Achieving Fiber-Optic Speeds over Copper Lines
A 100-year-old networking trick could boost transmissions over telephone infrastructure.
Alcatel-Lucent has developed a prototype technology that could dramatically increase the speed of data communications over the copper wires that make up the majority of the world’s telephone infrastructure. The technology combines three existing techniques, known as bonding, vectoring, and DSL phantom mode. It can reach speeds of 300 megabits per second at a distance of 400 meters from a communications hub, and 100 megabits per second at one kilometer.
Squeezing more speed out of copper connections is an important goal for telecommunications companies in the United States. They want to compete with the 50-megabit-per-second speeds offered by cable providers, but DSL connections transmit data through telephone lines–a fundamentally different technology from that used by cable companies. Alcatel-Lucent’s technology could help these companies extend high-speed Internet access before next-generation fiber-optic networks become widely available.
The first two components of the prototype system, vectoring and bonding, are standard ways to increase the speed of DSL broadband connections: vectoring cancels out noise in a DSL line, and bonding treats multiple lines as if they were a single cable, which increases bandwidth by a multiple almost equal to the number of cables involved. Neither technique is widely used in the United States, but bonding is deployed to a limited extent in both Asia and Europe, where high urban density makes it more economical.
The third component, “phantom mode,” is based on a networking trick invented in 1886 by electrical engineer and telephony pioneer John J. Carty, who later became a vice president at AT&T.
A digital signal is normally transmitted through two wires twisted together–one positive and the other negative. Carty realized that it is possible to send a third signal on top of four wires separated into two twisted pairs. The negative half of this “phantom” connection is sent down one twisted pair (which is already carrying a conventional signal), and the positive half down is sent down other twisted pair. At the destination, analog processors are used to extract all three signals–two real and one “phantom”–from the two pairs.
The challenge, says Stefaan Vanhastel, director of product marketing at Alcatel-Lucent, is that any additional bandwidth gained by creating a phantom channel can easily be swamped by the increased noise that the technique introduces. The noise arises because telephone wires are often bundled tightly into a single cable, allowing for electrical induction, or “cross-talk”, between them.
“The obvious solution is to remove the cross-talk, which is why we add [high-speed] DSL vectoring on top of this,” Vanhastel says. Vectoring eliminates cross-talk in bundled wires by sending a signal down the cable that is exactly the opposite of the cross-talk signal, cancelling the noise out.
Combining the three techniques has the potential to increase transmission speeds far above what’s possible with existing DSL connections. Typical ADSL connections top out at six megabits per second, while advanced, ADSL2+-powered connections to fiber-optic hubs are advertised at speeds of up to 24 megabits per second. Forthcoming VDSL technology could push speeds up to 50 megabits per second, and vectoring could take lines to more than 100 megabits per second. Phantom mode can further increase speeds, depending on the distance, by up to 200%, or simply boost the range of existing high speed connections.
There are a few catches, however. One is that a home or business must have at least two lines already connected (in the United States, many do). In addition, says Vanhastel, manufacturers have yet to introduce a three-channel modem for consumer use.
While Alcatel-Lucent is the first company to publicly announce a technology that combines these three techniques, it is not the first to deploy them in the lab or in field trials, according to Stanford University professor John Cioffi, CEO of DSL management company ASSIA. “I’ve seen it work in the field on customers’ lines, but I can’t say where and how,” Cioffi says. He notes that the cost of bonding and vectoring has deterred telecommunications companies from introducing them until now.
Other approaches are being used to get more speed out of copper connections. Last year Ericsson announced that it had induced a DSL line to transmit data at 500 megabits per second, but that achievement involved bonding six separate lines. Alcatel-Lucent limited its test to two bonded lines, says Vanhastel, because that is the largest number of lines a home or business could realistically be expected to have connected.
Alcatel-Lucent doesn’t believe it will roll out the combination technology until after 2011. Even so, that’s well ahead of the timetable for extending fiber-optic technology to all areas of the U.S.
One-hundred-megabit DSL is “what we can see” in the next five to 10 years, says Cioffi. That will be just in time to realize the Federal Communications Commission’s goal, announced in February, of rolling out 100-megabit-per-second broadband to 100 million U.S. homes by 2020.
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