Moving to VDSL3

As the telecom industry cuts its teeth on VDSL2, a small group of telecom companies, led by Israeli equipment maker ECI Telecom, is taking the next step, developing digital spectrum management, or DSM, technology with the aim of developing what the group calls VDSL3: a faster, further-reaching, higher-performing version of today's technology.

DSM, which has already been applied to some forms of DSL, is a way of managing the spectra of DSL signals to reduce interference and cross-talk (interference between DSL-carrying copper pairs housed in the same binder). Though it has already been applied successfully to other forms of DSL, its application to VDSL2 is expected to be especially useful, as VDSL2's relatively higher frequency (near 30 MHz) makes it more susceptible to cross-talk, forcing carriers to limit the length of VDSL2 loops.

The American National Standards Institute's DSM standards describe three types of DSM: impulse noise (say, from the sudden turning on of a nearby vacuum cleaner), spectrum balancing (which carves out spectrum for each user to avoid cross-talk) and vectoring (also called multiple input/multiple output). Only the first two have been commercially applied to DSL so far, and it's the third kind that ECI's group hopes to develop. Vectoring coordinates the spectra of multiple signals by processing them together.

“Vectoring coordinates cross-talk so well that it appears there is no cross-talk,” said John Cioffi, professor of engineering at Stanford University.

In theory, vectoring could potentially double the speed of VDSL2 loops between 100 and 200 meters long, said Ariel Shuper, product management director for ECI's broadband access division. It could also extend the reach of 100 Mb/s service by 250 to 300 meters.

“We're talking about significant enhancements,” Shuper said, though he doesn't expect his group's work to yield commercially available vectoring gear until “around 2009.”

In a DSM paper published more than a year ago, Alcatel (which declined to contribute to this article) declared vectoring “too complex” for the DSL chips of the day. “Either all the lines have to be processed on one chip or the various chips that process all the lines have to be endowed with fast data connections (to exchange the cross-talk information),” Alcatel wrote.

Because of the level of coordination involved, Shuper said his group is taking the right approach by tackling the issue simultaneously on both the chip and system levels, while also working to develop DSM algorithms for spectra management.

“The loops you want to coordinate do not [necessarily] come from the same line card,” Shuper said. “A system looks at all the line cards and coordinates them. A chip will not help. We don't need to increase the bandwidth anymore. The key will come from coordinating the system.”