Reading the O-RAN architecture diagram
The O-RAN ALLIANCE describes a logical architecture that adds open interfaces and intelligence to the functional split defined by 3GPP. A first look at its diagram can be confusing because it mixes radio functions, management functions and control functions. It helps to separate them into three groups: the radio network functions that carry user traffic, the intelligence layer that observes and steers the network, and the management and cloud layer that deploys and operates everything.
The radio network functions
- O-RU (O-RAN radio unit): the radio frequency and lower physical layer functions close to the antenna.
- O-DU (O-RAN distributed unit): real-time baseband functions such as higher physical layer, MAC and RLC processing.
- O-CU (O-RAN centralized unit): higher-layer protocol functions, split into control-plane and user-plane parts.
- Open fronthaul: the interface between the O-RU and O-DU, with specifications covering control, user and synchronization planes.
These units may run at different locations. An operator might place the radio units on towers, the distributed units at nearby sites or edge data centers, and the centralized units deeper in the network. Latency limits between units shape where each can sit.
The intelligence layer
The RAN Intelligent Controller, or RIC, is a platform that hosts applications which monitor and influence radio behavior. A near-real-time RIC works on timescales of roughly tens of milliseconds to a second and talks to RAN nodes over the E2 interface. A non-real-time RIC operates on longer timescales, supports model training and policy management, and talks to the near-real-time RIC over the A1 interface. Applications that run on them are called xApps and rApps respectively. The next article in this series covers them in more detail.
The management and cloud layer
The Service Management and Orchestration framework, or SMO, handles configuration, fault and performance management, and lifecycle operations. The O1 interface carries management data between the SMO and network functions, and the O2 interface connects the SMO to the cloud infrastructure, called the O-Cloud, that hosts virtualized or containerized functions. This is where Open RAN meets general cloud practice, including container platforms, orchestration and automation.
What is standardized and what is not
Interfaces are specified, but implementations differ. Vendors may support optional features, interpret specifications differently or add proprietary extensions. That is why interoperability testing matters, and why a buyer should ask which versions of which specifications a product supports. Specification releases are published by the O-RAN ALLIANCE and change over time, so verify details against the current documents.
As 6G work proceeds, many expect the interfaces and the intelligence layer to evolve. The architecture described here is a snapshot of current thinking rather than a final 6G design.
A worked example of the data flow
Consider a user streaming video in a busy cell. The O-RU receives the radio signal and passes frequency-domain data over the open fronthaul to the O-DU, which completes physical layer, MAC and RLC processing. The O-CU handles higher-layer protocols and forwards traffic toward the core. Meanwhile, the O-DU reports measurements over E2 to the near-real-time RIC, where an application may recommend a different cell for the user. The SMO watches the whole chain through O1 and keeps long-term records for analysis. Seeing one flow end to end makes the diagram far easier to read.
