A delay adjustment unit and optical network unit which efficiently adjust the delay time of upstream data to improve the quality of data transmission services. Upstream transmission data is segmented into small data blocks. A downstream receiver receives control information on which upstream data blocks are granted or not granted to the unit. It also received an equalization delay update request that commands the unit to update its current equalization delay parameter. When this equalization delay update request is received, a delay adjustment controller controls the delay of each data block, depending on whether the transmission of each subsequent data block is granted. An upstream transmitter then transmits the granted data blocks with the adjusted delay times.