Section: 5.15.11.1.1 Non-Hierarchical NSAC architecture
Content: The NSACF keeps track of the current number of UEs registered for a network slice so that it can ensure it does not exceed the maximum number of UEs allowed to register with the network slice. The NSACF also maintains a list of UE IDs registered with a network slice that is subject to NSAC. When an event related to a UE causes the current number of UEs registered with a network slice to increase, the NSACF first checks whether the UE Identity is already in the list of UEs registered with that network slice. If not, the NSACF checks whether the maximum number of UEs per network slice for that network slice has already been reached and if it has, the NSACF applies admission control policies. The AMF triggers a request to NSACF for NSAC for maximum number of UEs when the UE's registration status for a network slice subject to NSAC is changing, i.e. during the UE Registration procedure in clause 4.2.2.2.2 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3], UE Deregistration procedure in clause 4.2.2.3 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3], Network Slice-Specific Authentication and Authorisation procedure in clause 4.2.9.2 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3], AAA Server triggered Network Slice-Specific Re-authentication and Re-authorization procedure in clause 4.2.9.3 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3], AAA Server triggered Slice-Specific Authorization Revocation in clause 4.2.9.4 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3] and UE Configuration Update procedure in clause 4.2.4.2 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. NOTE 1: Early Admission Control (EAC) mode is applicable for Number of UEs per network slice admission control. The use of EAC in relation to the number of registered UEs is described in clauses 4.2.11.2 and 4.2.11.3 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. Since the UE may register or deregister for an S-NSSAI via 3GPP access and/or non-3GPP access as described in clause 5.15.5.2.1. The Allowed NSSAI for the access type may change while the UE is registering to a network. The AMF provides the Access Type to the NSACF when triggering a request to increase or decrease the current number of UEs registered with a S-NSSAI. The NSACF may take the Access Type into account for increasing and decreasing the number of UEs per network slice by storing the UE ID with the associated one or more Access Type(s), i.e. the NSACF is able to add or remove a registration for the UE ID for each Access Type and trigger the increase or decrease of the current number of UEs registered with a S-NSSAI based on a policy that takes the access type into account. If the Access Type provided by the AMF is not configured for NSAC in the NSACF, the NSACF always accepts the request from the AMF without increasing or decreasing the number of UEs. If the Access Type provided by the AMF is configured for NSAC in the NSACF and the maximum number is reached, the NSACF sends a reject response to the AMF including the access type. NOTE 2: For example, if the NSACF is configured to apply NSAC for 3GPP Access Type only, the NSACF counts registration via 3GPP access type only. If the NSACF is configured to apply NSAC for both Access Types, and the UE newly registers via 3GPP access while the UE is already registered via non-3GPP access (or vice versa), the NSACF updates the UE ID entry with both 3GPP Access Type and non-3GPP Access Type and the NSACF may count the UE once or twice based on its policy.