BEHIND THE VDSL RENAISSANCE

Not too long ago—in fact, less than a year—very high bit-rate digital subscriber line technology appeared destined to be nothing more than a nice niche technology. Although it would be useful in some applications, like rural network rebuilds or multi-dwelling units, the overwhelming majority of DSL users in the U.S. would be hooking into the Net and watching video streams over ADSL links, or some variation of that.

Over the last few months, however, VDSL — which is the fastest member of the DSL family but functions best only over short loop lengths—has made a resurgence in both mind share and actual deployments. Many factors are behind the revival, including a seemingly insatiable appetite for bandwidth by users in Japan and South Korea, a rush into video by U.S. carriers and a battle among various VDSL line-coding techniques.

Two years ago, MDUs were where VDSL technology was headed, said Dave Boulos, vice president of product management with Telco Systems. “Everyone was focused on [broadband LECS] and pushing bandwidth up the risers. What we're seeing now is that people are coming back to life on VDSL. They're realizing that it's matured into a realistic technology. Three years ago it was more of a science project.”

In fact, many of VDSL's first deployments were under controlled conditions with rural U.S. telcos that were forward thinking enough to be either expanding into video services at a time when the cable industry was just beginning to rebuild its own plant or pushing the so-called “edge out” strategy of providing CLEC services in neighboring towns. In either case, the deployments weren't always successful and hardly set a precedent for the entire industry to follow.

“Carriers were struggling with VDSL — you had to take your 9000-foot serving area and rebuild it into a 4000-foot serving area,” said Gary Bolton, vice president of product marketing with Catena Networks. “The whole video-over-DSL thing started to fall apart because rebuilding your network was just too high a hurdle.”

At the same time, the Asian high-speed access markets that would eventually spark much of the demand for VDSL were in their infancy as government regulators in South Korea and Japan began laying out the ground rules for near ubiquitous coverage of DSL. In the intervening years, though, both the South Korean and Japanese markets have seen an explosion of demand for bandwidth. In terms of pure speed, alone, the numbers are impressive.

Japanese gaming cafes have created such high demand that NTT and other carriers are providing VDSL-based service at 50 Mb/s downstream and 26 Mb/s upstream, according to Richard Sekar, vice president of marketing for VDSL chip vendor Ikanos. In Korea, the number is only slightly slower at 50 Mb/s downstream and 12 Mb/s upstream.

“There's a speed race going on in Asia, and the gaming market is really the high end of it,” he said.

Of course, carriers aren't deploying the biggest broadband pipes solely for shoot-'em-up, online fantasy role-playing and virtual racing geeks. Video, the same service driving U.S. carriers to explore beyond ADSL, is pushing carriers in some Asian markets toward VDSL. In Japan, for instance, the demand for video services is so great that NTT in 2000 launched a fiber-to-the-home program that was supposed to connect 16 million homes over the course of a decade. After three years, however, only 80,000 homes have direct fiber connections. Coming to rescue has been VDSL, which is providing bandwidth over the last portion of the last mile, often up a riser in a high-rise (see figure).

Deeper into the network, many point to the lack of ATM as a reason Asian carriers have been able to deploy VDSL. Without having to worry about stretching embedded ATM assets, carriers in Japan and South Korea have been able to jump to the newer Ethernet-based VDSL systems. In the U.S., where the earliest systems used ATM interfaces, carriers generally must figure out a way to use those existing switches.

“If you're a greenfield, you're not going to go ATM because of the cost,” Boulos said. “The real focus is moving to pure Ethernet VDSL.”

However, one of the biggest reasons VDSL has taken off in Japan and South Korea has nothing to do with technology. Instead, it is the living patterns of the respective countries' citizens and the resultant network designs.

“If you look at the Asian market and the way the subscriber base is distributed, VDSL makes a lot more sense,” said Sean Blakley, senior manager of product line management at AFC. “If you look at Japan, the base is so densely packed.”

