DSL is a means of providing high-speed connections over installed copper wires. DSL is one of the key teleworker solutions available.
Several years ago, Bell Labs identified that a typical voice conversation over a local loop only required bandwidth of 300 Hz to 3 kHz. For many years, the telephone networks did not use the bandwidth above 3 kHz. Advances in technology allowed DSL to use the additional bandwidth from 3 kHz up to 1 MHz to deliver high-speed data services over ordinary copper lines.
As an example, asymmetric DSL (ADSL) uses a frequency range from approximately 20 kHz to 1 MHz. Fortunately, only relatively small changes to existing telephone company infrastructure are required to deliver high-bandwidth data rates to subscribers. The figure shows a representation of bandwidth space allocation on a copper wire for ADSL. The area labeled POTS identifies the frequency range used by the voice-grade telephone service. The area labeled ADSL represents the frequency space used by the upstream and downstream DSL signals. The area that encompasses both the POTS area and the ADSL area represents the entire frequency range supported by the copper wire pair.
Another form of DSL technology is symmetric DSL (SDSL). All forms of DSL service are categorized as ADSL or SDSL, and there are several varieties of each type. ADSL provides higher downstream bandwidth to the user than upload bandwidth. SDSL provides the same capacity in both directions.
The different varieties of DSL provide different bandwidths, some with capabilities exceeding 40 Mb/s. The transfer rates are dependent on the actual length of the local loop (connecting the subscriber to the central office), and the type and condition of the cabling. For satisfactory ADSL service, the loop must be less than 3.39 miles (5.46 km).