What is Open RAN?

In our first two articles in this series, The road to cloud RAN from 1G to 5G and 20 radio access network (RAN) terms to know, we talked about Cloud RAN, which is a subset of virtualized RAN (vRAN) that uses cloud-native RAN components. Open RAN, the focus of this article, is an ecosystem that uses radio access network (RAN) components with open interfaces, thus allowing vendor interoperability across the RAN.

[ Learn why open source and 5G are a perfect partnership. ]

As we explained in our second article, vRAN and Cloud RAN are orthogonal to Open RAN: an Open RAN deployment can use virtualized and cloud-native or even physical components (although the industry has been gravitating towards cloud-native approaches for a while). The following diagram illustrates this concept by providing a high-level overview and correlation between these RAN terminologies.

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This article will primarily focus on the Open RAN ecosystem and the major industry alliances championing it, the O-RAN Alliance and Telecom Infra Project (TIP).

Why Open RAN?

RAN has historically been a closed proprietary system. Virtually every cellular deployment comprised a single vendor providing the antenna and the associated radio signal processing equipment—the Remote Radio Unit (RRU) and the Baseband Unit (BBU).

With the push toward RAN disaggregation (decoupling RAN software from the specialized hardware) and decomposition (breaking a BBU down into a Distributed Unit [DU] and a Centralized Unit [CU]), network operators have also been pushing for openness in the RAN infrastructure to avoid being locked into a single vendor for the entire RAN infrastructure. Having a choice of vendors for different RAN components opens the RAN market to competition, thus fostering innovation and, theoretically, driving down the cost for mobile service providers. However, this requires defining standardized, open interfaces between these components. In short, this results in a RAN ecosystem where Radio Units (RUs), DUs, and CUs built by different RAN equipment manufacturers would work together cohesively.

[ Check out three Portfolio Architectures for 5G RAN, on-premises, or hyperscaler telecom designs. ]

The Open RAN challenge

Open RAN is easier said than done. The first challenge in creating an Open RAN ecosystem is selecting the appropriate RAN functional split to determine the distribution of functions across the RU, CU, and DU.

The 3rd Generation Partnership Project (3GPP) (sort of) tackled this in Release 14 of its specification by defining eight split options (options 1 through 8) and recommending option 2, the Higher Layer Split (HLS) option, as the boundary between the CU's function and that of the rest of the RAN network. (See our previous article for information about the eight options.)

The Low Layer Split (LLS) option proved a little more challenging due to the complexities of disaggregating and virtualizing the Radio and Baseband signal processing functions. Thus 3GPP recommended further study on the LLS option, with a preference for options 6, 7, 8, or a combination thereof.

Without a clear consensus on the LLS, industry leaders, led by mobile network operators, took it upon themselves to agree on an LLS option to standardize and open the interface between the RU and DU. Enter the O-RAN Alliance.

O-RAN Alliance

Although they're often used synonymously, it is important to distinguish between the Open RAN movement and the O-RAN Alliance.

• "Open RAN" is an umbrella term for the ecosystem of open interfaces between decomposed RAN components (RU, DU, and CU), allowing vendor interoperability among them.
• The O-RAN Alliance is an operator-led industry body working to define specifications to realize the goals of enabling and fostering an Open RAN ecosystem.

The O-RAN Alliance finds its roots in the xRAN Forum, founded in 2016 by AT&T, SK Telecom, Deutsche Telekom, and some members of academia to standardize software-focused RANs. xRAN Forum later joined forces with the C-RAN Alliance, led by China Mobile, which was working to further the centralized RAN architectures. This resulted in what is now the O-RAN Alliance.

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The operator-led nature of the O-RAN Alliance is one of the features that sets it apart from some of its industry counterparts. Today, there are over 200 members in the O-RAN Alliance comprising network operators, academic institutions, and equipment manufacturers and vendors. However, each of its working groups is chaired by a network operator and works on technical specifications to further the basic tenets defined by the O-RAN Alliance. These tenets are centered around using virtualized RAN components with open and interoperable interfaces,  commercial off-the-shelf (COTS) hardware, and a cloud platform layer across the RAN domain.

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The O-RAN Alliance's structure comprises 11 work groups focused on technical specifications, design definitions, and test strategies for various aspects of RAN.

O-RAN has standardized the LLS by defining split option 7-2x, which is currently the leading Open FrontHaul split option, thus defining the Open RU (called O-RU) and Open DU (O-DU). Additionally, the O-RAN Alliance has worked on integrating 3GPP-defined interfaces such as F1, W1, E1, and Xn in an O-RAN Architecture.

