Introduction On ADSL2/ ADSL2+

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.

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.

The new standard has the features of broader broadband, further rate, and more flexible operation method. It would push ADSL technology to a brand new level.