The ABCs of Open RAN

April 21, 2021

By Ankur Sharma


Open RAN (Radio Access Network) has gained a great deal of momentum since the inception of the O-RAN Alliance in 2018, and it will come into play as mobile operator deploy their 5G networks. Radisys regularly supports Open RAN initiatives through leadership in the O-RAN Alliance, the Telecom Infra Project and the Small Cell Forum, in addition to participation in the Open RAN Policy Coalition and other standards organizations. In this blog, we break down the terms that are important to know in this new ecosystem.

Welcome to Open RAN 101

This is by no means a complete breakdown of all terminology associated with Open RAN. However, these key terms are a great way to better understand Open RAN endeavors and deployments.  

The term “Open RAN” is a term for RAN solutions developed from open, commercial off-the-shelf (COTS) hardware and open interface standards. This form of RAN is:

  • Disaggregated hardware from software
  • A CUPS architecture (Control and user plane separation)
  • Deployed in flexible options:
    • PNF (Physical network function like bare metal)
    • VNF (Virtualized network function like in SDN/NFV)
    • CNF (Containerized network function like in cloud native components)

The aforementioned standards bodies work to standardize architecture based on real-world deployment scenarios and to accelerate the deployment of Open RAN solutions. 

Open RAN Alphabet Soup: RU, DU, CU & EMS

In an Open RAN system, the RAN is divided into four important elements: The Radio Unit (RU), the Distributed Unit (DU), the Centralized Unit (CU) & the Element Management System (EMS).

RU: This part handles the RF (radio frequency) and Low-PHY (Low Physical layer). The RU converts radio signals to/from the antenna to a digital signal that can be transmitted over the fronthaul interface (i.e., eCPRI) to Distributed Unit (DU) which are ethernet packets. O-RRU (ORAN RRU) equipped with handling of DFE (digital front end), AFE (analog front end), time synchronization, MIMO & digital beamforming functionality towards Antenna interface.

DU: Open Distributed Unit (O-DU) is a most important entity which handles the L2 (lower layers) of the baseband processing like MAC (medium access control) and RLC (Radio link control). Radio resource allocation, user scheduling, interference control, QoS handling service are handled in the MAC layer. The RLC layer handles buffering, transmission and radio link management. The DU is responsible for all real time traffic scenario for all use cases.

CU: This unit is the interface to 5G Core network in North bound and supports DU in southbound. Handling of non-real time Layer 2 and Layer 3 workloads such as Radio Resource Control (RRC) as control plane entity and Packet Data Convergence Protocol (PDCP) along with newly defined Service Data Adaptation Protocol (SDAP) for user plane. It also enables the mobility and session management plus Quality of services enforcement through interfaces like NG-AP, Xn in northbound & F1 interface towards DU in southbound.

EMS: A centralized management system is an integral part of overall Open RAN architecture. This entity manages FCAPS (Fault, configuration, accounting, performance Security) functionality, NB interfaces toward operator’s OSS/BSS, ZTP (zero touch provisioning)/auto provisioning, container management, application software, and RU, DU, CU managements.

The setup of the disaggregated RAN architecture opens the door for operators to select hardware based on the needs of the particular node. Therefore, the CU, the DU and the RU can all be provided by different hardware vendors, and operators can decide the amount of processing power and additional functionalities that are necessary. 

Functional Splits

Functional splits in the RAN originated with 3GPP in Release 14 and continued in Release 15. The initial 5G use cases—enhanced mobile broadband (eMBB), ultra-reliable low latency communications (URLLC), and massive machine-type communications (mMTC)—needed an extremely flexible RAN architecture with control and user plane functions to allow for a feature-rich, responsive network that is able to support a range of use cases and other deployment configurations. The deciding factor for split function is real time vs. non-real time traffic scenarios, load and traffic steering CAPABILITY and enabling the effective coordination among DU-DU, DU-CU & CU-CU. Segmenting functionalities such as physical locations, workload types or both is integral to Open RAN and, in the bigger picture, 5G as a whole. As a result, multiple vendors provided different pieces and created the need for an interoperable framework.

With eight possible functional split options in 3GPP’s 5G-NR RAN2 specification, it was difficult to address the fronthaul requirements. After much consideration, 3GPP defined the following two 5G-NR split architectures in addition to the traditional monolithic one:

  • Option 2 – a high-level CU and DU split which is mainly a separated control and user plane
  • Option 7 (or 7.2) – a low-level split for URLLC and near-edge deployment
  • Option 6 (MAC-PHY split): The Small Cell Forum defined Option 6 in the nFAPI specification – the MAC and upper layers are located in the central unit and the PHY layer and RF are in the DU

These architectures needed additional development in order to reduce complexity in the remote radio unit (RRU), to add more intelligence to the local RAN, to increase cost competitiveness and resiliency and to support disaggregated spectrum and efficient transport utilization.

Plain and Simple: O-RAN Fronthaul Planes of Operations

The O-RAN Fronthaul specification developed by the O-RAN Alliance encourages Open fronthaul which enables any vendor of DU to be compatible with any vendor of RU. This Alliance also foresees all of the various scenarios regarding how a base station will interact with the radio, what the underlying 5G demands will be, time synchronization issues between two endpoints, and ensures that it is deployment-ready for operators. By addressing these various issues, the O-RAN standard is safeguarding interoperability between the base station and radio vendors. As a result, there are already commercial deployments leveraging the O-RAN Fronthaul specification. O-RAN Fronthaul defines multiple planes of operations, including: 

  • Control Plane (C-Plane): Messages define the scheduling, coordination needed for transferring data, including aspects of beamforming for 5G.
  • User Plane (U-Plane): Messages for efficient data transfer within the strict time limits of 5G numerologies.
  • Sync Plane (S-Plane): Addresses the timing and synchronization aspects between the DU and RU.
  • Management Plane (M-Plane): Defines the messages to manage the radio unit.

Get Smart: RAN Intelligent Controller

Following the O-RAN Alliance’s open Fronthaul specification contribution, the new, Open RAN for 5G networks needed to rise to the challenge of not only being disaggregated and capable of supporting a multi-vendor ecosystem, it also had to be smart.

In an effort to provide the network with automation and intelligence, the O-RAN Alliance introduced the RAN Intelligent Controller (RIC). For some time now, the telecom industry has been considering applying Artificial Intelligence (AI) and Machine Learning (ML) to the network – especially in the RAN – to enable increased efficiencies and lower operational expenses. The O-RAN Alliance aims to deliver both autonomous control and intelligence in the way radio networks are managed via RAN Intelligent Controller (RIC) nodes. The addition of AI and ML to the network in the RIC will lighten the load and need for human intervention within management and operations. Soon, operators will see their benefits in RAN and the industry will see the growth of new ecosystem players that are specialized in these algorithms. The RAN will become even more disrupted as these players find their seat at the table and transform the future of the RAN landscape.

Open RAN: No End in Sight

Open RAN-based solutions are in the early stages of deployment and new terms will continue to be added to the lexicon. Radisys will continue to be forefront of this RAN revolution. Visit our Open RAN portfolio for additional information about Radisys’ contributions to the industry and the innovation we bring to this space.

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