I P5 nFAPI P7 nFAPI P7 Cell n Cell 1 FAPI P7 FAPI P7 PHY n PHY 1 DU - Small Cell Radio Unit networks. The Small Cell Forum is in the process of building on 5G FAPI to develop 5G nFAPI supporting the 3GPP split option 6 interface (3GPP TR 38.801); this section high- lights concepts from 4G nFAPI to capture the latest views on 5G nFAPI. The relationship between nFAPI and FAPI is shown in Figure 4.6.7 and can be described such that an nFAPI interface provides a wrapper around FAPI to enable support for a non-ideal backhaul and multiple carriers (PHY) in one DU. This extension defines one instance of nFAPI per DU and supports scenarios where a DU may implement CA or is a neutral host small cell. The top-level architecture principles of nFAPI can be summarized as: nFAPI is a wrapper around FAPI. nFAPI is defined between one instance of CU and one instance of DU. nFAPI will have one instance of P5 (control). nFAPI can have more than one instance of P7 - one for each PHY in the DU. Additional nFAPI P5 procedures are defined to support configuration and control of the DU. Additional nFAPI P7 procedures are defined to support a non-ideal connection between CU and DU. Following the FAPI philosophy nFAPI defines a state machine for the DU to ensure con- sistent behavior and this is shown in Figure 4.6.8: A DU starts in IDLE mode, where the CU can query the DU's capabilities using the PARAM message sequence. A DU can be moved from IDLE to CONFIGURED state by the CU initiating the CONFIG message sequence. 4 NG-RAN Architecture PHY n DU Device PHY 1 DU PARAM.request PARAM.request DU Idle DU CONFIG.request CONFIG.request DU CONFIG.reques CONFIG.request, NMM.request Configured Configured NMM.indication DU START. request DU STOP.request START.request STOP.request NMM.request DU Running Running DL Node Sync Figure 4.6.8 nFAPI state machine. (Source: Reproduced by permission of © Small Cell Forum). A DU in CONFIGURED state can be configured further, or have its capabilities queried. A DU can be moved from CONFIGURED to RUNNING stat