indication message. This CRC information is separated from the MAC PDU since it gives the opportunity for the PHY to send the CRC.indication before the Data.indication, which can help the MAC when performing uplink scheduling. 4 NG-RAN Architecture sd UL-SCH L1 PHY L2/L3 software (from Actors) (from,FAPI) (from Actors) UL_DCI.request(PDCCH info sent in slot N) Control info sent to UE() UL_TTI.request(PUSCH in slot N+K) MAC PDU sent to UE() CRC.indication() RX_Data.indication() opt UCI expected UCI.indication() Figure 4.6.6 FAPI uplink data procedure. (Source: Reproduced by permission of © Small Cell Forum). If the UE was also due to transmit uplink control information then the PHY will also locate and decode this information (based on control received in the UL_TTI.request PUSCH PDU). This uplink control information is sent to the MAC by an UCI.indication message. FAPI has additional procedures to transmit and receive all logical channels; BCH, PCH, and RACH. These are primarily variations of the downlink and uplink data procedures, SO not detailed here. 4.6.5.2 nFAPI Network FAPI (nFAPI) (SCF 082) is an extension introduced by the Small Cell Forum to support a disaggregated small cell eNB for 4G. The MAC and above protocol layers are located in a CU, while the PHY and RF are situated in a DU. 15 nFAPI assumes the connec- tion between the CU and DU is non-ideal, where non-ideal means that there is a limitation in either latency, or throughput capabilities of the link between CU and DU. An example of a non-ideal connection that is particularly relevant to the small cell industry is Ethernet, which is a prime candidate for providing connectivity within indoor enterprise small cell 15 Note that CU and DU network nodes described in the present chapter are different from gNB-CU and gNB-DU nodes defined by 3GPP to support split option 7. 4.6 Small Cells Figure 4.6.7 nFAPI architecture. (Source: CU - Virtualized Small Cell Functions Reproduced by permission of © Small Cell Forum). DU Ctrl nFAPI P5 nFAPI P7 nFAPI P7 nFAP