e used to control user data scheduling, beamforming weight selection, and numerology selection, etc. User data messages carry radio data between O-DU and O-RU. Synchronization messages are used for timing synchronization between O-DUs and O-RUs via Ethernet, which rely on Precision Time Protocol (PTP) (IEEE 1588-2008). All three types of message are illustrated in Figure 4.5.6 (ORAN-WG4.CUS.0) O-RAN control- and user-plane protocols stacks are the same: the Ethernet physical layer as the first layer, then Ethernet MAC. If Ethernet is chosen as the fronthaul transport, the eCPRI or RoE protocol layer becomes the Ethernet payload to carry the control or user data message. For the fronthaul using IP/UDP, the IP and UDP layers are added after Ether- net MAC, and eCPRU/RoE is embedded as UDP payload. The fronthaul control message payload within the eCPRI/RoE is the same regardless of using either Ethernet or IP/UDP transport. Synchronization messages can only use the Ethernet protocol, but not IP/UDP. Figure 4.5.7 further illustrates the O-RAN protocol stack message frame in greater detail for both Ethernet and IP/UDP transports. Besides the transport protocol layer, the core of the O-RAN package is the payload. The O-RAN message payload consists of three building blocks: the transport header, the payload common header, and the payload section fields. In the interest of brevity, we use one control message and one data message to explain some key fields of these building blocks. Exhaustive description of all messages is beyond the scope of this section and can be found in the O-RAN specification (ORAN-WG4.CUS.0) As mentioned earlier, both eCPRI and IEEE1914.3 headers can to be used in the trans- port frame differentiated by their corresponding Ethertype. Both headers have the same length of 8 bytes. Below we describe how eCPRI and IEEE1914.3 can be used with O-RAN fronthaul. eCPRI / RoE eCPRI / RoE UDP (optional) UDP (optional) IP (optional) IP (optional) SyncE Eth L2 + VLAN Eth L2 + VLAN Eth L2 Eth L1 Eth L1 Eth L1 Fi