UPF — User Plane Function — the only function a data packet actually touches
The core network
Where it sits
What it is
What it is. The User Plane Function — the only function in the core that a data packet touches. TS 23.501 §6.2.3. Everything else in the core decides; the UPF does.
The functionality list, abridged:
Anchor point for intra-/inter-RAT mobility · allocation of the device's IP address/prefix in response to an SMF request · External PDU Session point of interconnect to Data Network · Packet routing & forwarding — including the uplink classifier and the branching point for multi-homing · Packet inspection · User plane part of policy rule enforcement — gating, redirection, traffic steering · traffic usage reporting · QoS handling for user plane — uplink/downlink rate enforcement, reflective QoS marking · uplink traffic verification (SDF to QoS flow mapping) · transport level packet marking · downlink packet buffering and downlink data notification triggering · packet duplication in the downlink and elimination in the uplink, in the GTP-U layer.
"Anchor point" is the word that carries the mobility story. A device's IP address belongs to the UPF that anchors its session — the PDU Session Anchor. As long as the anchor does not change, the address does not change, and the device can move across the whole RAN without the internet noticing. SSC mode is precisely the question of when the anchor is allowed to move.
"Downlink packet buffering and downlink data notification triggering" is what makes idle mode possible at all. A device that is CM-IDLE has no N3 tunnel to receive on; the UPF holds the packet, tells the SMF, and the device is paged. Without buffering here, going idle would mean dropping traffic.
It holds no state of its own. Every rule the UPF applies was installed by the SMF over N4 — that is the CP/UP split — which is exactly what lets an operator put a UPF at the edge of the network, or several in a chain over N9, without moving any control function.
Read on
This concept is read as part of one argument in ref-core, alongside the rest of its group.
Before this concept, the hierarchy says to learn the following — the full chain, in order: