The 5G Standard

SCF driving open 5G RAN/SCN products and networks

Mar 01,2021

By Prabhakar Chitrapu, Chair of SCF - Article first published in HIGHLIGHTS Issue 01, Sept. 2020.

SCF’s comprehensive suite of 5G FAPI/nFAPI (functional API) specifications stimulates innovation and competition across the small cell ecosystem by supporting multivendor interoperability within Small Cell Products and between different products of Small Cell Networks, architected in consistence with 3GPP’s disaggregated functional-splits.

Specifically, the suite of open 5G-FAPI specifications enable multi-vendor interoperability between vendors that provide the silicon chipsets and protocol software that go into Small Cell Products. Similarly, 5G-nFAPI suite of open specifications enables interoperability between vendors of different products of the 3GPP Split-Option-6 based RAN architecture, namely the Radio Unit (named by SCF as S-RU) and Distributed Unit (named by SCF as S-DU). The 5G-FAPI specification suite has been published recently, while the 5G-nFAPI specification suite is under active development. In the latter, SCF is collaborating with other organizations such as O-RAN Alliance and OAI.

SCF’s 5G-FAPI suite of specifications is the result of intensive cooperation across the ecosystem by a team led by Intel and Qualcomm. The suite consists of 3 interfaces, named as P4, P5, P7 & P19, between the software layers that implement the MAC & upper protocol layers and hardware that implement the PHY & RF layers of a 5G-Small Cell.

  • P5 & P7 interfaces: Control and Data path interfaces between MAC & PHY layers respectively
  • P19 interface: Control interface between the L2/L3 software and RF Front-End-Unit (FEU)
  • P4 interface: “Sensing” interface for the Network Monitor/listen Mode (NMM) function.

Together, these interfaces, comprising the 5G-FAPI suite of specifications, provide a robust set of APIs to enable fully open, multivendor 5G small cell designs down to chip level. The figure below illustrates these interfaces and the structure of the 5G-FAPI specifications.


The 5G-FAPI suite enhances the existing set of 3G and 4G specifications, which have played a significant part in growing the market for small cells by enabling a broad multivendor ecosystem. The development of the 5G FAPI suite is a proud achievement for the Forum and its members.

As mentioned at the beginning, a complementary suite of open specifications, namely 5G-nFAPI, is being actively developed by a broad spectrum of hardware and software vendors within the Forum. Active collaboration with O-RAN is being used to maximize the alignment between the specifications of both organizations. Figure below defines the Disaggregated 5G-SCN architecture based on 3GPP’s split-option-6, SCF’s 5G-nFAPI interface and SCF’s product terminology (S-RU, S-DU, S-CU).


About the 5G FAPI Suite

The latest FAPI specifications are the result of intensive cooperation between SCF members from across the ecosystem. They provide common logical interfaces for the main components of a 5G small cell, namely:

  • 5G implementation of PHY-API, which aligns to 3GPP P5 and P7 interfaces (the PHY links the radio channel to the MAC, which then supports functions like error correction and scheduling).
  • the RF and digital front end control interface (3GPP P19), including 5G enhancements to support tight timing requirements imposed by the use of millimeter wave spectrum.
  • the network monitor/listen mode API (3GPP P4).