Section: 5.2.21.2.2 Nnsacf_NSAC_NumOfUEsUpdate service operation
Content: Service Operation name: Nnsacf_NSAC_NumOfUEsUpdate Description: Updates the number of UEs registered with a network slice (e.g. increase or decrease) when the UE registration status for a network slice subject to NSAC has changed. Also, if the number of the UEs registered with the network slice is to be increased and the Early Admission Control (EAC) mode in the NSACF is activated for that network slice (see Nnsacf_NSAC_EACNotify service operation), the NSACF first checks whether the number of UEs registered with the network slice has reached the maximum number of UEs per network slice threshold. If the maximum number of UEs registered with the network slice has already been reached, the UE registration for that network slice via the same Access Type configured in the NSACF is rejected. If the EAC is not activated, the NSACF increases or decreases the number of UEs per network slice as per the input parameters below. Inputs, Required: S-NSSAI(s), UE ID (SUPI), NF ID, Access Type, update flag. Inputs, Optional: None. Inputs, Conditional: Notification endpoint for EAC Notification for the S-NSSAI, NSAC admission mode, PLMN ID, UE already registered indication, AMF NSAC service area. The S-NSSAI(s) parameter is a list of one or more network slices for which the number of UEs registered with a network slice is to be updated and checked if the maximum number of UEs per network slice threshold has already been reached. In the roaming case, the corresponding mapped S-NSSAI(s) of the HPLMN is also included. The UE ID parameter is used by the NSACF to maintain a list of UE IDs registered with the network slice. The NSACF also takes Access Type into account for increasing and decreasing the number of UEs per network slice as described in clause 5.15.11.1 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. The NF ID parameter is the NF instance ID of the NF (e.g. AMF or SMF + PGW-C) sending the request to the NSACF. The update flag input parameter indicates whether the number of UEs registered with a network slice is to be: - increased when the UE registers to a new network slice subject to NSAC. If the UE ID is already in the list of UEs registered with the network slice, the number of UEs registered with the network slice is not increased as the UE has already been counted as registered with the network slice. If the UE ID is not in the list of UE IDs registered with the network slice and the maximum number of UEs registered with the network slice has not been reached yet, the NSACF adds the UE ID in the list of UEs registered with the network slice and increases the number of the UEs registered with the network slice. If the UE_ID is not in the list of UEs registered with that S-NSSAI and the maximum number of UEs per network slice for that S-NSSAI has already been reached, then the NSACF returns maximum number of UEs per network slice reached result; - decreased when the UE deregisters for a network slice that is subject to NSAC. The NSACF decreases the number of the UEs registered with the network slice and removes the UE ID from the list of UEs registered with the network slice; - updated when the UE moves the PDU sessions across Access Types as described in the clause 4.11.5.9a. NSAC admission mode applies to inbound roamers. It is included by NF consumer to indicate applicable NSAC admission mode in VPLMN. Its values can be VPLMN NSAC admission mode, or VPLMN with HPLMN assistance NSAC admission mode. The PLMN ID is the serving PLMN of the UE. The UE already registered parameter indicates that the UE has already been registered in another NSACF service area. The AMF NSAC service area is included in a centralized NSAC architecture, if available. The NSACF may optionally return the current status of the network slice availability (e.g. a percentage out of the maximum number of UEs registered with a network slice) in the availability status parameter. This information may be used for NSACF signalling and load balancing in case multiple NSACFs are serving the same network slice. Outputs, Required: Result indication. The Result indication parameter contains the outcome of the update and check operation in the NSACF and may indicate one of the values 'maximum number of UEs for the S-NSSAI not reached' or 'maximum number of UEs for the S-NSSAI reached'. Outputs, Optional: Updated local maximum number of registered UEs, or Updated UE admission threshold. These parameters can only be returned by a primary NSACF.