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.