llers and a Network Orchestrator. Fronthaul and Backhaul Interfaces 9.4.1 Low Layer Fronthaul Low layer fronthaul refers to the low layer split point, which is not defined by 3GPP in detail. Figure 9.8 illustrates the O-RAN Alliance low layer fronthaul protocol stack and a low layer split, where part of the 5G L1 is in the O-RU (O-RAN-Radio Unit), while the rest of the radio protocols are in the 5G baseband. The baseband can be further split into O-CU (O-RAN-Control Unit) and O-DU (O-RAN-Distributed Unit) with a high layer split point. O-RAN specifies the transport protocols to be used in carrying the low layer split interface data, with two types of transport headers, either enhanced CPRI (eCPRI) header or IEEE-1914.3 header. Protocol Stack Protocol Stack Protocol Stack Protocol Stack User plane: Control plane: Sync plane: Mgmt plane: eCPRI/P1914.3 eCPRI/P1914.3 SyncE NetConf/SSH (UDP, IP) (UDP, IP) TCP/IP Ethernet L2/L1 Ethernet L2/L1 Ethernet L2/L1 Ethernet L2/L1 Figure 9.8 Low layer split point interface. 9.4 Fronthaul and Backhaul Interfaces For user plane transport, sample data is mapped either into UDP/IP or directly into Ethernet frames (without the IP layer) [4]. IPv6 and IPv4 are both included as alterna- tives. The control plane flow consists of real-time control data. The control plane stack is identical to the user plane stack. Due to the real-time control information, retrans- missions of control messages are not feasible, and a reliable transport protocol is not defined. Synchronization flow carries timing information to the O-RU, and the standard includes the Precision Timing Protocol (PTP) along with Synchronous Ethernet. The management plane is defined with the NetConf/SSH/TCP/IP protocol [18, 19]. Instead of transmitting time-domain samples as with CPRI, O-RAN with 5G transmits samples in the frequency domain. This reduces the bit rate required on the transport interface, as well as the possibility of compressing sample data. Compared to other 5G network transport segments, the low layer fronth