A word about Open RAN

Posted on Oct 22, 2022 by Kashif Islam and Syed Hassan

As mentioned in the first and second blogs of this series, Cloud RAN is a subset of Virtualized RAN in the sense that it refers to Cloud Native RAN components. Another terminology, Open RAN, refers to an ecosystem that employs RAN components with open interfaces, thus allowing vendor interoperability across the Radio Access Network. Also mentioned previously was that vRAN and Cloud-RAN are concepts orthogonal to Open RAN i.e. an Open RAN deployment can use virtualized/Cloud Native or physical components (although industry has been gravitating towards Cloud Native approaches for quite some time now). Building on these concepts, the following picture provides a high level overview and correlation between these RAN terminologies.

Open RAN, vRAN, Cloud RAN, and Traditional RAN Overlap

This blog will primarily focus on the Open RAN ecosystem and major industry alliances championing the Open RAN cause i.e. the O-RAN Alliance and Telecom Infra Project (TIP).

Why Open RAN?

Radio Access Network (RAN) has always been a closed proprietary system. Virtually every deployment comprised a single vendor providing the antenna and the associated radio signal processing equipment - i.e. the Radio Unit (RU) and BBU. With a push toward RAN disaggregation (decoupling of RAN software from the specialized hardware) and decomposition (the breakdown of a BBU into CU and DU), there was also a desire by network operators not to be locked-in to the same vendor for their entire RAN infrastructure. Using a choice of vendors for different RAN components opens up the RAN market to competition, thus fostering innovation and, theoretically, driving down the cost for mobile service providers. This would, however, only be possible by defining standardized, open interfaces between these components. In short, this results in an RAN ecosystem where the RU, DU, and CU by different RAN equipment manufacturers would cohesively work together.

The Open RAN Challenge

Open RAN is easier said than done. The very first challenge towards creating an Open RAN ecosystem is the selection of appropriate RAN Functional Split to determine the BBU functions distribution across the RU, CU, and DU. 3GPP (sort of) tackled this in Release 14 of its specification by defining 8 split options titled Option 1 through 8 and recommending Option 2 as Higher Layer Split (HLS) Option as the demarc between the function performed by the CU and the rest of the RAN network. The Low Layer Split (LLS) option proved a little more challenging due to the complexities involved in disaggregating and virtualizing the Radio and Baseband signal processing functions. Thus 3GPP recommended further study on the Lower Layer Splits option - with a preference for Options 6, 7, 8 or a combination thereof. Without a clear consensus on the Lower Layer Split, the industry leaders led by mobile network operators, took it upon themselves to agree on a Low Layer Split Option in an effort to standardize and open the interface between the RU and DU. Enter the O-RAN Alliance.

O-RAN Alliance

Oftentimes used synonymously, it is important to distinguish between the Open RAN movement and the O-RAN Alliance. The term Open RAN is used as an umbrella to refer to the ecosystem of open interfaces between various RAN components (RU, DU, and CU) allowing vendor interoperability between them. The O-RAN Alliance, on the other hand, is the leading industry body working on defining various 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 with a goal to standardize software-focused Radio Access Networks. xRAN forum later joined forces with the C-RAN Alliance, led by China Mobile, that was working on furthering the centralized RAN architectures in what is now known as the O-RAN alliance.

The operator-led nature of the O-RAN alliance is one of the features that set O-RAN alliance apart from some of its industry counterparts. Today there are over 200 members in the O-RAN alliance comprising network operators, academic institutions, and of course, equipment manufacturers and vendors. However, each of the Working Groups of the O-RAN alliance 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 the use of virtualized RAN components with open and interoperable interfaces, use of COTS hardware, and a cloud platform layer across the RAN domain.

The current O-RAN Alliance’s structure comprises 11 working groups focused on technical specifications, design definitions and test strategies for Usecases and Overall Architecture, RAN Intelligent Controller (RIC), Open Cloud architecture and Orchestration for RAN Components, Open Fronthaul, xHaul architecture, OAM and security, among others. O-RAN has not only standardized the Low Layer Split by defining Split Option 7-2x, it has also worked on open interfaces in other 3GPP defined specifications such as F1, W1, E1, and Xn interfaces. More details about the various workgroups can be found at the O-RAN website. The figure below shows the current O-RAN Working Groups and a brief statement about their charter.

O-RAN Alliance Working Groups

In addition to the 11 workgroups, 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 4 such groups named Standard Development Focus Group (SDFG), Testing and Integration Focus Group (TIFG), Open Source Focus Group (OSFG) and next Generation Research Group (nGRG).

As can be see from the workgroup and focus groups, the work performed by the O-RAN alliance goes beyond just the open FrontHaul interface (i.e. the use of Split Option 7-2.x between O-RU and O-DU) and includes specification for O-Cloud Platform as well as Service Management Orchestration framework (SMO). The below picture summarizes the O-RAN alliance’s view of an Open RAN network and the interfaces between RAN components.

O-RAN Alliance View of RAN Components and Connectivity Interfaces

Note: The figure shown above is purposely drawn differently than the one typically shown by the O-RAN Alliance. More specifically, the O-Cloud is shown as a horizontal cloud platform layer, whereas in this picture O-Cloud is aligned with the RAN components that will be hosted on the cloud platform. More details on this horizontal cloud platform will be provided in the next blog.

It must be mentioned that the work on RIC, O-Cloud specifications and SMO framework is still in its relative infancy. There are specifications still being developed and vendor implementations starting to take shape.

Telecom Infra Project (TIP)

Telecom Infra Project (TIP) is a group that includes operators, equipment manufacturers and others that has the sole goal to facilitate connectivity solutions. TIP employs a project framework instead of working groups, and as such OpenRAN (notice the absence of space between Open and RAN) is a sub-project 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 respective solutions.

It must be noted that OpenRAN is just one of the projects undertaken by TIP. A bulk of TIP;s work also revolves around Fixed Broadbands, Non-Terrestrial Connectivity solutions, Wireless Backhaul, Open Core and others. One key distinction that can be made is that the O-RAN alliance is more of a technical specifications body, whereas TIP focuses more on the implementation and adoption of the O-RAN architectures.

Open RAN Summary

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

  • RAN has been a closed, proprietary system with a single vendor providing antenna and associated radio processing equipment i.e. the RU 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 CU and DU, are increasingly being implemented using cloud-native approach. Hence Open RAN implementations, in most cases, also adhere to Cloud RAN architectures.
  • 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, but are not limited to Open Fronthaul specification, the use of Open Cloud (O-Cloud) to host RAN application, RAN Intelligent Controller (RIC), Packet Switched xHaul Architectures, as well as Service Management Orchestration framework (SMO).
  • TIP is the 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