13 October 2011—As ideal as optical fiber is for transmitting huge quantities of data over long distances, running fiber to millions of individual homes is one expensive construction project. For now, the quest for faster broadband for the masses still involves finding better ways to use the existing copper infrastructure, such as digital subscriber lines (DSL), which evolved from telephone dial-up service. Alcatel-Lucent has just released technology that it says will more than triple the 20- to 30-megabit-per-second data speeds that most DSL subscribers and cable modem users are limited to today.
In late September, at the Broadband World Forum, in Paris, Alcatel-Lucent announced that it is starting commercial rollout of an enhanced version of a very high-speed DSL technology, called VDSL2, with vectoring. Vectoring is a technique that reduces copper-wire interference simultaneously for multiple customers in order to push broadband speeds over 100 Mbps. While running fiber all the way to the home can produce speeds of several hundred megabits per second, VDSL2 vectoring can use legacy telephone access networks.
Michael Peeters, chief technology officer of Alcatel-Lucent’s wire-line division, says carriers can introduce VDSL2 vectoring without overhauling any existing infrastructure. In most networks, a home is connected to a street cabinet, which is then connected to a central office by optical fiber, he says. Customers still have copper running over the last few hundred meters to their homes, and VDSL2 vectoring is designed to optimize the data capacity of those short connections.
In a typical copper loop, Peeters explains, the signal is transmitted over a copper pair to minimize the electromagnetic interference. ”But still, part of the signal of one customer’s wire often leaks into another customer’s, and that’s when you get what’s known as cross talk,” he says. Cross talk limits bandwidth by corrupting the signals that are transmitting information.
Cross talk is theoretically easy to completely eliminate, as long as you can estimate how much one line is leaking into another. Then you can cancel out that interference with calculations performed by hardware installed in the cabinet, Peeters says. Cross talk estimation uses what’s called an error vector, which is sent from a user’s home equipment back to the cabinet. The receiving equipment there knows what the signal should look like, and the difference is a good measure of the amount of cross talk between a wire and all the wires around it, Peeters says.
In the new VDSL2 scheme, the cross talk for every line in a cabinet is estimated using these error vectors and then processed to minimize the interference for them all.
”Computation for VDSL2 vectoring can be demanding,” Peeters says, because there’s a large matrix of cross talk measurements to consider. ”For 48 lines, it requires as much processing power as a PlayStation 3,” he says. But due to the decreasing cost of silicon and the high cost of digging for fiber, he estimates that the cost of installing VDSL2 with vectoring is still no more than one-third the cost of running fiber all the way to the home.
Stanford researchers—led by IEEE Fellow John Cioffi, who’s been driving DSL innovation since the early 1990s—began developing vectoring methods in 2001 for eliminating cross talk for many sets of wires at once. Cioffi has long been convinced that copper-wire broadband isn’t going away anytime soon. Industry seems to agree: His new broadband technology company, Assia, provides services that are now being used to boost speeds on 45 million DSL lines worldwide, up from about 5 million in 2006.
Cioffi says that even in the early 90s, when he proposed the first standard for improving copper-wire networking—asymmetric digital subscriber line technology—people were talking about fiber to the home. But in most of the developed world, fiber lines all the way to the home still represent only around 5 percent of broadband subscribers. The price tag is a big reason. ”The cost of running fiber to every home in the U.S. would be around $250 billion,” Cioffi says, ”and worldwide, it would be trillions.”
Telecoms can’t afford to do the infrastructure overhaul themselves, and with today’s debt problems, it is not realistic to expect government subsidies for massive fiber construction projects, he says.
Belgacom agrees with the economics. The Belgian telecom operator will be the first to use VDSL2 vectoring throughout its national network and plans to have 85 percent of its 1 million subscribers connected by 2013.
Belgacom already uses VDSL2 without vectoring for 900 000 of its subscribers, hooked into the 19 000 street cabinets that support the standard. Basic VDSL2 provides 20- to 30-Mbps broadband data rates at a range of up to 400 meters, Peeters says. Wim De Meyer, vice president of business transformation at Belgacom, says 60 percent of its subscribers are within 400 meters of a street cabinet and could see their DSL bandwidth go up to 100 Mbps. And even though data rates decrease as a user’s distance increases, he still thinks customers who live up to 1200 meters from a cabinet will see download speeds of 40 Mbps or more.
De Meyer says that this year he has seen an increased interest in the use of the copper instead of optical fiber to boost broadband services. ”In today’s economic climate, people are talking about how to use the existing infrastructure,” he says. Belgacom started installing fiber to street cabinets in 2003. ”Now we have over 16 000 kilometers of fiber between central offices and the street cabinets,” he says. Adopting Alcatel-Lucent’s latest vectoring technology for the last few hundred meters of copper makes sense for Belgacom, because the telecom firm will need to update equipment only in street cabinets to roll out the upgrade.
Alcatel-Lucent has already begun testing its vectoring technology with a handful of other carriers that are already using VDSL2.