N3 Interface
The reference point between the (R)AN and the UPF carrying user-plane data encapsulated in GTP-U tunnels.
N3 is pure user plane: it carries subscriber data packets between the gNB and the UPF, wrapped in GTP-U tunnels. Each PDU session's traffic gets its own GTP-U tunnel identified by a TEID, so the UPF and gNB can keep different subscribers' and sessions' packets straight over the shared transport.
This is the same GTP-U mechanism LTE used on S1-U, which is why N3 is one of the most familiar pieces of 5G to anyone with EPC experience. A few things to keep in mind operationally: GTP-U adds encapsulation overhead, so MTU and fragmentation matter on the N3 path; and because it's the bearer for all user traffic, N3 transport problems show up as throughput or packet-loss issues rather than signalling failures. When a UE moves, the tunnel endpoints get re-pointed (via N2 signalling and the SMF/UPF) so the data keeps flowing to the right place.
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