of FAPI is shown in Figure 4.6.3 showing the high-level FAPI principles: 4.6 Small Cells Control L2/L3 Software Sched L1 API Figure 4.6.3 FAPI architecture. (Source: Reproduced by permission of © Small Cell Forum). FAPI is defined between one instance of MAC and one instance of PHY. In scenarios such as CA there will be multiple instances of FAPI, one per carrier. The PHY configuration messages are defined by the logical interface P5. This configura- tion is semi-static and is being generated by a PHY control entity. The PHY data-plane messages are defined by the logical interface P7. This configuration is generated once per slot and consists of: Slot configuration messages, which include information required by the PHY to encode and decode data. Downlink data messages to transfer MAC PDUs to the PHY. Uplink data messages to transfer MAC PDUs, Uplink Control Information (UCI), Sounding Reference Signal (SRS), and Random Access Channel (RACH) PDUs from the PHY. To support consistent PHY behavior, FAPI defines a state machine for the PHY and this is shown in Figure 4.6.4: A PHY starts in IDLE mode, where the MAC can query the PHY's capabilities using the PARAM message sequence. A PHY can be moved from IDLE to CONFIGURED state by the MAC initiating the CONFIG message sequence. A PHY in CONFIGURED state can be configured further, or have its PHY capabilities queried. A PHY can be moved from CONFIGURED to RUNNING state by the MAC initiating the START message sequence. A PHY in RUNNING state is transmitting over the air and provides a small cell service. To stop the small cell the MAC must initiate the STOP message sequence. Once a PHY is operational the interaction between the MAC and PHY occurs at the periodicity of a slot. Since 5G supports several different subcarrier spacing values these per-slot messages can be exchanged every 125 us, 250 us, 500 us, or 1 ms. In fact, if a carrier 4 NG-RAN Architecture CONFIG.request CONFIG.request CONFIGURED PARAM.request START.request PARAM.request STOP.request RUNNING CONFIG.r