equest Figure 4.6.4 FAPI state machine. (Source: Reproduced by permission of © Small Cell Forum). is supporting several different bandwidth parts (BWP), with different subcarrier spacing, then the MAC may be exchanging FAPI messages with the PHY at different periodicities for different BWP. FAPI reduces this complexity by providing all per-slot control messages in a single down- link control message, DL_TTI.request, and all downlink MAC PDUs in a single downlink data message, TX_Data.request. Similarly, for the uplink there is a single uplink control message, UL_TTI.request, and all similar uplink is grouped together into one message, RX_Data.indication, UCI.indication, SRS.indication, and RACH.indication. The FAPI procedure to transmit data on the DL-SCH is shown in Figure 4.6.5: The MAC sends a DL_TTI.request message to the PHY. In this message there will be one PDSCH PDU for each UE with data in slot N, where the PDSCH PDU provides configura- tion information instructing the PHY how the MAC PDU, received in TX_Data.request, will be encoded. For two codeword transmissions then only one PDSCH PDU is included, containing the modulation and coding scheme of both codewords. The DL_TTI.request also includes PDCCH PDUs, which specify Downlink Control Information (DCI) sent to the UE to indicate data are scheduled. It should be noted that not every DL-SCH trans- mission requires a DCI; for example, data using configured grant scheduling. The MAC sends a TX_Data.request message to the PHY. In this message there will be one MAC PDU for PDSCH PDU with one codeword, and two MAC PDUs for PDSCH PDU with two codewords. The PDUs in TX_Data.request and _TTI.request contain an iden- tifier, included in both messages, used to link the MAC PDU and PDSCH PDU together. The structure of TX_Data.request is flexible to support different underlying small cell architectures by allowing the MAC PDU data to be referenced by a pointer to memory, a list of pointers to memory locations, or the actual data. The PHY encodes the MAC PDUs receiv