The streams from a plurality of VBR coders or generic data sources (31) are multiplexed to a single stream by a multiplexer (32), while inserting null packets to make their bit-rates uniform. A null-packet eliminator (60) removes the null packets from the stream, while signaling their positions, and inputs the stream to an ACM modulator (62) which codes the stream with the maximum ruggedness allowed by the rate of the incoming useful packets, and transmits the stream over a satellite channel. In the receiving station, the stream is demodulated by an ACM demodulator (64), and the null packets are re-inserted into the stream by a re-inserter (66), based on the received signaling. A bit-rate control circuit (72) receives from the transmitting station, over a return channel, a signaling of the quality of the received signal (QoS) and modifies the bit-rate of the VBR coders or generic data sources (31) accordingly.