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.
Let’s start with the basic architecture, which you can check out in our Open RAN videos. Using 4G LTE as an example, the two interfaces from the RAN point of view 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 of these interfaces are standardized by 3GPP and open, so no issues here. However, the simplified 4G network is more like what is shown in this diagram, if we go just a little bit in detail. There are 2 more interfaces that are the key reason the Open RAN movement started.
The first is the fronthaul. As we discussed in our Open RAN concept video, there are two components in the RAN. The virtualized BBU software that runs on COTS servers and the Remote Radio Head or RRH. 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 the 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 these eCPRI and N-FAPI are specific to 5G, they can be used for 4G as well, along with other ethernet-based open fronthaul options.
The second interface to note is the X2 interface. Even though this interface has been defined by 3GPP, it is an optional interface. Many incumbent vendors intentionally did not implement this initially and, when they did implement it, they used many proprietary messages over this interface thereby ensuring that multi-vendor networks were difficult for an operator to deploy.
While we won’t cover everything the X2 interface does in this blog post, the main thing to note is that X2 is quite useful for a 4G network, even though it is an optional interface. For multi-vendor networks to function seamlessly, this interface becomes essential, especially for managing interference. It becomes even more important in case of 5G.
So, if the X2 interfaces are not open, then operators are forced to deploy 5G today using their existing 4G LTE vendors. 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 every operator does not have a spare chunk of spectrum available for these kinds of innovative solutions.
Open interfaces would be very helpful in such a scenario, and we hope this explanation helped clarify why the Open RAN movement is still necessary even though we use 3GPP defined interfaces for many different connections, be it air interface or connecting to the core and to the outside word.
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