A communication system includes a first communication unit, which is arranged to communicate with a remote communication system over a first communication link having a first, variable data rate. A second communication unit is arranged to exchange data with the remote communication system by communicating with the first communication unit over a second communication link having a second data rate. The first and second communication units are arranged to modify the second data rate responsively to variations in the first data rate, so as to match the second data rate to the first data rate.