Section: 4.6.3.1 Session management based network slice admission control
Content: A serving PLMN or the HPLMN, or SNPN can perform network slice admission control for the S-NSSAI(s) subject to NSAC to monitor and control the total number of established PDU sessions per network slice. The SMF performs network slice admission control on the S-NSSAI during the PDU session establishment procedure. If the maximum number of PDU sessions on a network slice associated with an S-NSSAI has been already reached, the SMF rejects the PDU session establishment request using S-NSSAI based congestion control as specifed in subclause 6.2.8 and 6.4.1.4.2. The SMF performs network slice admission control on the S-NSSAI for a PDU session that is associated with the non-3GPP access, when the UE requests to transfer a session from the non-3GPP access to the 3GPP access with the Allowed PDU session status IE as described in subclause 5.6.1.4. If the maximum number of PDU sessions on a network slice associated with an S-NSSAI has been already reached, the SMF rejects the request to establish the user-plane resources (see 3GPP TS 29.502[ 5G System; Session Management Services; Stage 3 ]  [20A]). Based on operator policy, the session management based network slice admission control is not applicable for the PDU session for emergency services, or the session management based network slice admission control result is ignored for the PDU session for emergency services. Based on operator policy, the session management based network slice admission control is not applicable for the PDU session for priority services, or the session management based network slice admission control result is ignored for the PDU session for priority services. NOTE 1: The SMF can use the Sbi-Message-Priority field, as defined in TS 29.500[ 5G System; Technical Realization of Service Based Architecture; Stage 3 ]  [20AA], to determine whether the PDU session is for priority services. The session management based network slice admission control is not applicable to PDU session established for onboarding services in SNPN. NOTE 2: For the MA PDU session during the PDU session establishment procedure, the SMF performs network slice admission control only when it is newly established over the associated access type. NOTE 3: For a set of redundant PDU sessions, the SMF performs network slice admission control for each PDU session independently.