Section: 5.15.20 Support of Network Slice Instance Replacement
Content: The Network Slice Instance Replacement is used when a PDU Session for a given S-NSSAI is established using a selected Network Slice instance and the S-NSSAI corresponding to this Network Slice instance is associated with multiple Network Slice instances. In this case, the network may change the Network Slice instance for the S-NSSAI if the selected Network Slice instance is no longer available (e.g. due to overload). The AMF may subscribe with the NSSF for notifications when any of the Network Slice instances served by the AMF is congested or no longer available. In case of roaming, the NSSF of VPLMN subscribes with the NSSF of the HPLMN for notifications. When the NSSF notifies the AMF that a Network Slice instance is congested or no longer available, for some of PDU Sessions associated with the Network Slice instance that is no longer available, the AMF may delete old NSI ID corresponding to the Network Slice instance that is no longer available and the SMF of the PDU Session(s) selected by using such old NSI ID is informed by the AMF to release the PDU Session(s). Subsequently, the SMF triggers the impacted UE(s) to establish new PDU session(s) associated with the same S-NSSAI as described in clause 5.6.9.2 for PDU Session(s) of SSC Mode 2 and SSC Mode 3. The AMF selects a new Network Slice instance for the given S-NSSAI during PDU Session Establishment.