hierarchical architecture support the following:

• When the AMF triggers an NSAC request to the NSACF, the AMF includes the UE already Registered indication if the AMF can determine that the UE has been registered with the S-NSSAI in one NSAC service area before. If the AMF does not include the UE already Registered indication, the registration request to the indicated S-NSSAI is determined as initial registration, i.e. the UE has not been registered in any service area before. The AMF determines the UE already Registered indication based on the received Allowed NSSAI information from the source AMF (in case of inter AMF handover) or from SMF+PGW-C (in case of mobility from EPS to 5GS).
• There are two types of UE admission control: quota-based control or threshold-based control. A PLMN is configured to deploy only one type of UE admission control. Based on the type of UE admission control configured for the PLMN, the NSACF handles the NSAC request as described below:
• For NSACFs supporting quota-based control, if the NSACF receives a request to increase the number of UEs and the number of UEs registered for a network slice has reached the local maximum number of UEs provisioned in the NSACF, or upon receiving a request to decrease the number of UEs and no UE entry is present in the NSACF, the NSACF interacts with the Primary NSACF for the handling of the NSAC request for the UE. The Primary NSACF may return in the response an updated local maximum number of registered UEs value to the NSACF based on the status of registered UEs for the Network Slice and which enables the NSACF to handle locally the request. Alternatively and if the request to increase the number of UEs includes the UE already Registered indication, the Primary NSACF may admit the UE request and store the UE entry which allows for service continuity. The Primary NSACF may also reject the request. If the NSACF receives a request to increase the number of UEs and the local maximum number of UEs is not reached, the NSACF handles the request locally and sends a response to the AMF without interaction with the Primary NSACF.
• For NSACF supporting threshold-based control, the NSACF is initially configured with a UE admission threshold and a local maximum number of Registered UEs to be admitted. Threshold-based control refers to an admission threshold, defined in percentage, against provisioned local maximum number in NSACF (e.g. an admission threshold of 80% refers to the case when 80% of the provisioned local maximum number should be used). If upon a receiving a request to increase the number of UEs without a UE already Registered indication and if UE admission threshold is at or above the threshold level configured at the NSACF, the NSACF immediately rejects the NSAC request. If the received request includes the UE already Registered indication and if UE admission is at or above the threshold level configured at the NSACF, the NSACF accepts the request to enable UE admission and allow for service continuity as long as the local maximum number of Registered UEs have not been reached. If the local maximum number of registered UEs value have been reached, the NSACF interacts with the Primary NSACF for the handling of the NSAC request for the UE. The NSACF does not include the UE already Registered indication in this case. The Primary NSACF may return an updated UE admission threshold value to the NSACF in the response which enables the NSACF to handle the request locally. Alternatively, the Primary NSACF may handle and store the UE entry. The Primary NSACF may also reject the request.
• For both options, the Primary NSACF supports the following capabilities depending on the NSACF configuration:
• Returning a new updated local maximum number of Registered UEs for the NSACF to admit if the NSACF is configured to support the quota-based UE admission control; or
• Returning a new updated UE admission threshold for the NSACF to apply if the NSACF is configured to support the threshold-based UE admission control;
• The Primary NSACF handles, stores entries only related to UEs which the NSAC request includes the UE already Registered indication, that are already admitted in an existing NSAC service area but cannot be admitted in the new NSAC service area due to no remaining local maximum number of registered UEs, as long as the overall PLMN number of registered UEs at the Primary NSACF is not exhausted. The Primary NSACF informs the NSACF in its response;
• Based on the response from the Primary NSACF, the NSACF determines whether to accept or reject the NSAC request for UE registration. In addition, the NSACF may also update the local maximum number of Registered UEs or admission threshold respectively if the related updated value is received;
• At any time, the Primary NSACF can update the NSACFs local maximum number of Registered UEs or admission threshold through the Nnsacf_NSAC_LocalNumberUpdate operation as described in clause 4.2.11.6 of TS 23.502 [3]. The updated values provided from the Primary NSACF to the 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 registered UEs at all NSACFs. Based on the obtained information, the Primary NSACF can update the NSACF with local maximum number of registered UEs.