DSL circuits are full duplex, meaning that data can flow in both directions at the same time.
Traditional analog telephone conversations, faxes and modem transmissions are limited to a 3,400-Hertz analog voice channel. The maximum possible modem speed using this analog voice channel more recently approaching the 56 kbps range. Poor line quality and distance often make it impossible for a modem to function at the speed it was designed for. DSL technology is distance sensitive and louses strength as it travels over copper wire. In most areas the maximum distance for DSL service is about 18,000 feet.