Very-high-bit-rate digital subscriber line 2 (VDSL2) is an access technology that exploits the existing infrastructure of copper wires that were originally deployed for traditional telephone service as a way of delivering very-high-speed internet access. The main high-speed link (e.g. a fibre optic connection) terminates at a hub near the customers' location. The existing copper wire infrastructure is then used to carry the high speed connection for the short remaining distance to the customers. It can be deployed from central offices, from fiber-optic connected cabinets located near the customer premises, or within buildings. It has been defined in standard ITU-T G.993.2 finalized in 2005.
VDSL2 is an enhancement to very-high-bit-rate digital subscriber line (VDSL), Recommendation G.993.1, and is the newest and most advanced currently deployed standard of digital subscriber line (DSL) broadband wireline communications. 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, and first published on 17 February 2006. Several corrections and amendments were published from 2007 to 2011.
VDSL2 permits the transmission of asymmetric and symmetric aggregate data rates up to 200 Mbit/s downstream and upstream on twisted pairs using a bandwidth up to 30 MHz. It deteriorates quickly from a theoretical maximum of 250 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+.
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.
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.
|Profile||Bandwidth (MHz)||Number of downstream carriers||Carrier bandwidth (kHz)||Maximum aggregate downstream transmit power (dBm)||Max. downstream throughput (Mbit/s)|
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.
Although technically feasible at the moment vectoring is incompatible with local-loop unbundling but future standard amendments could bring a solution.
Main article: List of VDSL and VDSL2 deployments