In 5G, the gNB is not a single box. Under the 3GPP higher-layer split (TS 38.401), a Central Unit owns the upper radio-protocol layers, and one or more Distributed Units own the real-time lower ones — RLC, MAC, and PHY. F1-U is the user-plane leg between them, and it carries GTP-U tunnels running the NR User Plane Protocol (NR-U / NRUP, TS 38.425). The CU-UP pushes downlink data toward a radio it cannot see; the DU reports back what it managed to send. This walkthrough reads a real srsRAN F1-...