System and method for transmitting a data stream from a source (10) to a destination (12) over a communication channel. A transmitter (20) comprises input processing means (21, 23; 26, 27) for assembling data packets for the data stream, and a multiplexer (29) for assembling a data frame to be transmitted over the communication channel, in which each data frame has at least one fixed slot. The multiplexer (29) further provides at least one freely allocatable time slot in each data frame. Retransmission control means (25; 28) connected to the multiplexer (29) retransmit a specific data packet,which is not properly received by the destination, using one of the freely allocatable slots.