Add yet another group to the chorus calling for open RAN standards. This time, it’s the Small Cell Forum (SCF) that’s calling for the cellular industry to adopt open air interface and networking standards to avoid fragmentation and ensure interoperability of equipment.
Importantly, however, the SCF is addressing something that is central to the 5G architecture: small cells, which are moving from the periphery of the network to the heart of the network.
As SCF puts it: “Densification absolutely requires an open ecosystem to make deployment of large numbers of cells cost-effective. These interfaces are the enabler of the innovation and price competition that comes when operators can select equipment from many suppliers.”
The group previously explained its positioning, but this week, during a plenary meeting in Santa Clara, California, it put out a new call for the industry to prevent fragmentation.
Of course, the move to open interfaces has been more advanced in the small cell layer than the macro network. The forum points out that several suppliers already offer architectures in which a number of small cells are clustered around a centralized, virtualized controller.
But here’s the problem, as explained by SCF’s Tech Group Lead, Prabhakar Chitrapu, via email: “Split RAN/SC architectures have multiple (7 to be specific) options, as identified by 3GPP. Of these, 3GPP has focused on Option-2 (RLC-PDCP) and ORAN on Option-7.2 (PHY-PHY). Option-6 (PHY-MAC) is not being addressed by any of these organizations. SCF seeks to fill this gap.”
Furthermore, Chitrapu said, PHY-MAC interface is important for the industry, because it is an interface that has been highly successful and implemented in the 4G world, where it is called FAPI and nFAPI. “It is therefore considered very important that we extend these interface specifications for 5G, as 5G-FAPI and 5G-nFAPI,” Chitrapu said.
According to Chitrapu, the difference between FAPI (functional application platform interface) and nFAPI (network FAPI)—also applicable for their 5G counterparts 5G-FAPI & 5G-nFAPI—is as follows:
- 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 is a “network” interface and is between a Distributed Unit (DU) and Centralized Unit (CU) of a Split RAN/SC network solution. An open specification of this interface (nFAPI) will help network architects by allowing them to mix DU & CU from different vendors.
The SCF acknowledged in a blog post last month that there’s a risk there will be too many competing interfaces and work that was supposed to end fragmentation will actually contribute to it.
The Telecom Infra Project (TIP) and the ORAN Alliance are two well-known efforts that are underway to build an open radio access system, and SCF says it has an extensive partnership program that includes several open RAN efforts, including ORAN and TIP. It’s possible that efforts will converge, but in the meantime, SCF figures 3GPP standardization is the best way to ensure the end result is fully harmonized.