A data transmitter (10) divides incoming data for transmission into data blocks and passes them in frame transmission order to a radio link stage (16) via a serial frame buffer (14). The buffer (14) holds the data frames until the radio link stage (16) is able to transmit them. The incoming data has associated with it various parameters. The radio link stage (16) has allocated to it radio link resources. The parameters and resources, which change independently of each other from time to time, are supplied to a controller (18) which calculates high and low buffer levels therefrom. The controller (18) controls the passing of the data frames through the frame buffer (14) to maintain the number of frames in the buffer at any instant of time at a level between the calculated high and low levels.