Section: 4.11.5.9a Network Slice Admission Control in 5GS for maximum number of UE with at least one PDU session and one PDN connection
Content: Support of NSAC in conjunction with interworking with EPC for maximum number of UE with at least one PDU session/PDN connection is described in clause 5.15.11.5a of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. For option 1, the same mechanisms in clause 4.2.11.2 (Number of UEs per network slice availability check and update procedure) is used with the following additions: - The SMF+PGW-C invokes this procedure when the UE establishes the first PDU Session/PDN Connection and when the last PDU Session/PDN Connection associated with the network slice is released. In this case, the AMF in figure 4.2.11.2-1 is replaced with SMF+PGW-C. - Step 2: The SMF+PGW-C includes in the message the S-NSSAI, identity of SMF+PGW-C, UE ID and update flag. The update flag may include either 'increase', 'decrease' or 'update' values. - Step 2: The AMF ID is replaced by the SMF+PGW-C ID. - Step 3: The 'update' value indicates that for existing UE registration the Access Type is to be replaced with a new Access Type during inter access mobility. If the update flag parameter from the SMF+PGW-C indicates update value, the NSACF locates the existing entry with UE ID and NF ID and replaces the Access Type in the existing entry. The NSACF may take the Access Type into account for UE counting based on local policy. If there was one UE entry in the new Access Type with same NF ID then NSACF may not increase the count again. If there was one UE entry in the new Access Type with different NF ID then NSACF may add this new NF ID in the UE entry and not increase the count. If there was one entry with same NF ID in the old Access Type then NSACF may remove the UE entry and decrease the count. Editor's note: Step 3 needs to be further discussed/completed to cover both cases when the access type is considered and not considered. - The NSACF determines whether or not to accept the request as described in clause 5.15.11.5a of TS 23.501[ System architecture for the 5G System (5GS) ]  [2] NOTE 1: EAC mode is not applicable here. For option 2, the same mechanisms in clause 4.2.11.4 (Number of PDU Sessions per network slice availability check and update procedure) are used with the following additions: - The NSACF also counts the maximum number UE with at least one PDU session/PDN connection based on the update value received from SMF+PGW-C. - If the update flag parameter from the SMF/SMF+PGW-C anchoring the PDU session/PDN connections indicates update, the NSACF locates the existing entry with UE ID and NF ID and replaces the Access Type in the existing entry. The NSACF may take the Access Type into account for UE counting based on local policy. If there was one UE entry in the new Access Type with same NF ID then NSACF may not increase the count again. If there was one UE entry in the new Access Type with different NF ID then NSACF may add this new NF ID and not increase the count. If there was one entry with same NF ID in the old Access Type then NSACF may remove the UE entry and decrease the count. - The NSACF determines whether to accept the request as described in clause 5.15.11.5a of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. - The NSACF performs other actions in addition to the above to keep track of the last PDU Session/Connection released so as to decrease the count for the number of Registered UEs with at least one PDU Session/one PDN Connection. NOTE 2: EAC mode is not applicable here. If Hierarchical NSAC architecture is deployed in the network, there are the following additional enhancements: - For option 2, the NSAC for number of UEs and NSAC for number of PDU sessions/PDN connections shall be handled within the same NSACF. - For both options, when the local maximum number of UEs with at least one PDU Session/PDN Connection or local threshold is reached, the NSACF may interact with the Primary NSACF before it returns the response back to the SMF+PGW-C. For more details on handling between the NSACF and Primary NSACF see clause 4.2.11.2a. NOTE 3: For both options, given that the SMF+PGW-C can not provide the UE already registered indication to NSACF, hence when the NSACF interacts with Primary NSACF, the Primary NSACF will not manage the UE entry by itself. - For NSAC for number of PDU Sessions, when the local maximum number is reached, the NSACF may interact with the Primary NSACF before it returns the response back to the SMF+PGW-C. For more details on handling between the NSACF and Primary NSACF see clause 4.2.11.4a.