Some vendors even point to the different urbanization patterns as a reason Asia has become a more fertile ground for VDSL. In the U.S., as people become more prosperous, there's a tendency to want larger houses on larger lots, which pushes them out of the city and into more sprawling suburbs, said Bolton. “In Asia it's the opposite,” he said. “The wealthier you are, the higher in the high-rise you live.”

The result of such density is that the majority of users are within 12,000 feet of a central office or fiber node. A significant number are within 6000 feet, where VDSL has its greatest speed advantage over other copper-based technologies.

“If you look at the loop length outside the U.S., 90% of the consumers are within 9000 to 12,000 feet,” said Sekar, whose company has had its greatest success in Asia. “VDSL is available at very high speeds at up to 6000 feet. It goes to 12,000, but it converges to ADSL speed.”

With urban fiber deployment, those loop lengths are shortened even more because VDSL can be deployed in neighborhood nodes or in MDU basements where the network transitions from fiber to copper. “[Fiber to the premises] does not have to mean that the fiber has to go all the way up to the premises,” he added. “VDSL has a broader role.”

Whether that will translate to the U.S. market with the anticipation of RBOCs' en masse deployment of FTTP is uncertain.

Proponents of VDSL believe it can be deployed in the U.S. just as cheaply as any variant of ADSL. Providing VDSL at up to 50 Mb/s downstream and 12 Mb/s upstream for the same price as ADSL 2 Plus, which can go up to 24 Mb/s downstream, gives carriers the speed they need to trump any cable modem competition, according to VDSL backers. At the same time, the recent push by more than a dozen vendors to support discrete multi-tone (DMT) line coding for VDSL will push costs even lower.

“Today you can buy VDSL for $100 per line in volume, compared to $80 to $90 for ADSL,” Sekar said. “Long before VDSL attains the 40 million lines that ADSL has [globally], it will reach the same price.”

VDSL also is the better technology for MDUs, which is still a significant portion of the North American market, said Jeff Weber, vice president of technology for Motorola Next Level, which was the vendor for most of the first domestic VDSL deployments. In Toronto, for example, Bell Canada has been testing VDSL-based video service as a way to blunt the impact local cable companies are having with their high-speed Internet service. “ADSL might be the more appropriate technology for rural LEC, but there's definitely still life in VDSL,” he said. “We have customers using both.”

There also is some hope that a newly emerging standard, VDSL 2, will be able to push current VDSL speeds beyond its distance limitations.

Detractors—and there are many in the U.S. vendor community—think ADSL 2 Plus will provide plenty of bandwidth even if fiber is run to within 1000 feet of most homes. Additionally, large carriers have not indicated they want anything to do with VDSL.

Texas Instruments, one of the largest suppliers of ADSL chip sets, is prepared to get into VDSL but is holding off for the moment because it doesn't see a significant demand.

“We think things are going to move to VDSL speeds, but there's a lot of life left in 24 [Mb/s] downstream,” said Kenton Epard, marketing manager of xDSL central office products for TI's Broadband Communications Group. “The VDSL standards aren't fully complete yet either, though we are a big supporter of DMT in VDSL.”

The line code issue that VDSL vendors are currently debating was settled in September 1996 in the ADSL world when a joint committee of RBOCs made the decision to use DMT. ADSL 2 Plus uses the same line coding, which some say makes it more cost effective than VDSL.

“You've got to have an application driver to need more than ADSL 2 Plus,” Bolton said.

That driver may come from more widespread adoption of high-definition television. Though limited in reach today, falling prices on HDTV sets, as well as a federal government mandate for broadcasters to begin giving up their analog spectrum, is expected to push its demand.

In its uncompressed state, HDTV requires 19 Mb/s of bandwidth per channel. However, with current compression techniques that space can be narrowed to 12 Mb/s without sacrificing quality. MPEG-4, the next generation of compression that is just entering the market, can further squeeze HDTV down to less than 10 Mb/s. