In the downlink data transmission, the latency includes: transmission time from O-DU to O-RU, queueing time at O-RU, O-RU data processing time, and the time until the data are transmitted over the air. O-DU must make sure that the data reach the antenna at the right time. Even though the O-RU has buffer to store the received data, the O-DU cannot send the data too early, otherwise they may overflow the buffer. Using the time reference points, we define the downlink delay as Tla = T12 + T2a. The transmit window is then defined as the time period that allows the transmitter to send all the data to the receiver. It is not an exact transmission time, but rather a specified boundary T1a (= T12 + T2a) Ta4 (= Ta3 + T34) Figure 4.5.5 Definition of reference points for delay management. (Source: Reproduced by permission of © O-RAN). 4 NG-RAN Architecture Table 4.2.1 Uplink transmit and receive window. Downlink Uplink Transmit window Tlamax - Tlamin Ta3max - Ta3min Receive window T2amax - T2amin Ta4max - Ta4min that the transmitter has to observe. Similarly, the receive window is the time period that the receiver is able to accept data, with some buffer time to process the data. In the uplink direction, when O-RU receives a piece of sampled data from the antenna, it starts processing when radio samples of one symbol are collected. After the processing is completed, data are packed and sent through the fronthaul interface toward the O-DU for further processing. The time for the radio data to reach the O-DU should be sufficiently short to leave enough time for the O-DU to process the data within the HARQ timing requirement. Similar to downlink, we define the uplink delay as Ta4 = Ta3 + T34. The uplink transmit and receive windows are expressed in Table 4.5.2. The transmit and receive windows defined above are key parameters in managing the fronthaul transport network latency and jitter. Once the transmit and receive windows are calculated based on the dynamic or static latency measurement, the O-DU and O-RU can ensure that t