5G Terminal Interaction with AMF and SMF (1)

In the 5G (NR) system, AMF and SMF are two independent core network functional units. They are directly connected via the N11 interface; the 5G terminal (UE) connects to them directly or indirectly through N1, N2, N3, N4, and N11 interfaces, and the information exchanged is as follows:

I. Messages exchanged with SMF via the N1 interface include:

• A single N1 termination point is located in the AMF; the AMF forwards SM-related NAS information to the SMF based on the PDU session ID in the NAS message. Subsequent SM NAS exchanges (e.g., SM NAS message responses) received by the AMF via access (e.g., 3GPP or non-3GPP access) are transmitted through the same access.
• The serving PLMN ensures that subsequent SM NAS exchanges (e.g., SM NAS message responses) received by the AMF via access (e.g., 3GPP or non-3GPP access) are transmitted through the same access.
• The SMF handles the session management part of the NAS signaling exchanged with the UE.
• The UE can only initiate PDU session establishment in the RM-REGISTERED state.
• When an SMF is selected to serve a specific PDU session, the AMF must ensure that all NAS signaling related to that PDU session is handled by the same SMF instance.
• After successful PDU session establishment, the AMF and SMF store the access type associated with that PDU session.

II. Messages exchanged with SMF via the N11 interface include:

• The AMF reports the UE's reachability to the SMF based on the SMF's subscription, including:
• location information of the UE relative to the area of interest indicated by the SMF.
• The SMF indicates to the AMF when the PDU session is released.
• After successful PDU session establishment, the AMF stores the identifier of the SMF serving the UE, and the SMF stores the identifier of the AMF serving the UE, including the AMF set. When attempting to connect to the AMF serving the UE, the SMF may need to apply the behavior described in Section 5.21 for "other CP NFs".

III​. Messages exchanged with SMF via the N2 interface include:

• Certain N2 signaling (e.g., handover-related signaling) may require the joint action of the AMF and SMF. In this case, the AMF is responsible for ensuring coordination between the AMF and SMF. The AMF can forward SM N2 signaling to the corresponding SMF based on the PDU session ID in the N2 signaling.
• The SMF should provide the PDU session type and PDU session ID to the NG-RAN so that the NG-RAN can apply the appropriate header compression mechanism to packets of different PDU types. See TS 38.413 [34] for details.

IV. N3 interface interaction messages with the SMF include:

Selective activation and deactivation of existing PDU session UP connections are defined in clause 5.6.8 of TS 23.501.

V. N4 interface interaction messages with the SMF include:

When the UPF learns that a UE has received downlink data but there is no downlink N3 tunnel information, the SMF will interact with the AMF to initiate a network-triggered service request procedure. In this case, if the SMF learns that the UE is unreachable, or that the UE is only reachable for regulatory priority services, and the PDU session is not for regulatory priority services, the SMF should not send a downlink data notification to the AMF;