In a method of controlling downlink transmission timing at a radio network controller (RNC) in a communication system, a relative window is determined. The relative window represents a time difference between an arrival time of a data frame at the RNC from a core network, and a target transmission time for transmitting the data frame from the RNC to a Node-B. Boundary thresholds that adjoin the relative window are set, and a target arrival time that a data frame is to be received at the RNC is compared to the thresholds. The RNC may send a request to advance or retard sending of the data frame to the core network, depending on whether target arrival time of the data frame is outside one of the thresholds. A data frame received within the relative window is transmitted from the RNC to a Node-B.