DSL

ADSL2 is inching closing to being a working reality. Clearly, the new standard offers some nice improvements to the design and development of ADSL equipment. But will it challenge VDSL services?

by Patricia Fusco ISP-Planet Managing Editor [December 4, 2002]

First, let's review the nomenclature for asynchronous digital subscriber line standards. "Study Group 15," a task force within the International Telecommunication Union (ITU), is responsible for developing ADSL standards. The group has categorized projects under various designations, such as G.dmt, G.lite, G.voice, and G.bond.

Most ISP operators are familiar with the work the group has already completed within the G.dmt and G.lite categories, which include the G.992.1 full-rate ADSL and G.992.2 splitterless versions of ADSL standards. These standards include annexes that define how ADSL works under various conditions. For example, G.992.1 Annex A is for ADSL over plain old telephone services (POTS), G.992.1 Annex B is for ADSL over integrated services digital network (ISDN), and G.992.1 Annex C is for ADSL over ISDN in Japan.

ADSL2 is a new standard that will eventually supersede existing ADSL standards. G.dmt.bis and G.lite.bis are designations for G.992.3 full-rate ADSL and G.992.4 for splitterless ADSL. The beauty of ADSL2 is that it is interoperable with existing ADSL deployments—it will perform both ADSL and ADSL2 modes of operation. This is essential to current ADSL providers—providers need to be able to continue to use the equipment they have invested in.

ADSL2+ is an extension of the new ADSL2 standard that should be approved by the ITU early in 2003. ADSL2+ is a hot topic because it is capable of doubling the transmission speed of typical ADSL connections from 1.1 MHz to 2.2 MHz. This doubles downstream data rates to over 20 Mbps, but these data speed rates will only be attainable on loops shorter than 8,000 feet. Here's where things get a bit jumbled—ADSL2 is often called ADSL+, but most experts expect that ADSL2+ will be the term used within technical circles, as this name highlights that it is in fact an extension of ADSL2.

Aware, Inc. is beating a path toward the development and marketing of semiconductor intellectual property for broadband communication technologies like ADSL2 and ADSL2+. The company has been licensing DSL intellectual property and software to semiconductor manufacturers since 1993. David Benini, Aware's director of product marketing, said that it's the features that will really drive the success of the new standards in the global market.

"ADSL2 and ADSL2+ are backward compatible to ADSL," Benini said. "It might be five years before a carrier decides to deploy video over DSL services. With the new chipset, carriers can continue to deploy regular ADSL connectivity while they prepare to offer new services like VoDSL."

Rate and reach
While voice over DSL (VoDSL) may not be possible now, the feature set of ADSL2 remains very appealing. Although doubling downstream bandwidth is a boon, the new standards also extend the reach of DSL services. ADSL2 has been engineered to improve the rate and reach of ADSL by overcoming narrowband interference over long lines. ADSL2 accomplishes this through its improved modulation efficiency, which enhances the signal processing algorithms.

For example, on longer phone lines, ADSL2 will provide a data rate increase of 50 kbps—a significant increase. This data rate increase also produces an increase in reach of about 600 feet, which translates to an increase in coverage area of about six percent, or 2.5 square miles.

Benini said the immediate payoff from deploying ADSL2 and ADSL2+ is rate and reach enhancements, but the standards also provide dramatically improved diagnostics capabilities.

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