DSL uses a sophisticated modulation scheme to pack data onto copper wires. DSL is sometimes referred to as a last-mile technology because it is used only for connections from a telephone switching station to a home or office, not used between switching stations. DSL is also called an always on connection because it uses existing 2-wire copper telephone line connected to the premise and will not tie up your phone as a dial-up connection does. There is no need to dial in to your ISP as DSL is always on. The two main categories of DSL for home subscribers are called ADSL and SDSL.