{
  "id": 5395653,
  "title": "F1-U flow control: how a 5G base station's two halves stay in step",
  "url": "https://urgent.news/2026/09/03/f1-u-flow-control-how-a-5g-base-stations-two-halves-stay-in-step",
  "topic": "tech",
  "section": "Tech",
  "published": "2026-09-03T19:24:30.000Z",
  "source": {
    "name": "Dev.to",
    "slug": "dev-to",
    "url": "https://dev.to/eventhelix/f1-u-flow-control-how-a-5g-base-stations-two-halves-stay-in-step-3bi"
  },
  "original_language": "en",
  "account": "In 5G, the gNB is composed of two units: the Central Unit (CU) and one or more Distributed Units (DU). The CU owns the upper radio-protocol layers, while the DU handles the real-time lower layers such as RLC, MAC, and PHY. F1-U is the user-plane leg that connects these two units, carrying GTP-U tunnels for the NR User Plane Protocol (NR-U / NRUP). This article analyzes a real F1-U capture from srsRAN, examining a 29-frame window following a voice bearer. NR-U is a two-way loop, with data flowing in opposite directions between the CU and DU. The protocol uses PDU type to determine direction, with Type 0 carrying downlink user data and Type 1 carrying delivery status. The delivery status is timer-driven, with the DU repeating the same report roughly every 10 ms even when there is no new data to report. The Desired Buffer Size is used for flow control, effectively bottomless for low-rate voice bearers. A shrinking Desired Buffer Size indicates congestion, prompting the DU to ask the CU to slow down. PDCP Sequence Number is crucial for delivery tracking, with the DU echoing back the sequence number for each frame. By correlating PDCP SN and PDCP SN, one can pair a bearer's two tunnels, such as TEID 0x06 and TEID 0x18 as one DRB. The article provides a full walkthrough of this 82-second capture, analyzing the flow control process in four beats. VisualEther was used to decode and caption every arrow in the diagram, creating a sequence diagram from the raw Wireshark PCAP. The full walkthrough is available at https://www.eventhelix.com/5G/f1u-nr-u-flow-control/.",
  "summary": "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…",
  "key_points": [
    "5G gNB consists of Central Unit (CU) and Distributed Units (DU)",
    "F1-U connects CU and DU, carries GTP-U tunnels for NR User Plane Protocol (NR-U / NRUP)",
    "Desired Buffer Size used for flow control, effectively bottomless for low-rate voice bearers"
  ],
  "editors_take": null,
  "illustration": null,
  "coverage": {
    "outlets": 1,
    "also_reported_by": []
  },
  "ai_generated": true,
  "disclaimer": "Summaries, key points and the editor’s take are written by software from other outlets’ reporting and may contain errors — always check the linked original."
}