Info Image

The Ultimate Guide to Open RAN: Why Do Interfaces Need to Be Open?

The Ultimate Guide to Open RAN: Why Do Interfaces Need to Be Open? Image Credit: Moon Splinters/Bigstockphoto.com

A question that often comes up in Open RAN discussions is this: why do we need the Open RAN movement if the networks use 3GPP-based interfaces, which are already open and standardized? Here is our take on this important and relevant question.

3GPP interfaces

Let’s start with the basic wireless architecture. Using 4G/LTE as an example, the two interfaces in the RAN are:

  • The air interface, also known as Uu or LTE-Uu interface that uses the RRC protocol

  • The S1 interface, between the RAN and the Core

Both interfaces are standardized by 3GPP and open, so no issues here. However, the simplified 4G network is more like what is shown in detail in the figure below. There are two more interfaces that are the key reason the Open RAN movement started.

Source: Parallel Wireless

There are two components in the RAN. The virtualized BBU software (DU/CU) that runs on COTS servers and the Remote Radio Head (RRH) or RU. The interface between them is known as fronthaul, and it uses the CPRI protocol. This protocol generally has vendor-specific implementation and is not necessarily open. Open RAN-focused organizations are trying to get rid of this CPRI in the fronthaul by using other open alternatives. For example, O-RAN Alliance defines eCPRI to use with Split 7. Small Cell Forum, on the other hand, has defined N-FAPI to use with Split 6. Even though you may think that eCPRI and N-FAPI are specific to 5G, they can be used for 4G as well, along with other ethernet-based open fronthaul options.

X2 interface

The second interface to note is the X2 interface. Even though this interface has been defined by 3GPP, it is an optional interface. Many legacy RAN vendors intentionally did not implement this initially and, when they did implement it, they used many proprietary messages over this interface. This ensured that multi-vendor networks were difficult for an operator to deploy, which basically meant that an MNO was locked in into one specific vendor.

The main thing to note is that X2 is quite useful for a 4G network, even though it is an optional interface. This interface is essential for multi-vendor networks to function seamlessly, especially for managing interference.

Source: Parallel Wireless

As you may be aware, all the 5G deployments today are 5G Non-Standalone (NSA) deployments. People who are familiar with technical terminology also call this Option 3, 3A, 3X or by the 3GPP defined name, EN-DC. What this means in simple terms is that the 5G New Radio is used for the access network, but it only works in conjunction with the 4G LTE access network and the 4G core, also known as the EPC.

So, if the X2 interfaces are NOT open, then operators are forced to deploy 5G today using their existing 4G LTE vendors, hence the lock-in continues into 5G as well. In some cases, the operators have come up with innovative solutions where they have provided a new or existing small chunk of 4G spectrum to the new 5G vendor to break this 4G dependency, but not every operator has a spare chunk of spectrum available for these kinds of innovative solutions.

Open interfaces would be very helpful in such a scenario, and this is why the Open RAN movement is still necessary even though we have well defined 3GPP interfaces for many different connections, be it air interface or connecting to the core and to the outside world.

The interface options for MNOs to create an open RAN network

Option one is to have their vendors open up interfaces between the RAN components like the radio and the BBU/DU/CU software. The greatest example of this is Nokia in the Rakuten deployment, when they opened up their radios to another vendor’s software. But there is no guarantee that legacy vendors will continue to open up their radios to another vendors’ software. This is where TIP comes in, by creating an ecosystem of hardware and software vendors, initiating PlugFests and developing blueprints. TIP is not writing specs, but rather TIP is promoting, educating and deploying OpenRAN globally, starting in LATAM in 2016, then with Vodafone in Asia, Europe and DRC.

The second option is to use O-RAN Alliance-defined interfaces. The O-RAN Alliance was formed after the merger of the C-RAN alliance and XRAN. Today, it has more than 160 mobile operators, vendors, and research & academic institutions operating in the Radio Access Network industry. The O-RAN Alliance publishes new RAN specifications, releases open software for the RAN, and supports its members in integration and testing of their implementations. The O-RAN Alliance develops, drives and enforces standards to ensure that equipment from multiple vendors like RUs and DU/CU software interoperates with each other. The Alliance creates standards where none are available - for example, the Fronthaul specifications for the RAN functional splits to ensure interoperability. In addition, it creates profiles for interoperability testing where standards are available - for example, the X2 interface.

