Understanding the F1 Interface in 5G NR: CU-DU Split and Protocol Stack Explained

Last updated on 13 Oct 2025

Getting to Know the F1 Interface in 5G NR Networks

The F1 Interface is a key component of the 5G NR (New Radio) architecture. It facilitates communication between the Central Unit (CU) and the Distributed Unit (DU), playing a vital role in the flexible and disaggregated design of the 5G Radio Access Network (RAN). This design supports scalable and cloud-based implementations.

The illustration above shows how the F1 interface interlinks various logical units of the RAN, detailing both control plane (F1-C) and user plane (F1-U) connections, along with the corresponding protocol stacks as defined by 3GPP standards.

The F1 Interface’s Role in 5G RAN

In 5G, the gNodeB (gNB)—basically the equivalent of a base station—can be broken down into two functional parts:

Central Unit (CU) – Takes care of higher-layer protocols and control logic.

Distributed Unit (DU) – Focuses on managing real-time radio functions that are closer to users.

This split provides more network flexibility, virtualization, and scalability. The F1 interface links these two units, making sure that the radio operations (DU) and network intelligence (CU) work together seamlessly.

Functional Purpose

The F1 interface:

Creates a logical division between control and data processing.

Promotes multi-vendor interoperability between CU and DU.

Offers flexible deployment options—whether centralized or distributed.

Facilitates efficient fronthaul and midhaul transport.

Architecture Overview

The F1 interface is located between the DU and CU, as depicted in the image.

The DU connects to the radio site (antennas) through the fronthaul link.

The CU is split into two logical sub-units: * CU-CP (Control Plane) * CU-UP (User Plane)

These sub-units connect to the DU via two logical interfaces:

F1-C: Control Plane Interface

F1-U: User Plane Interface

This separation allows independent handling of control signaling and user data, enhancing both efficiency and scalability.

F1-C (Control Plane) Interface

The F1-C interface connects the DU with the CU-CP (Central Unit - Control Plane). It carries all control signaling between these two components.

Defined by:

📘 3GPP TS 38.473

Key Protocol Stack Layers (see the image):

F1-AP (F1 Application Protocol) – Manages control procedures like configuration, setup, and resource allocation.

SCTP (Stream Control Transmission Protocol) – Ensures reliable, message-oriented transport with congestion control.

IP Layer – Deals with packet routing between DU and CU.

Data Link Layer – Focuses on error detection and frame synchronization.

PHY Layer – Manages physical transmission over the fronthaul network.

Main Functions of F1-C:

Establishing and releasing F1 connections.

Managing DU configuration and status.

Handling control signaling for mobility (like handovers).

Reporting performance measurements.

Managing faults and synchronization.

F1-U (User Plane) Interface

The F1-U interface connects the DU with the CU-UP (Central Unit - User Plane). It’s responsible for transporting user data traffic—think voice, video, and application data.

Defined by:

📘 3GPP TS 38.425

Protocol Stack Layers (see the image):

GTP-U (GPRS Tunneling Protocol - User Plane) – Encapsulates and tunnels user data packets between DU and CU-UP.

UDP (User Datagram Protocol) – Offers fast, connectionless transport for GTP-U.

IP Layer – Handles packet routing and addressing.

Data Link Layer – Maintains reliable link control.

PHY Layer – Transmits data over physical media.

Main Functions of F1-U:

Transmitting and forwarding user data.

Managing QoS (Quality of Service) for each data flow.

Handling buffer management and flow control.

Facilitating path switching during mobility or handover.

F1 Interface Protocol Summary

Why the F1 Interface is Important for 5G

The F1 interface is essential for the disaggregation of 5G RAN, enabling architectures like Cloud RAN (C-RAN) and Open RAN (O-RAN).

Key Advantages:

Flexible Deployment: The CU can be centralized in a data center, while the DU remains at cell sites for low-latency tasks.

Vendor Interoperability: The standardized 3GPP specs ensure that different vendors can work together.

Network Optimization: It allows for real-time optimization through centralized control.

Scalability: Supports dynamic adjustments of CU/DU functions for better load balancing.

Operational Efficiency: Simplifies maintenance and enhances automation in networks.

How the F1 Interface Aids Network Virtualization

In today’s 5G deployments, both the CU and DU can be virtualized and run on general-purpose hardware. The F1 interface is pivotal in this virtualization by:

Separating network functions into manageable logical units.

Enabling centralized orchestration through SDN (Software Defined Networking) and NFV (Network Function Virtualization).

Reducing latency by allowing real-time processing closer to users (at the DU).

Facilitating network slicing, which enables the efficient handling of different service types (like IoT, URLLC, eMBB).

F1 Interface in the Context of Other 5G Interfaces

To grasp the F1 interface better, it’s helpful to see how it fits with other 5G RAN interfaces:

Interface | Between | Purpose

N2 | CU-CP ↔ 5G Core (AMF) | Control plane signaling

N3 | CU-UP ↔ 5G Core (UPF) | User data transport

E1 | CU-CP ↔ CU-UP | Coordination between control and user planes

F1 | CU ↔ DU | Coordination and data exchange within gNB

O1/O2 | Management systems ↔ Network functions | Network management and orchestration

So, F1 serves as a midhaul interface, connecting the central and distributed elements of the RAN.

Deployment Scenarios for the F1 Interface

The F1 interface's flexibility means it can adapt to various deployment models:

Centralized RAN (C-RAN): * CU is centralized in a data center. * DU is located at remote radio sites. * Cuts operational costs and simplifies updates.

Distributed RAN: * CU and DU are co-located for ultra-low latency applications. * Ideal for dense urban or industrial settings.

Open RAN Integration: * Works with F1 alongside O-RAN’s E2 and A1 interfaces. * Encourages vendor neutrality and software-driven innovations.

Challenges in Implementing the F1 Interface

While the F1 interface has many advantages, it does come with challenges:

Synchronization: Accurate time synchronization is crucial between the DU and CU.

Latency Constraints: The transport network must support low-latency midhaul connections.

Security: IP-based communications need solid encryption and authentication measures.

Interoperability Testing: Multi-vendor integration requires thorough testing.

Conquering these challenges is key to ensuring a robust and flexible deployment of 5G RAN.

Wrapping Up

The F1 Interface is foundational to the 5G RAN architecture, defining how Central Units (CUs) and Distributed Units (DUs) communicate. It’s governed by 3GPP TS 38.473 (Control Plane) and TS 38.425 (User Plane), creating a standardized, flexible framework that separates control and user functions.

By allowing for virtualization, interoperability, and scalability, the F1 interface empowers operators to build next-generation networks that are efficient, flexible, and future-ready. Whether through centralized, distributed, or Open RAN setups, the F1 interface is a cornerstone of 5G innovation and performance.