DSL Variations

This page discusses the main flavors of xDSL (ADSL, SDSL, VDSL, and more).

The xDSL "family tree" includes two main branches -- symmetric and asymmetric. Symmetric DSL services provide identical data rates upstream and downstream; asymmetric DSL provides relatively lower rates upstream but higher rates downstream.

Four main variations of xDSL exist:

• ADSL, G.Lite and RADSL
• HDSL, SDSL, and SHDSL
• VDSL (VADSL, BDSL)
• IDSL

ADSL

Asymmetric Digital Subscriber Line was designed to provide higher downstream data rates at the expense of upstream rates. Many typical uses of the Web -- such as file downloads and general web browsing -- benefit from greater downstream bandwidth but require relatively little in the opposite direction.

Universal ADSL (also known as G.Lite) is a form of ADSL that improves on one of the weaknesses of regular ADSL -- installation. Regular ADSL generally requires a technician visit to the client site to install the splitter device that divides the frequency spectrum for voice and data. G.Lite does not require that this splitter be installed, but it does so at the expense of lower data rates. G.Lite supports a maximum of 1,544 kbps (384 kbps upstream) whereas regular ("full-rate") ADSL can support more than 8,000 kbps (as shown earlier).

Another related technology, Rate-Adaptive DSL (RADSL), is an implementation of ADSL that automatically configures the modem at startup to adjust its rate according to the quality of the phone line. Like G.Lite, RADSL supports a much lower maximum date rate (1,088 kbps) than regular ADSL.

HDSL, SDSL and SHDSL

A symmetric solution, High Bit / Data-Rate DSL offers the same bandwidth both upstream and downstream. HDSL requires two phone lines to deliver the basic data rate (1,544 kbps), and it can deliver a maximum rate of 2,048 kbps using three lines.

Symmetric DSL improves on the older HDSL technology by implementing the same basic data rate (1,544 kbps) while requiring only a single phone line.

Another variation, Symmetric High-Bit-Rate DSL attempts to improve on both HDSL and SDSL by only requiring a single line and by integrating low-level services of interest to small businesses.

VDSL

Very High Data-Rate DSL needs shorter cable lengths than most other forms of DSL (maximum 4,500 feet as compared to 18,000 feet for regular ADSL), but it also achieves the highest data rate (roughly 51,840 kbps). VDSL is also known as BDSL. It was originally named VADSL ('A' for asymmetric) but later was extended to support both symmetric and asymmetric varieties of DSL.

The bandwidth levels supported by VDSL are needed to support certain high-end applications such as High-Definition Television (HDTV) that requires, for example, up to 20,000 kbps.

IDSL

ISDN DSL implements -- as the name suggests -- a hybrid DSL/ISDN solution. As such, IDSL offers only limited data rates (128 kbps, although multiple circuits may be bonded).

The xDSL Family Tree

Note: In Europe, different standard organizations oversee development of these technologies than in the United States. Although the implementations of xDSL remain nearly identical, occasionally a customer may see terms like "VDSLe" or "ADSLe" that refer to the European ('e') specification instead of the analogous American ones.

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