When the UE is outside of the SMF Service Area, an I-SMF is inserted between the SMF and the AMF. The I-SMF has a N11 interface with the AMF and a N16a interface with the SMF and is responsible of controlling the UPF(s) that the SMF cannot directly control. The exchange of the SM context and forwarding tunnel information if needed are interacted between two SMFs directly without involvement of AMF.
Depending on scenario, a PDU Session in non-roaming case or local breakout is either served by a single SMF or served by an SMF and an I-SMF. When a PDU Session is served by both an SMF and an I-SMF, the SMF is the NF instance that has the interfaces towards the PCF and CHF.
188.8.131.52 SBA architecture
184.108.40.206 Non-roaming architecture
Figure 220.127.116.11-1: Non-roaming architecture with I-SMF insertion to the PDU Session in reference point representation, with no UL-CL/BP
Figure 18.104.22.168-2: Non-roaming architecture with I-SMF insertion to the PDU Session in reference point representation, with UL-CL/BP
22.214.171.124 Roaming architecture Word-p. 307
Figure 126.96.36.199-1: Roaming 5G System architecture with SMF/I-SMF - local breakout scenario in reference point representation
Figure 5.34.4-1: User plane Architecture for the Uplink Classifier controlled by I-SMF