UPF
User Plane Function: the 5GC user-plane node performing packet routing, forwarding, QoS enforcement, traffic measurement, and acting as the anchor point for intra/inter-RAT mobility.
The UPF is where user traffic actually moves. It sits between the RAN (N3) and the data network (N6), forwarding packets, enforcing the QoS the SMF asked for, counting bytes for charging, and acting as the mobility anchor so a UE's IP address survives as it moves between cells.
What makes it interesting operationally is that it's the one core function you can move around freely. Pull a UPF out to the network edge near a factory or a stadium and you cut the round-trip for that traffic — this is the basis of local breakout and a lot of MEC deployments. You can also chain UPFs: an anchor UPF (PSA) holds the IP, while an intermediate UPF (I-UPF) handles routing closer to the access. It runs zero application logic; everything it does, it does because the SMF programmed it over N4.
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