Terminology » O-RAN architecture Interfaces: O1,O2,A1,E2,F1,Open Fronthaul

O-RAN architecture Interfaces: O1,O2,A1,E2,F1,Open Fronthaul

5g open ran o-ran interfaces O1 interface O2 interface A1 interface E2 interface F1 interface open fronthaul

Introduction : Various interfaces are needed in O-RAN to enable open, modular, and interoperable communication between its disaggregated components such as the O-RU, O-DU, O-CU, and RIC. Unlike traditional RANs, where all functions were tightly integrated in proprietary hardware, O-RAN separates these functions into different logical units provided by potentially different vendors. Each interface (like O1, O2, A1, E2, F1, and Open Fronthaul) ensures that data, control, and management messages can flow seamlessly and securely between these components, regardless of vendor implementation.

O-RAN Architecture Interfaces

These interfaces also serve different purposes across the control, user and management planes; for example, the Open Fronthaul connects the DU and RU for real time radio data, E2 allows the RIC to optimize RAN behavior using AI/ML and O1/O2 enable centralized configuration and orchestration through the SMO.

O-RAN Architecture Interfaces Image Courtesy : O-RAN Alliance

Interface	Position in architecture	Role or Purpose
O1 interface	between SMO / orchestration and RAN elements (RU / DU / CU)	management, configuration, monitoring (OAM) etc.
O2 interface	between SMO and infrastructure / cloud resources (O-Cloud)	resource management (compute, virtualization) etc.
A1 interface	between Non-RT RIC (in SMO) and Near-RT RIC	higher level policy/model communication.
E2 interface	between Near-RT RIC and RAN nodes (O-DU, O-CU)	real/near real time control & telemetry.
F1 interface	between O-DU and O-CU	splits control & user plane (F1-C / F1-U) in conventional 3GPP / O-RAN architectures.
Open Fronthaul	between O-RU and O-DU / sometimes part of DU-RU split	usually called the FRONT-HAUL / FH interface, further subdivided into CUS Plane (Control + User + Sync) and M Plane (management).

Conclusion: In summary, O1 and O2 manage operations and infrastructure, while A1 and E2 enable policy control and real time interactions. The O1, O2, A1, and E2 interfaces are the connective tissue of the O-RAN ecosystem. By separating management (O1/O2) from control (A1/E2), O-RAN enables a layered, modular implementation of orchestration, policy enforcement and real time adjustments.

In short, these standardized open interfaces are essential for multi vendor interoperability, network flexibility, and intelligent automation; the core goals of the O-RAN architecture.