• It eliminates vendor lock-in and results in significant Capex and Opex reductions and faster deployments.

A question that often comes up in Open Radio Access Networks (Open RAN) discussions is this: why do we need the Open RAN approach if the networks use 3GPP-based interfaces, which are already open and standardised? 

Here is the explanation.

Role of 3GPP Interfaces

Let’s start with looking at 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

In theory, both interfaces are standardised by 3GPP and open. However, the simplified 4G network has two more interfaces that are the key reason the Open RAN movement started.

Let’s have a look at the two main components in the virtualised RAN. 

The virtualised 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 open which means, it doesn’t allow to mix and match components which hinders any interoperability. The lack of interoperability causes vendor lock-in.

Role of X2 Interface

The second interface to look at is the X2 interface. Even though it has been defined by 3GPP, it is an optional interface. X2 is useful for a 4G network as multi-vendor networks need to function seamlessly, especially for managing interference between different radios.

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, basically; again, an MNO was locked in into one specific vendor.

As you may be aware, all the 5G deployments today are 5G Non-Standalone (NSA) deployments. 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 (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. 

Open interfaces would be very helpful in such a scenario to enable vendor diversity, 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 the outside world. Open RAN enables vendor interoperability.

Interface options for MNOs

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 other vendors’ software. This is where TIP comes in, by creating an ecosystem of hardware and software vendors, initiating PlugFests and developing blueprints and promoting, educating and deploying OpenRAN globally. 

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. Besides, it creates profiles for interoperability testing where standards are available – for example, the X2 interface. 

In 2020, 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 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, 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 way by allowing any small cell CU/DU to connect to any small cell radio unit or S-RU.

Summary

Global MNOs are realising the economic benefits of an open architecture that can only be fully realised 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 utilising 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 and faster deployments.

• Eugina Jordan is the Vice-President for Marketing at Parallel Wireless.

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