Second-generation systems (VDSL2; ITU-T G.993.2 approved in February 2006)[5] use frequencies of up to 30MHz to provide data rates exceeding 100Mbit/s simultaneously in both the upstream and downstream directions. The maximum available bit rate is achieved at a range of about 300 metres (980ft); performance degrades as the local loopattenuation increases.
Conceptual development
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 1.5Mbit/s. Specifically, it explored the feasibility of symmetric and asymmetric data rates exceeding 10Mbit/s on short phone lines.
VDSL2 standard is an enhancement to ITU T G.993.1 that supports asymmetric and symmetric transmission at a bidirectional net data rate up to 400Mbit/s on twisted pairs using a bandwidth up to 35MHz.
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. The 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
VDSL2
ITU G.993.2
VDSL2
200 Mbit/s
100 Mbit/s
2006-02-17
VDSL2-Vplus
ITU G.993.2 Amendment 1 (11/15)
VDSL2 Annex Q VPlus/35b
300 Mbit/s
100 Mbit/s
2015-11-06
VDSL2
A VDSL2 cabinet on top of a copper cabling cabinet in Italy
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,[5] 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 35MHz on its latest version.[6] It deteriorates quickly from a theoretical maximum of 350Mbit/s at source to 100Mbit/s at 500 metres (1,600 feet) and 50Mbit/s at 1,000 metres (3,300 feet), but degrades at a much slower rate from there, and outperforms VDSL. Starting from 1,600m (1mi) 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 1–4Mbit/s (downstream) over distances of 4–5km (2.5–3.1mi), gradually increasing the bit rate up to symmetric 100Mbit/s as loop-length shortens. This means that VDSL2-based systems, unlike VDSL systems, are not limited to short local loops, multitenant units (MTUs), or multidwelling units (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. Hybrid Access Networks[8] can be used to combine xDSL with wireless networks. This enables network operators to provide faster Internet access services over long lines.
Vplus/35b
Vplus is a technology to achieve higher speeds over existing VDSL2 networks. It was developed by Alcatel-Lucent and standardised in November 2015 in ITU G.993.2
Amendment 1 as VDSL2 profile 35b.[2] It promises to deliver speeds of up to 300Mbit/s downstream and 100Mbit/s upstream on loops shorter than 250m. On longer loops, Vplus falls back to VDSL2 17a vectoring performance.[9]
Vplus uses the same tone spacing as VDSL2 17a to allow vectoring across Vplus (35b) and 17a lines, and thus mixed deployments and a smooth introduction of Vplus.[9]
Profiles
The VDSL1 standard has three bandplans: Annex A (Asymmetric BandPlan), Annex B (Symmetric BandPlan) and Annex C (Fx BandPlan). Annex A and Annex B were formerly called Plan 998 and Plan 997 respectively.
VDSL1 Annex C is intended for use in Sweden only and it uses a variable separating frequency between the second downstream band, and the second upstream band. All VDSL1 bandplans have spectrum up to 12MHz, so the length of the copper loops must be shorter than ADSL.[10][11]
The VDSL2 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.[12]
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.[13][14] The technology is analogous to G.INP and Seamless Rate Adaptation (SRA).[15]
Although technically feasible, as of 2022, vectoring is incompatible with local-loop unbundling, but future standard amendments could bring a solution.[citation needed]
VDSL2+ Supervectoring
Supervectoring[de] is an evolution of the vectoring technology invented and widely implemented by Deutsche Telekom,[16][17] which further increases crosstalk and interference resistance and allows for stable internet at home connections at 250Mbit/sdownstream and 100Mbit/s upstream.
