How 5G Split Base Stations Coordinate Data Flow Over the F1-U Interface

In 5G networks, a base station (gNB) is divided into a Central Unit (CU) and one or more Distributed Units (DU), which communicate over the F1-U user-plane interface using GTP-U tunnels. A real-world packet capture from srsRAN, spanning 37,871 frames over roughly 82 seconds, was analyzed to study how data and feedback flow between these two halves. The interface uses two PDU types: Type 0 carries downlink user data from the CU to the DU, while Type 1 carries delivery status reports back from the DU to the CU every ~10 milliseconds. A key flow-control mechanism is the Desired Buffer Size field, which signals the DU's capacity — a shrinking value indicates congestion and instructs the CU to reduce its transmission rate. In the analyzed voice bearer, the buffer size remained constant at about 5.9 MiB, meaning flow control was never triggered under the light load conditions observed.