he fronthaul I/Q data streams are sent or received within the corresponding window. 4.5.4 Fronthaul Interface In this section we describe the O-RAN fronthaul interface, which has been defined to support the functional split 7-2 described above. O-RAN specifications allow several options for the transport protocol. The choice of the transport protocol is up to a vendor and a service provider. Regardless of the protocol used, the transport network should fulfill the QoS requirements defined in the standard. For example, even though the underlying transport media can be shared (e.g. with other radio access technology), the latency requirements of the 5G radio data need to be protected. One way to achieve this is to assign the highest priority to the fronthaul traffic flow, compared with other types of traffic carried by the same transport network. This is further elaborated on in Section 6.6, while some high-level considerations on the transport network usage with O-RAN fronthaul are provided below. Ethernet is the most commonly used fronthaul transport, even though other protocols such as IP/UDP can also be used. The fronthaul traffic identifier can be used for accelerating data package processing. When Ethernet is used to carry both user-plane and control-plane traffic, the fronthaul packets can be identified by a specially allocated Ethertype field. Either eCPRI Ethertype or IEEE1914.3 Ethertype can be used. When IP/UDP is used as fronthaul transport protocol, the fronthaul messages may be identified by the designated UDP port configured via the management plane during system startup. 4.5 Lower-Layer Split The fronthaul transport is considered secured, and therefore no additional security mea- sures are specified in O-RAN to protect the control-plane and user-plane traffic. That is, it is assumed that the security is provided by the transport network itself. 4.5.4.1 Messages The fronthaul traffic falls into three categories in term of message types: Control plane User data Synchronization. The control messages ar