When dealing with a cloudified 5G radio access network (RAN) what is the difference between centralized RAN (C-RAN) or cloud RAN (also C-RAN), virtualized RAN (vRAN), open RAN and O-RAN?
Turns out, it depends how involved and strict you want to get with definitions, which, indeed, have shifted slightly over time.
So let’s get into it. Here are the simple and more complicated definitions of the elements that make up the cloud RAN.
A Simple Take
“Whenever you talk about vRAN, you’re literally talking about disaggregating the hardware and the software,”Mohammed Al Khairy, director of product marketing at Qualcomm said. This should -- in theory -- remove the need for proprietary telco vendor black boxes, running unknown software, and allow mobile operators to use general purpose servers.
“You can use any software that works with any hardware,” Al Khairy claimed.
This will allow operators more choice in the hardware and software they use to run the radio network. "Once you've done the virtualization you can really break these components into pieces," he continued.
Open RAN, he said, means that the interfaces between these components can be standardized. "So you can bring different hardware talking to different hardware and software" from separate vendors, he explained.
Analysys Mason has predicted that the industry will start to see more macro V-RAN deployments in 2023 and 2024. In addition, the firm expects that some vRAN deployments will start to support open RAN from 2025.
A complex answer
There are typical "strict" definitions that the industry had in the early days, analyst Roy Chua at AvidThink told Silverlinings via email. He said that the categorization usually breaks down into:
Centralized (C-RAN or CRAN)
Centralized baseband units (BBUs) are connected via fronthaul fiber networks to the remote radioheads (RRHs), Chua said.
“Centralization of BBUs provided opportunities to improve coordination between RRHs in neighboring cells, optimizing spectrum use. However, C-RAN is also confusingly used to refer to RAN built on cloud-native infrastructure,” the analyst wrote.
Virtualized RAN (vRAN)
Distributed RAN, and centralized-RAN have historically been implemented using vendor-proprietary hardware-based appliances from RAN incumbents like Ericsson, Huawei, and Nokia.
The vRAN movement applies NFV principles to the RAN, virtualizing proprietary RAN systems into software running on commercial-off-the-shelf (COTS) hardware — for instance, a virtual BBU (vBBU) on x86 or Arm-based servers. vRAN provides additional flexibility and potentially lowers the cost of the RAN through leveraging commodity hardware, Chua said.
Cloud-native RAN (cloud RAN, also confusing called C-RAN)
“This is now the more common use/definition of ‘C-RAN’," Chua said. It involves taking the virtualized RAN functions, breaking them into cloud-native architectures and leveraging cloud-type services (micro-services, message queues, RESTful APIs), so not the monolithic virtualization, but rather cloud-architected components.
Open RAN (sometimes also called O-RAN)
Get ready because it's complicated. Open RAN refers generically to architectures that, firstly, have disaggregated components, the analyst stated. The RRH + BBU (or vBBU) combination is now split into a centralized unit (CU), a distributed unit (DU) and a radio unit (RU).
The DU is connected to the RU via the frontal, and the CU is connected to DU via a mid-haul link. In turn, the CU is connected via a backhaul to the mobile core. Note that the CU, DU and RU don’t always have to be separately located, and a single CU can serve multiple DUs, just as a DU can serve multiple RUs.
Secondly, in open RAN, the interfaces between the CU, DU and RU need to be agreed-upon and openly defined, allowing the interchanging of CU, DU and RU from different vendors.
Thirdly, the functional splits in terms of the work performed by the various components (CU, DU, RU) have to be decided upon in a specific implementation while conforming to the options (split options 1 to 8) as defined by the 3GPP. And then, of course, the O-RAN moniker is applied to signal compliance with the standard interfaces between the CU/DU/RU as defined by the O-RAN Alliance.
The modern approach
“Today, most of the new RAN architectures are increasing virtualized (a.k.a. NFV), so vRAN is the more common (e.g. like what Verizon has deployed and what Ericsson is touting),” the analyst wrote. “vRAN refers mostly to use of commodity hardware to run RAN functions (with exception of the RU, and the DU still needs some acceleration help),” he said.
“Then the cloud-RAN term is used to indicate that the vRAN functions have been re-architected, so the code components in the CU, DU are broken up into micro-services that provides greater agility, flexibility, scalability, and [are] sometimes touted to have higher resiliency. And cloud-RAN is understood to run on commodity hardware components or at least x86/ARM architectures,” Chua said.
“O-RAN is then reserved for vRAN or cloud-RAN systems that adhere to the O-RAN Alliance standard interfaces,” the analyst finished.
Whew. Got that?
We RAN, RAN so far...
So there you go, hope that clears up some of the definitions happening in the cloudified RAN market on a board level. We’ll probably revisit this in a year to find that the exact same acronyms now have different meanings!