Section: 4.2.11.5.1 Network Slice Admission Control Support for Roaming by VPLMN
Content: This clause describes the case of VPLMN NSAC admission mode. For NSAC for roaming UEs, a maximum number of allowed UEs per mapped S-NSSAI in HPLMN and/or a maximum number of allowed PDU Sessions in LBO mode per mapped S-NSSAI in HPLMN is allocated to the VPLMN for each S-NSSAI in HPLMN and stored in one NSCAF in the VPLMN responsible for NSAC for the S-NSSAI in the HPLMN, subject to NSAC. Enforcement for the maximum number of UEs registered with a network slice is done in the VPLMN by the NSACF in the VPLMN as per the description in Figure 4.2.11.2-1 with the following differences: - Step 2, in the Nnsacf_NSAC_NumOfUEsUpdate_Request service operation where the AMF provides both the S-NSSAI in VPLMN and the corresponding mapped S-NSSAI in HPLMN to the NSACF in the VPLMN. - Step 3, the NSACF in the VPLMN performs NSAC for both the S-NSSAI in VPLMN and the corresponding mapped S-NSSAI in HPLMN based on the SLA between VPLMN and HPLMN. For LBO PDU sessions, enforcement for the maximum number of PDU Sessions established for an S-NSSAI is performed in the VPLMN by the NSACF in the VPLMN as per the description in Figure 4.2.11.4-1 with the following differences: - Step 2, in the Nnsacf_NSAC_NumOfPDUsUpdate_Request service operation where the V-SMF provides both the S-NSSAI in VPLMN and the corresponding mapped S-NSSAI in HPLMN to the NSACF in the VPLMN. - Step 3, the NSACF in the VPLMN performs NSAC for both the S-NSSAI in VPLMN and the corresponding mapped S-NSSAI in HPLMN based on the SLA between VPLMN and HPLMN. An NSACF at VPLMN can optionally fetch the maximum number of registered UEs to be enforced and the maximum number of LBO PDU sessions to be enforced rather than have them pre-configured. In this case, the following is performed: - For a centralized NSAC architecture in the VPLMN, the NSACF, being a centralized NSACF, issues the Nnsacf_NSAC_QuotaUpdate Request to fetch from the HPLMN centralized NSACF or primary NSACF the maximum number of registered UEs and the maximum number of LBO PDU sessions to be enforced. - For a hierarchical NSAC architecture in the VPLMN, the NSACF issues the Nnsacf_NSAC_NumOfUEsUpdate_Request or Nnsacf_NSAC_NumOfPDUsUpdate_Request to the VPLMN primary NSACF for NSAC for the maximum number of registered UEs, or NSAC for the maximum number of LBO PDU sessions. The VPLMN primary NSACF in turn issues the Nnsacf_NSAC_QuotaUpdate Request, to fetch from the HPLMN centralized NSACF or primary NSACF the maximum number of registered UEs, or the maximum number of LBO PDU sessions to be admitted; this information in turn may be distributed to the VPLMN NSACF. The NSACF in VPLMN discovers the primary or central NSACF in HPLMN as defined in clause 6.3.22 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2], or optionally be configured with the needed information.