SMF
Session Management Function: the 5GC function responsible for PDU session establishment, modification, and release, including UPF selection and IP address allocation.
Whenever a UE asks for connectivity, the SMF is what turns that request into a working data path. It allocates the UE's IP address (or prefix), picks which UPF (or chain of UPFs) the traffic should flow through, and programs that UPF over N4 using PFCP — installing the forwarding, QoS, and usage-reporting rules per PDU session.
It pulls policy from the PCF and turns abstract rules into concrete QoS flows on the user plane. The SMF also owns the lifecycle: it handles session modification when policy or location changes, releases sessions cleanly, and generates the charging records. A useful mental model is that the AMF decides whether and where a subscriber is connected, and the SMF decides what their sessions look like and how packets are treated. One subscriber can have several SMFs serving different PDU sessions at once.
Frequently asked questions
- What is the relationship between the SMF and the UPF?
- The SMF is the controller and the UPF is the data-plane worker. The SMF makes all the decisions — UPF selection, IP allocation, QoS, traffic steering — and pushes them to the UPF as packet-handling rules over the N4 interface using PFCP. The UPF just forwards and meters packets according to those rules. It's the same control/user-plane split idea as CUPS in the 4G EPC.
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