O-RAN Architecture: O-CU, O-DU & O-RU Explained

Introduction : There are three primary components in O-RAN architecture such as O-RU (Radio Unit), O-DU (Distributed Unit) and O-CU (Central Unit). Each plays a defined role in handling physical layer, MAC/RLC and higher layers respectively. This page covers their functional breakdown, interactions, splitting options and deployment considerations in the context of modern 5G networks.

O-RAN Architecture Components

O-RAN architecture consists of following components. It include O-RU, O-DU, O-CU, Near Real Time RIC, Non Real Time RIC, SMO and O-Cloud. Let us understand main functions or responsibilities of these components including their position in the system.

O-RU (Radio Unit)

• Location : At or near cell site/antenna tower; physically close to radio antennas.
• OSI functionalities : physical layer (Layer 1) mainly Low-Phy
• Main functions :
  • Handles RF, analog/digital front end processing including antenna, filtering, amplification
  • Executes Low-PHY (lower physical layer) tasks such as FFT/iFFT, cyclic prefix, digital front end, PRACH extraction etc.
  • Interfaces with O-DU over open fronthaul

O-DU (Distributed Unit)

• Location : Usually in edge or aggregation site; logically between O-RU and O-CU.
• OSI functionalities : layers 1 + 2 (High PHY, MAC, RLC)
• Main functions :
  • Scheduling, error correction (HARQ/ARQ), resource allocation etc.
  • Aggregates multiple O-RUs and manages real time processing for lower/mid layers
  • Implements F1 interface towards O-CU for control and user-plane data (when split).
  • Works with near-RT RIC via E2 interface

Image Courtesy : O-RAN Alliance

O-CU (Central Unit)

• Location : Deployed further towards core / central data centers; may be virtualized.
• OSI functionalities : layers 2 + 3 (PDCP, RRC, SDAP; user/control layers)
• Main functions :
  • Split into O-CU-CP (Control Plane) and O-CU-UP (User Plane). CP handles control signalling (e.g. RRC, control-plane PDCP) and UP handles user data plane (PDCP-user plane, SDAP) to better scale and separate load.
  • Acts as gateway between RAN side functions (DU/RU) and the 5G Core network
  • Handles mobility management, session management, bearer setup etc.
  • Interfaces with near-RT RIC and non-RT RIC (for metrics, policies etc.).

Near Real Time RIC

• Location : Deployed on edge cloud / regional DC; sits logically above O-DU/O-CU, connected via E2 interface.
• OSI functionalities : between layer 3 and application / control plane / management functions (control/optimization plane)
• Main functions :
  • Provides near real time control & optimization of RAN behavior with latency typically 10 ms to 1 second
  • Hosts xApps which are modules/applications that perform functions such as traffic steering, interference management, dynamic scheduling etc.
  • Uses policy/enrichment data from non-RT RIC and metrics from O-CU / O-DU to make decisions.
  • Provides primitives to adjust RAN parameters dynamically.

Non Real Time RIC

• Location : Part of SMO (Service Management & Orchestration) in a more centralized/cloud domain.
• OSI functionalities : above layer 3 (application / management / policy plane)
• Main functions :
  • Handles optimization / analytics over longer time scales (beyond ~1 second).
  • Policy generation, ML/AI model training, performance trends, enrichment data.
  • Provides guidance (policies, target KPIs) to near-RT RIC via A1 interface.
  • Manages part of lifecycle of RAN functions, configuration etc.
  • Also supports cross domain orchestration (many cells / regions) via SMO.

SMO

• Location : Central management / control domain; connects to all RAN components and controllers.
• OSI functionalities : above layer 7 (management / orchestration / orchestration plane)
• Main functions :
  • Oversees lifecycle management of O-RU, O-DU, O-CU etc.
  • Manages Fault, Configuration, Accounting, Performance, Security of RAN elements.
  • Provides infrastructure management (O-Cloud) and ensures multi-vendor interoperability etc.
  • Uses interfaces such as O1, O2, etc. for management and orchestration.

O-Cloud

• Location : Physical/virtual infrastructure (servers, compute, storage, networking / virtualization) that host virtualized network functions (O-DU, O-CU-UP/CP, RICs etc.)
• OSI functionalities : supports all layers as infrastructure (hosting compute, storage, virtualization, underlying stack)
• Main functions :
  • Provides compute, storage, networking virtualization needed for cloud deployment of RAN functions.
  • Works with SMO / O2 interface for infrastructure orchestration.
  • Provides resource or infrastructure management functions for reliability, fault tolerance, performance isolation etc.

Conclusion: By modularizing radio functionality across these units (i.e. O-RU, O-DU and O-CU), operators gain flexibility in placement, scaling and vendor selection. Balanced deployment of these components enables efficient fronthaul, optimized performance and smoother evolution toward programmable & virtualized RAN.