Digital Subscriber Line (DSL)

DSL is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term Digital Subscriber Line is widely understood to mean Asymmetric Digital Subscriber Line (ADSL), the most commonly installed technical varieties of DSL. DSL service is delivered simultaneously with regular telephone on the same telephone line as it uses a higher frequency band that is separated by filtering.

The data throughput of consumer DSL services typically ranges from 384 KB/s to 20 MB/s in the direction to the customer, depending on DSL technology, line conditions, and service-level implementation. Typically, the data throughput in the reverse direction, i.e. in the direction to the service provider is lower, hence the designation of asymmetric service, but the two are equal for the Symmetric Digital Subscriber Line (SDSL) service.

Voice and data

DSL uses a second, higher frequency band (greater than 25 kHz) above the low frequency regime (5 kHz and below) used by voice communications. On the customer premises, a DSL filter is installed on each outlet for telephone handsets to remove the high frequency band, eliminating interference with the operation of the telephone set, and enabling simultaneous use.

DSL technologies

DSL technologies (sometimes summarized as xDSL) include:

• ISDN Digital Subscriber Line (IDSL), uses ISDN based technology to provide data flow that is slightly higher than dual channel ISDN.
• High Data Rate Digital Subscriber Line (HDSL / HDSL2), was the first DSL technology that uses a higher frequency spectrum of copper,
  
• Symmetric Digital Subscriber Line (SDSL / SHDSL), the volume of data flow is equal in both directions.
• Symmetric High-speed Digital Subscriber Line (G.SHDSL), a standardised replacement for early proprietary SDSL.
• Asymmetric Digital Subscriber Line (ADSL), the volume of data flow is greater in one direction than the other.
• Asymmetric Digital Subscriber Line 2 (ADSL2), an improved version of ADSL
• Asymmetric Digital Subscriber Line 2 Plus (ADSL2+), A version of ADSL2 that doubles the data rates by using twice the spectrum.
• Asymmetric Digital Subscriber Line Plus Plus (ADSL++), technology developed by Centillium Communications for the Japanese market that extends downstream rates to 50 Mbit/s by using spectrum up to 3.75 MHz.
• Bonded DSL Rings (BDR), A shared ring topology at 400 Mbit/s
• Rate-Adaptive Digital Subscriber Line (RADSL), designed to increase range and noise tolerance by sacrificing up stream speed
• Very High Speed Digital Subscriber Line (VDSL)
• Very High Speed Digital Subscriber Line 2 (VDSL2), an improved version of VDSL
• Etherloop Ethernet Local Loop

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