ink Uplink Figure 5.8 Lower layer functional slit alternatives. interface between the DU (with RF and lower L1) and the CU (upper L1 and other higher layer functions), but the eCPRI/F2 interface is not covered by the 3GPP specifications. In CPRI, multiple different options for the lower layer split functional split with eCPRI were defined, as well as studied also in 3GPP [4]. The options include the following alter- natives: Option 7.1, with the split in the downlink direction between the beamforming and Inverse Fast Fourier Transform (FFT) block, and in the uplink between FFT and beamforming Option 7.2 with placement between the layer mapping and resource element mapping in the downlink and FFT and port reduction in the uplink direction Option 7.3 is for the downlink only, between scrambling and modulation functionality There is an impact on the resulting data rates for the fronthaul interface as addressed in Chapter 9, but clearly the needed transmission capacity is always largely reduced com- pared to sending pure RF samples to the cell site. Between the different options, there is the trade-off always between how easy the network upgrades are compared to which operations are easier to be performed closer to the RF than further away. Work done in the O-RAN Alliance has taken option 7.2 as the basis for the work on the open fronthaul interface (Figure 5.8). 5.4.1 NG-Interface The NG interface connects the gNB (5G-RAN) to the 5G Core. It is based on the use of IP transport based on GTP-U and UDP. Similar to the S1-interface between LTE and 5.4 5G RAN Architecture GTP-U GTP-U Figure 5.9 NG-interface control and user plane protocol stacks. the EPC core, the NG interface is also defined to be an open multivendor interface. The NG-interface has been designed in an access-agnostic way, to facilitate convergence toward and use of the 5G core for other access technologies as well. This would simply require from the end devices the ability to use 3GPP-defined NAS signaling for connec- tion setup/registration. The NG-int