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 is 56K. Distance and poor line quality often make it impossible for a modem to function at the speed it was designed for. By the year 2001, newer forms of DSL featuring speeds up to 60 Mbps, or 1,000 times faster than a 56Kbps modem, will start to become available on a commercial basis. DSL is distance sensitive and loses strength as it travels over copper wire. In most areas, the maximum distance for high speed DSL service is 18,500 feet.