Very-high-bit-rate digital subscriber line
Very-high-bit-rate digital subscriber line (VDSL or VHDSL)[1] is a digital subscriber line (DSL) technology providing data transmission faster than asymmetric digital subscriber line (ADSL) over a single flat untwisted or twisted pair of copper wires (up to 52 Mbit/s downstream and 16 Mbit/s upstream),[2] and on coaxial cable (up to 85 Mbit/s down- and upstream)[3] using the frequency band from 25 kHz to 12 MHz.[4] These rates mean that VDSL is capable of supporting applications such as high-definition television, as well as telephone services (voice over IP) and general Internet access, over a single connection. VDSL is deployed over existing wiring used for analog telephone service and lower-speed DSL connections. This standard was approved by ITU in November 2001.
Second-generation systems (VDSL2; ITU-T G.993.2 approved in February 2006) use frequencies of up to 30 MHz to provide data rates exceeding 100 Mbit/s simultaneously in both the upstream and downstream directions. The maximum available bit rate is achieved at a range of about 300 meters; performance degrades as the loop attenuation increases.
Development
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The concept of VDSL was first published in 1991 through a joint Bellcore-Stanford research study. The study searched for potential successors to the then-prevalent HDSL and relatively new ADSL, which were both 1.5 Mbit/s. Specifically, it explored the feasibility of symmetric and asymmetric data rates exceeding 10 Mbit/s on short phone lines.
VDSL standards
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A VDSL connection uses up to seven frequency bands, so one can allocate the data rate between upstream and downstream differently depending on the service offering and spectrum regulations. First generation VDSL standard specified both quadrature amplitude modulation (QAM) and discrete multi-tone modulation (DMT). In 2006, ITU-T standardized VDSL in recommendation G.993.2 which specified only DMT modulation for VDSL2.
| Version | Standard name | Common name | Downstream rate | Upstream rate | Approved in |
|---|---|---|---|---|---|
| VDSL | ITU G.993.1 | VDSL | 55 Mbit/s | 3Mbit/s | 2001-11-29 |
| VDSL | ITU G.993.2 | VDSL2 | 100 Mbit/s | 100 Mbit/s | 2006-02-17 |
Deployment
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