Next-Gen DSL Goes Commercial With First Step toward 100 Mbps

Rob Gallagher, principal analyst, Informa

Rob Gallagher, principal analyst, Informa

September 26, 2011 – Following a long period of development and debate over the future of DSL technology the curtain will lift on the next era in twisted-pair networking with Alcatel-Lucent’s commercial rollout of a 100 megabit-per-second platform before year’s end.
While that speed is limited to line distances of up to 400 meters or about 1,300 feet, Alcatel-Lucent’s new VDSL2 Vectoring technology is just the first step in what promises to be a series of advances moving out of Bell Labs and other research facilities to commercialization over the next year or two. For example, noted Keith Russell, senior marketing manager for Alcatel-Lucent’s fixed access portfolio, a technology known as “Phantom Mode DSL,” now making its way through testing and trials, will work with vectoring to achieve much higher throughput.

“One of the things we want to make clear is our vectoring solution is not the last innovation for DSL,” Russell said. “There’s plenty of innovation still to come.”

While telcos will have to install new line cards in DSLAMs to accommodate vectoring, costs of implementing the technology are estimated to be a tenth of the costs of fiber-to-the-home. Telcos reported to be in various stages of field trials with VDSL2 vectoring include Belgacom, Orange France, Swisscom, Telekom Austria and Turk Telekomunikasyon. Interest is said to be high in North America as well, where Verizon, having capped its fiber buildout at least for the time being, is looking at DLS upgrades and AT&T is looking to get more throughput for its popular U-verse service.

Vectoring is the foundation to the next generation of DSL technology insofar as it takes care of one of the most vexing limitations on twisted-pair capacity by providing a means of canceling the crosstalk interference that occurs among multiple line pairs in a binder group. The new technology isn’t just about getting to 100 mbps at 400 meters; it’s about improving performance over VDSL2 links, including bonded pairs, at all distances where VDSL2 is in operation, Russell said.

“VDSL frequencies can only travel so far, and that doesn’t change with vectoring,” he noted. “Crosstalk is one of the reasons we’re not reaching the maximum potential of VDSL2.”

Vectoring, standardized by the International Telecommunications Union with the designation “G.vector,” is in development worldwide with several prototype implementations designed to work within DSLAMs across multiple ports and line cards on display at trade shows over the past year or so. But Alcatel-Lucent is the first to bring the technology to market for VDSL2, which, based on the firm’s field analysis, currently operates on average at about 40 mbps over 400 meters.

While the principles behind vectoring have been understood for a long time, the ability to perform crosstalk cancellation across dozens or even hundreds of lines was beyond the reach of processors until fairly recently. According to Alcatel-Lucent documents, vectoring at this density, which entails analyzing the interference from all lines and generating anti-phase signals matched to each in real time, requires in the range of 2,600 billion MAC (multiply-accumulate) operations per second.

The technology can produce throughput gains at up to 1,200 meters or about 4,000 feet. In the U.S. where fiber-to-the-node (FTTN) deployments typically terminate at points where the maximum copper line distance is about 3,000 feet, the improvement with vectoring raises the per-pair line rate from about 30 mbps to about 40 mbps, Russell said.

This is a propitious moment for commercialization of VDSL2 multi-line vectoring, noted Rob Gallagher, principal analyst and head of broadband & TV research at Informa, “Service providers and governments have stated their intent to boost broadband speeds to consumers and businesses alike,” he said. “But the challenges associated with comprehensive fiber-to-the-home deployments have been a major obstacle. VDSL2 vectoring promises to make superfast broadband speeds available to many more people, much faster than many in the industry had thought possible.”

The opportunities for telcos to hit the 100 mbps level uniformly across all households served by a fiber node without extending fiber deeper are fairly limited in the U.S. but are more abundant in Europe and Asia where FTTN penetration is often much deeper. Nonetheless, even at the 3,000-foot node distances, there’s a great benefit not only by virtue of the 33 percent improvement in throughput but also because the service provider can be more certain that all lines will perform at the advertised rate, Russell noted.

This has to do with the software refinements Alcatel-Lucent has applied to enable the algorithms to adjust to the idiosyncrasies of under-performing lines, as when moisture has collected inside the sheaths. “We’ve designed our own silicon for vectoring, which means we have a lot of flexibility in how we deploy and manage the processing,” Russell said.

“If you look at all the binders going out to subscribers, there’s very wide differentiation in the levels of service because of differentiation in the levels of crosstalk,” he continued. “That variance makes it hard for operators to offer higher rates uniformly. With vectoring at 3,000 feet you don’t get the huge rate gains that you get at shorter distances, but there’s huge advantage because of the predictable bandwidth. We make adjustments to make sure you get the highest possible vectoring gains on problematic lines.”

Vectoring is a completely automatic process that can be implemented at the line card level to apply crosstalk cancellation across a 48-port VDSL2 card in small nodes or at the system level in larger nodes, where a single vectoring processor card performs cancellation across four line cards or a total of 192 ports. Next year the system-level processor capacity will be expanded to support cancellation on eight line cards or 384 ports, Russell said.

While most of the processing for vectoring is performed at the DSLAM, CPE has a role to play in receiving the test signals that let the system know what the crosstalk effects are at each end point. All but the oldest versions of VDSL2 modems can be upgraded via software downloads to accommodate vectoring, Russell noted.

Alcatel-Lucent’s next step in the DSL performance expansion process will entail introduction of vectoring with traditional bonding technology, which itself uses crosstalk cancellation techniques to nearly double the data rate by sharing the throughput over two pairs in a single cable. With vectoring, bonded VDSL2 links will deliver close to 200 mbps at the 400-meter range.

Then, at a date yet to be announced, comes Phantom Mode DSL, which depends on a new technique used in combination with bonding and vectoring to achieve even higher data rates. Last year Bell Labs announced it had lab tested Phantom Mode DSL at 300 mbps over 400 meters and at 100 mbps over one kilometer on a bonded pair. Some vendors by bonding more pairs together have used the Phantom Mode technique to go as high as 800 mbps in lab tests.

This is another technology that has been understood from a theoretical standpoint for some time, going back to 2004 when software developer ASSIA, Inc. produced a paper on the subject. “Phantom Mode technology leverages vectoring to provide business services over copper that will be competitive with the fastest optical services,” observed John Cioffi, chairman and CEO of ASSIA.

As explained by Cioffi and others, the technology exploits the fact that each pair consists of a signal carrier and what amounts to a ground wire which normally doesn’t transmit a payload signal. Phantom Mode uses the two ground wires in a bonded pair to support transmission of a third payload signal at a data rate equivalent to that of each of the signal pairs, which increases the overall throughput of the vectoring-enabled bonded pair by 50 percent. (Vectoring is critical to Phantom Mode insofar as, without it, the crosstalk between the ground wires would be too intense to achieve a significant increase in overall throughput.)

Alcatel-Lucent is close to announcing its first customers for VDSL2 vectoring. Meanwhile, according to informed sources, field trials of Phantom Mode are already underway at Swisscom, which suggests the day for product availability may not be too far off.