Earlier this year, these two groups announced a liaison agreement to ensure their alignment in developing interoperable Open RAN solutions. Because TIP is agnostic about the specifications it uses to create the solutions service providers are looking for, it has to work with various standards bodies to ensure smooth operation. But the liaison agreement with O-RAN Alliance allows for the sharing of information, referencing specifications and conducting joint testing and integration efforts. If you look at the TIP OpenRAN 5G NR Base Station Platform requirements document, you see normative references to the O-RAN Alliance specifications. Within TIP, only companies that are members of both the TIP and the O-RAN Alliance can participate in any discussions related to O-RAN specifications.

Option three is specifically for small cells. The Small Cell Forum, or SCF, has created its own ecosystem of Open RAN with small cells in mind. Recently they have been focusing heavily on creating open interfaces. Earlier this year, they expanded the set of specifications they released last year, to enable small cells to be constructed piece-by-piece using components from different vendors, in order to easily address the diverse mixture of 5G use cases. These open interfaces are called FAPI and nFAPI, which stands for network FAPI. FAPI helps equipment vendors to mix PHY & MAC software from different suppliers via this open FAPI interface. So, FAPI is an ‘internal’ interface. On the other hand, nFAPI, or more specifically 5G-nFAPI, is a ‘network’ interface and is between a Distributed Unit (DU) and Centralized Unit (CU) of a split RAN small cell network solution. This will help network architects by allowing them to mix distributed and central units from different vendors. In short, the SCF nFAPI is enabling the Open RAN ecosystem in its own way by allowing any small cell CU/DU to connect to any small cell radio unit or S-RU.

Source: Small Cell Forum

Summary

Global MNOs are realizing the economic benefits of open architecture that can only be fully realized when the interfaces are open. The industry is setting up teams and focusing on innovation and engagements in Open RAN architecture, be it through opening up 3GPP interfaces, or utilizing O-RAN Alliance or Small Cell Forum common and open interfaces. While earlier MNOs used to buy hardware and software together from one specific vendor, they are now splitting the orders for hardware and software, and open interfaces allow them to do it. This eliminates vendor lock in and results in significant CAPEX and OPEX reductions, competency and faster deployments.

Author

Eugina, a female executive and an immigrant, started her telecom career as a secretary and now has gone on to become the CMO of the prominent industry organization, Telecom Infra Project (TIP).

She has over 20+ years of strategic marketing leadership experience, leading marketing and communications for small and Fortune 500 global technology companies like Starent and Cisco.

Previously, she served as the VP of Marketing of the major telecom industry disruptor Parallel Wireless and was instrumental in creating the Open RAN market category.

She is a well sought-after speaker at many technology and telecom events and webinars. She is a well-known telecom writer contributing to publications like The Fast Mode, RCR Wireless, Developing Telecoms and many others.

She is also an inventor, holding 12 patents that include 5G and Open RAN.

She is a founding member of Boston chapter of CHIEF, an organization for women in the C-Suite, to strengthen their leadership, magnify their influence, pave the way to bring others, cross-pollinate power across industries, and effect change from the top-down.

Her passion is to help other women in tech to realize their full potential through mentorships, community engagement, and workshops. Her leadership development book “Unlimited: How to succeed in a workplace that was not designed for you” is due for release in May 2023.

Ms. Jordan resides in Massachusetts with her husband, teenage son, and three rescue dogs. She loves theater and museums. She volunteers for dog rescues and programs that help underprivileged children and women.

Ms. Jordan has a Master’s in Teaching from Moscow Pedagogical University, and studied computer undergrad at CDI College in Toronto, Canada.

PREVIOUS POST

Bringing More Cellular Capacity to Venues at a Lower Cost - the Lens Antenna Advantage

NEXT POST

The Ultimate Guide to Open RAN: Open RAN Components and RAN Functional Splits