DSL - VDSL Access Technology

VDSL is the technology with a high rate. Operating at speeds up to 52Mbps, VDSL is the next generation of DSL technology with higher throughput and requirements for implementing, which are simpler than ADSL. VDSL began its life being called VADSL, but was renamed VDSL by the ANSI working group T1E1.4. The main reason T1E1.4 decided VDSL on VADSL was that, unlike ADSL, VDSL is both symmetric and asymmetric. VDSL is nearly ten times faster than ADSL and is over thirty times faster than HDSL. In the tradeoff for increased speed loop length: VDSL has a shorter reach in the loop.

In the following table, it shows a comparison of the various DSL technologies available today. We see that VDSL is highest in terms of bandwidth technology and supports applications both asymmetric and symmetric, and is ideal for broadband full service.

DSL Type Symmetric/ Asymmetric Loop Range (kft) Downstream (Mbps) Upstream (Mbps)
ISDL Symmetric 18 0.128 0.128
SDSL Symmetric 10 1.544 1.544
HDSL (2 pairs) Symmetric 12 1.544 1.544
ADSL G.lite Symmetric 18 1.5 0.256
ADSL Asymmetric 12 6 0.64
VDSL Asymmetric 3 26 3
Asymmetric 1 52 6
Asymmetric 3 13 13
Asymmetric 1 26 26

Like other DSL technologies, VDSL uses higher frequency spectrum of copper above standard frequencies used for lifeline service to the plain old telephone (POTS) and Integrated Services Digital Network Services (ISDN). This is commonly referred to technology as data and video-on-voice. This technology enables Telco’s existing copper infrastructure for the provision of broadband services over the same physical plant.

VDSL spectrum is specified to range from 200 kHz to 30 MHz. Real spectral distribution vary with the line rate or based on asymmetric or symmetric rates that are used. Baseband for POTS and ISDN service use is preserved by the use of passive filters commonly called as dispatchers.

Asymmetric VDSL

VDSL is designed to offer a multitude of asymmetric broadband services, including Digital Television Broadcasting, Video on Demand (VoD), High-Speed Internet Access, Distance Learning and Telemedicine, to name a few. The delivery of these services requires the downstream channel to have a higher bandwidth that the channel upstream and is asymmetrical.

For example, HDTV requires 18 Mbps for video content downstream. Upstream, however, it does not require the transmission of signaling information (e.g., change of channel or program selection), which is of the order of kbps.

The following table specifies the rate VDSL standards established in the specification / ANSI S1.4 of T1. The downstream rates are derived from the sub-multiples of Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) speed of 155.52 Mbps canonical, i.e. 51.84, 25.92 Mbps and 12 Mbps, 96 Mbps.

Typical Service Range Bit Rate (Mbps) Symbol Rate (Mbps) Comments
Short range, 1 kft 6.48 0.81 baseline
4.86 0.81 optional
3.24 0.81
Medium range, 3 kft 3.24 0.405 baseline
2.43 0.405 optional
1.62 0.405
Long range, 4.5 kft 3.24 0.405 baseline
2.43 0.405 optional
1.62 0.405

Symmetric VDSL

VDSL is also designed to provide symmetrical services for small and medium business customers, business enterprise, high-speed data applications, video conferencing and tele-applications, etc.

Symmetric VDSL can be used to provide short-haul T1 replacements NXT1 rate and support a host of other business applications.

The following table contains the symmetric VDSL standards for service established in the ANSI T1E1.4. A rate of 6.48 Mbps to 25.92 Mbps, it should be noted that VDSL provides symmetrical service between the standard T1 (1.536 Mbps) and T3 (44.376 Mbps) rates, fill the gap simplest copper twisted pair. Although ANSI has not specified distance and long-term rates for symmetric services 6 Mbps to 1.5 Mbps on loops from 3 kft to 10 kft may be supported.

Typical Service Range Bit Rate (Mbps) Downstream Symbol Rate (Mbps) Upstream Symbol Rate (Mbps)
Short range, 1 kft 25.92 6.48 7.29
19.44 6.48 7.29
Medium range, 3 kft 12.96 3.24 4.05
9.72 3.24 2.43
6.48 3.24 3.24