Very-high-bit-rate digital subscriber line

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This VDSL modem used in Taiwan provides four Ethernet ports and an internal filter for voice-data separation.

Very-high-bit-rate digital subscriber line (VDSL[1] or VHDSL) and Very-high-bit-rate digital subscriber line 2 (VDSL2[2]) are digital subscriber line (DSL) technologies providing data transmission faster than asymmetric digital subscriber line (ADSL).

VDSL offers speeds of up to 52 Mbit/s downstream and 16 Mbit/s upstream,[3] over a single flat untwisted or twisted pair of copper wires and up to 85 Mbit/s down- and upstream on coaxial cable [4] using the frequency band from 25 kHz to 12 MHz.[5] 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 the International Telecommunication Union (ITU) in November 2001.

Second-generation systems (VDSL2; ITU-T G.993.2 approved in February 2006)[6] 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[edit]

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[edit]

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 on
VDSL ITU G.993.1 VDSL 55 Mbit/s 3 Mbit/s 2001-11-29
VDSL ITU G.993.2 VDSL2 100 Mbit/s 100 Mbit/s 2006-02-17

VDSL2 frequencies.png

VDSL2[edit]

VDSL2 is an enhancement to VDSL. Designed to support the wide deployment of triple play services such as voice, video, data and high-definition television (HDTV) VDSL2 is intended to enable operators and carriers to gradually, flexibly, and cost-efficiently upgrade existing xDSL infrastructure.

The protocol is standardized in the International Telecommunication Union telecommunications sector (ITU-T) as Recommendation G.993.2. It was announced as finalized on 27 May 2005,[6] and first published on 17 February 2006. Several corrections and amendments were published from 2007 to 2011.[2]

VDSL2 permits the transmission of asymmetric and symmetric aggregate data rates up to 300+ Mbit/s downstream and upstream on twisted pairs using a bandwidth up to 35 MHz. It deteriorates quickly from a theoretical maximum of 350 Mbit/s at source to 100 Mbit/s at 0.5 km (1,600 ft) and 50 Mbit/s at 1 km (3,300 ft), but degrades at a much slower rate from there, and outperforms VDSL. Starting from 1.6 km (1 mi) its performance is equal to ADSL2+.[7]

ADSL-like long-reach performance is one of the key advantages of VDSL2. LR-VDSL2 enabled systems are capable of supporting speeds of around 1–4 Mbit/s (downstream) over distances of 4–5 km (2.5–3 miles), gradually increasing the bit rate up to symmetric 100 Mbit/s as loop-length shortens. This means that VDSL2-based systems, unlike VDSL systems, are not limited to short local loops or MTU/MDUs only, but can also be used for medium range applications.

Bonding (ITU-T G.998.x) may be used to combine multiple wire pairs to increase available capacity, or extend the copper network's reach.

Profiles[edit]

The standard defines a wide range of profiles that can be used in different VDSL deployment architectures; in the central office, in the cabinet or in the building for example.[8]

Profile Bandwidth
(MHz)
Number of
downstream
carriers
Carrier
bandwidth
(kHz)
Maximum aggregate
downstream transmit
power (dBm)
Max. downstream
throughput
(Mbit/s)
8a 8.832 2048 4.3125 +17.5 50
8b 8.832 2048 4.3125 +20.5 50
8c 8.5 1972 4.3125 +11.5 50
8d 8.832 2048 4.3125 +14.5 50
12a 12 2783 4.3125 +14.5 68
12b 12 2783 4.3125 +14.5 68
17a 17.664 4096 4.3125 +14.5 100
30a 30 3479 8.625 +14.5 200
35b 35.328 8192 4.3125 +17.5 250

VDSL2 Vectoring[edit]

Vectoring is a transmission method that employs the coordination of line signals for reduction of crosstalk levels and improvement of performance. It is based on the concept of noise cancellation, much like noise-cancelling headphones. The ITU-T G.993.5 standard, "Self-FEXT cancellation (vectoring) for use with VDSL2 transceivers" (2010), also known as G.vector, describes vectoring for VDSL2. The scope of Recommendation ITU-T G.993.5 is specifically limited to the self-FEXT (far-end crosstalk) cancellation in the downstream and upstream directions. The far end crosstalk (FEXT) generated by a group of near-end transceivers and interfering with the far-end transceivers of that same group is cancelled. This cancellation takes place between VDSL2 transceivers, not necessarily of the same profile.[9][10]

Although technically feasible at the moment vectoring is incompatible with local-loop unbundling but future standard amendments could bring a solution.

See also[edit]

References[edit]

External links[edit]