The current O-RAN work groups are:

• WG1: Use Cases and Overall Architecture Work Group: Defines the overall O-RAN architectures and real-world use cases while working with other working groups
• WG2: The Non-Real-Time RAN Intelligent Controller and A1 Interface Work Group and WG3: The Near-Real-Time RIC and E2 InterfaceWork Group: These define the Non-Real-Time and Near-Real-Time RAN Intelligent Controller (RIC). RIC is the software application used to control and optimize RAN functions. In summary, a Non-RT RIC defines RAN policies, whereas Near-RT RIC enforces these policies for a better RAN experience.
• WG4: The Open Fronthaul Interfaces Work Group: Dedicated to defining the open interface (and standardized split option) between O-RU and O-DU
• WG5: The Open F1/W1/E1/X2/Xn Interface Work Group: Focuses on midhaul open interfaces (F1 in 5G, W1 in 4G), between CUs Control/Data plane (F1), gNB to gNB (X2), gNB to eNB (Xn)
• WG6: The Cloudification and Orchestration Work Group: Drives the decoupling of RAN software from specialized hardware and develops reference designs to allow the use of commodity hardware
• WG7: The White-box Hardware Work Group: Specifies reference designs for decoupled open RAN software and hardware; for example, Open FH Gateway (FHGW) specifications were defined by WG7
• WG8: Stack Reference Design Work Group: Defines the software architecture for Open CU and Open DU in line with O-RAN Alliance and 3GPP specifications
• WG9: Open X-haul Transport Work Group: Defines the transport network architecture, technologies, and services across the front-, mid-, and backhaul to transport mobile traffic
• WG10: Operations, Administration, and Management (OAM) Work Group: Gathers OAM requirements and defines OAM architecture for the O-RAM ecosystem
• WG11: Security Work Group: Focuses on securing the O-RAN ecosystem

There are also a few research and focus groups within the O-RAN Alliance. These groups focus on overarching technical work that applies to the whole ecosystem. Currently, there are four such groups: Standard Development Focus Group (SDFG), Testing and Integration Focus Group (TIFG), Open Source Focus Group (OSFG), and next Generation Research Group (nGRG).

The O-RAN Alliance's work goes beyond just the open fronthaul interface (using split option 7-2.x between O-RU and O-DU). It includes a specification for the O-Cloud Platform and Service Management Orchestration framework (SMO). The O1 and O2 Interfaces define the interaction between O-Cloud, SMO, and RAN components. The diagram below summarizes the O-RAN Alliance's view of an Open RAN network and the interfaces between RAN components.

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Note: We purposely drew this figure differently than the one typically shown by the O-RAN Alliance, which shows the O-Cloud as a horizontal cloud platform layer. In this picture, O-Cloud is aligned with the RAN components hosted on the cloud platform. Our next article will provide more details on this horizontal cloud platform.

The work on RIC, O-Cloud specifications, and the SMO framework is still in its infancy. However, specifications are being developed, and vendor implementations are starting to take shape.

[ Learn how to design a 5G Core platform that scales well. ]

Telecom Infra Project (TIP)

The Telecom Infra Project (TIP) is a group that includes operators, equipment manufacturers, and others that has the sole goal of facilitating connectivity solutions. TIP employs a project framework instead of working groups. As such, OpenRAN (notice the absence of a space between Open and RAN) is a subproject of TIP that defines a reference architecture for Open RAN. TIP and the O-RAN Alliance often work together and have a liaison agreement to ensure interoperability and compatibility between their solutions.

OpenRAN is just one of TIP's projects. Much of TIP's work revolves around Fixed Broadbands, Non-Terrestrial Connectivity solutions, Wireless Backhaul, Open Core, and others. One key distinction is that the O-RAN Alliance is more of a technical specifications body, whereas TIP focuses more on implementation and adoption of the O-RAN architectures.

Summary

To summarize, here are the basic things to understand and remember about Open RAN:

• RAN has long been a closed, proprietary system with a single vendor providing the antenna and associated radio processing equipment (the RRU and BBU).
• Open RAN refers to the ecosystem that aims to provide open interfaces and interoperability between RAN components across vendors, thus enabling mobile operators to avoid vendor lock-in.
• Open RAN components, specifically the O-CU and O-DU, are increasingly being implemented using a cloud-native approach. Hence Open RAN implementations, in most cases, also adhere to Cloud RAN principles.
• The O-RAN Alliance, led by network operators, is the leading industry body championing the Open RAN cause and defines the specification to further the Open RAN ecosystem.
• O-RAN Alliance specifications include the Open Fronthaul specification, the use of Open Cloud (O-Cloud) to host RAN applications, RAN Intelligent Controller (RIC), Packet Switched xHaul Architectures, and the Service Management Orchestration framework (SMO), among other things.
• TIP is an implementation and adoption-focused community of companies and organizations. OpenRAN is the TIP project group that works to accelerate real-world deployments of open, disaggregated RAN.

Read other articles in this series:

• The road to Cloud RAN from 1G to 5G
• 20 radio access network (RAN) terms to know
• What does Red Hat OpenShift have to do with Cloud RAN?
• Comparing Red Hat OpenShift models for Cloud RAN deployments

This article originally appeared on the Cloudify{ing}.Network{ing} blog and is republished with permission.