Open RAN: Understanding the Distributed Unit (DU) and the Central Unit (CU)

Open RAN has ushered in a fresh chapter for telecommunications. By championing flexibility, interoperability, and cost efficiency, Open RAN is a progressive approach to network architecture. Central to this vision are two primary components: the Distributed Unit (DU) and the Central Unit (CU).

This article provides an in-depth exploration of these integral units within the Open RAN ecosystem. Consider this a simplified and accessible guide to these concepts, optimised for those new to the field.

Understanding the Distributed Unit (DU)

The Distributed Unit (DU) handles the lower layers of the protocol stack, which includes the Upper Physical, MAC, and RLC layers. Its key responsibilities include:

• Data Organisation and Management: The DU prepares data for transmission. It organises data into the right format for efficient radio transmission.
• Interaction with the Radio Unit (RU): The DU directly manages data communication with the Radio Unit, translating the organised data into radio waves for transmission and vice versa.
• Latency Reduction and Efficiency: By being closer to the RU and managing the lower protocol layers, the DU helps ensure minimal latency, which is especially crucial for real-time data exchange applications.

Metaphorically, think of the DU as the skilled technician of a symphony, tuning the instruments to perfection before the performance.

Acronym Explainers

• Upper Physical Layer: The part of the physical layer closest to the MAC layer. It deals with modulation, encoding, and other processes essential for preparing data for radio transmission.
• MAC (Medium Access Control) Layer: Responsible for controlling how data packets are placed on the network. It decides when data can be transmitted and helps prevent collisions by managing access to the physical medium.
• RLC (Radio Link Control) Layer: Ensures the reliable transmission of data between user equipment and the network. It handles segmentation, reassembly, and error correction of data packets.

Understanding the Central Unit (CU)

The Central Unit (CU) oversees the higher layers of the protocol stack, particularly the SDAP, PDCP, and RRC layers. Its primary roles include:

• Network Management: The CU manages broader data flow within the network, dictating how data packets travel through the infrastructure.
• Coordination with DUs: To maintain coherent communication between the core network and the RUs, the CU instructs the DUs, guiding their data management tasks.
• Strategic Decision-Making: For overarching network decisions—such as user mobility management and establishing user-specific communication bearers—the CU is pivotal.

To draw a parallel, the CU is akin to a symphony conductor, ensuring each section comes together harmoniously for a flawless performance.

Acronym Explainers

• SDAP (Service Data Adaptation Protocol) Layer: Maps between QoS flows and data radio bearers, marking QoS flow IDs on packets.
• PDCP (Packet Data Convergence Protocol) Layer: Handles user data and control information transmission. Tasks include header compression, security (ciphering and integrity protection), and in-sequence delivery of PDUs.
• RRC (Radio Resource Control) Layer: Responsible for the establishment, configuration, maintenance, and release of radio bearers. It deals with handovers, system information broadcasts, paging, and control of user equipment measurements.

The Importance of Integration and Openness

A key benefit of the Open RAN model lies in its commitment to vendor neutrality. This interoperability stands regardless of the specific equipment or software suppliers, breaking down proprietary barriers and fostering a more open, competitive ecosystem.

This vendor neutrality is enhanced by the E2 interface defined by the O-RAN Alliance, which—together with O1 and O2—enables a standardised approach to management, orchestration, and support for real-time telemetry exposure and control of CU and DU nodes. This approach brings AI/ML-driven network intelligence directly into the RAN.

This vendor neutrality offers network operators the flexibility to integrate best-in-class solutions without being tethered to a single provider.

Acronym Explainers

• 3GPP (3rd Generation Partnership Project): A collaboration between telecommunications standards associations, providing specifications for 3G, 4G, and 5G cellular systems.
• O-RAN Alliance: An industry-wide initiative shaping future RANs with more intelligence, openness, flexibility, and efficiency.
• E2 Interface: The interface between the RAN Intelligent Controller (RIC) and radio nodes, allowing real-time control and optimisation of the RAN.
• O1 Interface: Connects O-RAN components to the non-real-time RIC and SMO for management, orchestration, configuration, fault management, and performance monitoring.

Would You Like to Know More?

Open RAN is a true game-changer, capable of redefining, reshaping, and revitalising the industry. If you’re keen to look deeper and understand how Open RAN can revolutionise your operations—or if you have queries about its implementation—we’re here to assist. Click here to read more about Accelleran’s Private 5G Solution.