What Is the Difference Between ADSL, SDSL & XDSL?

Regular DSL

The principle behind DSL is that voice only uses between 300 Hz and 3.3 kHz of all the available frequency on a telephone line. Frequency is the number of sound waves produced per second, and the frequency spectrum is measured as the number of cycles per second, where "cycle" is another word for "wave." The higher the frequency, the more waves there are per second. The unit of measurement for frequency is "hertz," which is abbreviated to Hz. A thousand hertz is a kilohertz and abbreviated to kHz. DSL divides the available spectrum on a telephone line into a voice channel, which occupies 0 to 4 kHz and a data channel, which occupies between 26 kHz and 1,100 kHz.

Internet users download more than they upload, so DSL services were soon modified to be Asymmetric Digital Subscriber Line, or ADSL. This uses the data channel more efficiently because it allocates more space to the downstream (data traveling to the home) than to the upstream (data traveling from the home). The upstream channel occupies between 36 kHz and 138 kHz and the download channel uses 138 kHz to 1,100 kHz. This is the most common form of DSL in operation today.

SDSL, Symmetric Digital Subscriber Line, has no voice channel. It is a data-only line used for wholesale transfers, which is also called "trunking." SDSL isn't used any more because it was replaced by Single-Pair High-Speed Digital Subscriber Line, or SHDSL. Many people refer to SHDSL incorrectly as SDSL. It uses all the frequency spectrum available on the cable.

The "X" in xDSL doesn't stand for anything. It is a group abbreviation for all the DSL variants. So xDSL means ADSL, SDSL and SHDSL. In addition to these versions of DSL are HDSL (High-bit-rate Digital Subscriber Line), MSDSL (Multi-rate Symmetric DSL), PDSL (Powerline DSL) and VDSL (Very-high-bitrate DSL). ADSL's main employment is carrying broadband Internet to homes. SHDSL is a bulk carrier for Internet telephony. MSDSL is also sometimes used to deliver home broadband Internet to homes; data throughput can be varied under this system enabling providers to set different tariffs. VDSL achieves high speeds over short distances. It is still in its infancy, but it is seen as a method to deliver HDTV to the general public. Powerline DSL carries digital signals over cables carrying the power supply. Electricity supply companies use this system to communicate with smart meters.