Description
The F1-U interface is the user plane counterpart to the F1-C within the 3GPP disaggregated gNB architecture. It provides the data path for user traffic between the CU's user plane component (CU-UP) and the DU. The interface is designed to carry encapsulated user data packets using the GPRS Tunneling Protocol for the user plane (GTP-U) over a UDP/IP transport layer. This tunneling mechanism allows for the multiplexing of data flows from multiple UEs and bearers over a single F1-U association between a CU-UP and a DU.
Operationally, for each established Data Radio Bearer (DRB), one or more GTP-U tunnels are set up over the F1-U interface. The CU-UP, which houses the PDCP layer for the user plane, performs functions like header compression, ciphering, and integrity protection. It then forwards the processed PDCP Protocol Data Units (PDUs) to the DU via the appropriate GTP-U tunnel. The DU, responsible for the RLC, MAC, and PHY layers, receives these PDCP PDUs, segments them if necessary (at RLC), schedules them for transmission (at MAC), and finally sends them over the air interface to the User Equipment (UE). The reverse path is followed for uplink data.
The architecture of the F1-U is defined to be independent of the F1-C, allowing the user plane traffic to be routed over potentially different network paths for optimization. This separation is crucial for enabling high-throughput, low-latency data delivery. The interface supports data forwarding during handover procedures to minimize data loss. Furthermore, the F1-U's design aligns with network slicing requirements, as different QoS flows can be mapped to different DRBs and subsequently transported over the F1-U with appropriate prioritization within the transport network.
Purpose & Motivation
The F1-U interface was developed to facilitate the user plane separation mandated by the CU-DU split in 5G. In monolithic 4G eNBs, user data processing and radio transmission were tightly coupled within the same hardware, limiting deployment options. The F1-U enables the physical and logical separation of the user plane processing function (in the CU-UP) from the radio transmission/reception function (in the DU).
This separation solves several key problems. It allows network operators to centralize user plane processing in cost-effective data centers (CU pools), achieving statistical multiplexing gains and simplifying the implementation of advanced features like edge computing. The DUs can remain as simpler, potentially mass-produced radio units deployed at cell sites. This architecture reduces fronthaul bandwidth requirements compared to a more stringent split like CPRI, as the F1-U carries partially processed packet data rather than raw IQ samples.
The creation of the F1-U, alongside the F1-C, was motivated by the need for RAN virtualization and cloudification. By defining a standard packet-based user plane interface, 3GPP enabled the CU-UP to be implemented as a Virtual Network Function (VNF) or Cloud Native Network Function (CNF) on commercial off-the-shelf servers. This drives down costs, increases service agility, and paves the way for AI/ML-based optimization in the RAN. The F1-U is thus essential for realizing the economic and performance promises of 5G networks, including support for enhanced Mobile Broadband (eMBB) and Ultra-Reliable Low-Latency Communications (URLLC) services.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (10 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the foundational architecture for the F1-U interface was introduced, defining it as the user plane interface terminated by the gNB-CU-UP on the network side and the gNB-DU on the radio access side. This release specified the functional split where the gNB-CU-UP hosts the user plane part of the PDCP protocol, connecting to the gNB-DU which hosts the RLC and MAC layers. Furthermore, the release enabled support for multiple Transport Network Layer Associations (TNLAs) on the F1 interface, including F1-U, to enhance reliability and load distribution.
In Release 16, the new functionality for the F1-U interface was the introduction of trace function support over the F1 interface. This enhancement allows for the management and orchestration of trace activities across the NG-RAN architecture involving the gNB-CU-UP and gNB-DU. The update integrates this capability into the existing F1 interface principles without altering the fundamental protocol stacks or functional division.
- Trace function Support over F1 Interface TS 38.470CR0040
In Release 17, the F1-U interface was enhanced to support Multicast and Broadcast Services (MBS) by introducing new procedures and contexts for multicast traffic. Specifically, the release included corrections and clarifications for establishing F1-U tunnels dedicated to Multicast MRB (MBS Radio Bearer) sessions. These updates defined the required "Multicast F1-U Context" to properly manage the user plane resources for multicast distribution between the gNB-CU-UP and the gNB-DU.
In Release 18, a key enhancement for the F1-U interface was the introduction of procedures for F1-U failure recovery specifically for Multicast Broadcast Service (MBS) sessions, as indicated by the correction for "MBS F1-U failure Recovery." Furthermore, the specifications were updated to include a reference to the Iuant interface within the NR architecture context, ensuring its proper integration.
In Release 19, the primary update for the F1-U interface was the correction and refinement of the stage-2 specification related to Artificial Intelligence/Machine Learning (AI/ML) management. This work, based on the framework defined in 3GPP TS 37.460, focused on enhancing the AI/ML aspects within the F1 user plane protocols and procedures.
- Correction on AIML F1-U stage2 TS 38.470CR0170
Explore further
Broader topics and technologies where F1-U plays a role.
Defining Specifications
3GPP specifications that define or reference F1-U, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.401 vj10 | NG-RAN Architecture Specification | Rel-19 |
| TS 38.470 vj10 | F1 Interface Introduction | Rel-19 |