5.15.11.5a Support of Network Slice Admission Control in 5GS for maximum number of UEs with at least one PDU Session/PDN Connection

When EPS counting is required for a network slice and NSACF is configured with maximum number of UEs with at least one PDU Session/PDN Connection, the NSACF keeps track of the current number of UEs with at least one PDU session/PDN connection established on a network slice to ensure it does not exceed the maximum configured number.

To support the NSAC for maximum number of UEs with at least one PDU Session/PDN Connection, the SMF+PGW-C may be configured with one of the following options:

• Option 1: Triggering an Nnsacf_NSAC_NumOfUEsUpdate_Request to NSACF for NSAC for maximum number of UEs when the UE establishes first PDU Session/PDN connection associated with the network slice in the SMF+PGW-C, or when the last PDU Session/PDN connection associated with the network slice is released. The NSACF performs admission control as described in clause 5.15.11.5 and the number of registered UE is replaced with number of UE with at least one PDU session/PDN connection. Also, if the maximum number of UEs with at least one PDU Session/PDN connection has already been reached and SMF+PGW-C receives the rejection from NSACF, unless operator policy implements a different action, the SMF+PGW-C rejects the PDU Session/PDN connection indicating the cause being the number of UEs in the network slice has been exceeded. The AMF is not configured for this S-NSSAI to be subject to NSAC; or
• Option 2: Triggering an Nnsacf_NSAC_NumOfPDUsUpdate_Request as described in clause 5.15.11.5 to NSACF and the NSACF performs admission control for the number of UEs with at least one PDU Session/PDN connection as follows:
• The NSACF supports handling both for the number of UEs with at least one PDU Session/PDN Connection and number of PDU session for the S-NSSAI that is subject to EPC interworking and NSAC. In this case the AMF is not configured for this S-NSSAI to be subject to NSAC. As an optimization option, the SMF+PGW-C can be configured not to trigger the Nnsacf_NSAC_NumOfUEsUpdate_Request to NSACF.
• When the NSACF receives request to increase the current number of PDU Session/PDN Connection established for the network slice, the NSACF checks whether this is the first PDU Session/PDN Connection associated with the network slice. If this is the first PDU Session/PDN Connection associated with the network slice the NSACF checks whether the maximum number of UEs with at least one PDU Session/PDN Connection has been reached. If the maximum number has not been reached then the NSACF increases the number of UE with at least one PDU session/PDN connection and add an entry for UE ID. If the maximum number of UEs has already been reached, unless operator policy implements a different action, the SMF+PGW-C rejects the PDU Session/PDN connection indicating the cause being the number of UEs in the network slice has been exceeded.
• When the NSACF receives request to decrease the current number of PDU Session/PDN Connection established for the network slice, the NSACF locates the UE entry, checks whether this is the last PDU Session/PDN Connection associated with the network slice for the UE. If it is the last PDU Session/PDN Connection the NSACF decreases the number of UE with at least one PDU session/PDN connection and remove the associated UE entry.

NOTE 1: A PLMN can deploy one of the above two options for a slice when EPS counting is required for a network slice and NSACF is configured with maximum number of UEs with at least one PDU Session/PDN Connection.

NSACF is configured with the information of whether the NSAC for number of UEs with at least one PDU session/PDN connection is based on Option1 or Option 2.

In both options, the SMF+PGW-C provides the Access Type to the NSACF when triggering a request to increase or decrease or update the number of UEs with at least one PDU Session/PDN Connection and/or the number of PDU Sessions for an S-NSSAI.

In the case of roaming, same mechanisms in clause 5.15.11.3 are used and number of registered UE is replaced with number of UE with at least one PDU Session/PDN Connection. For home routed PDU Session/PDN Connection only HPLMN admission mode can be used in this case.

If hierarchical NSAC architecture is deployed, when the local maximum number or local threshold is reached the NSAC may interact with the Primary NSACF before it returns the response back to the SMF+PGW-C. For more details on handling at the NSACF and Primary NSACF see clause 5.15.11.1.2.

NOTE 2: When NSAC for number of UEs with at least one PDU session or one PDN connection is used, the session continuity is guaranteed at inter-system mobility as the admission is granted during the establishment of the PDU Session/PDN Connection.