sical distance between the O-DU and O-RU. Therefore, the fronthaul, like any other time-critical transport network, has a stringent end-to-end latency requirement. It is determined by the radio signal frame timing requirements of the air interface. 4.5 Lower-Layer Split Another issue is jitter, which is caused by the network traffic load variation and Ethernet switch buffering. The latency and jitter of the fronthaul interface impact O-RU and O-DU processing budget and timing accuracy. While it is impossible to completely eliminate them, mitigation of these negative factors with a reasonable cost is an important design considera- tion. The final target is to guarantee that the RAN meets overall system timing and accuracy requirements. To build a fronthaul network that meets the latency requirements of NR, the deployment use case, network topology, traffic loading, and switching performance should be taken into account. For better understanding of the fronthaul latency issue, an end-to-end latency model is required. We use the latency model defined in eCPRI (which can be used together with O-RAN fronthaul) to analyze different transport network and use case scenarios. The eCPRI model defines timing reference points that are used for delay management, which breaks up the latency into multiple time periods to analyze the latency for downlink and uplink transmission. The reference points defined by eCPRI are reflected in Figure 4.5.5 (eCPRI). These reference points are: R1/ R4 - Transmit/Receive interface at O-DU R2/R3 - Receive/Transmit interface at O-RU Ra - Antenna interface at O-RU. The latency measurement framework shown above helps to budget (in terms of time) each processing step in both downlink and uplink: from the radio data generation until its transmission over the air, and from radio data reception at the antenna to radio data decoding. When data are sent across the fronthaul interface between O-DU and O-RU, many uncer- tain factors such as buffer size, processing time at O-RU, and data transmission over