Asymmetrical digital subscriber line is a high-throughput transmission system built upon the conventional telephone subscriber loop infrastructure originally designed for the “plain old telephone service” (POTS). Via a telephone subscriber loop, ADSL connects a user to a central office. The transmission throughput of ADSL is asymmetrical. In the downstream direction (from a central office to a user), the transmission throughput of ADSL is between 1 and 8 Mbps (megabits per second), while in the upstream direction (from a user to a central office) it is between 100 and 800 kbps (kilobits per second).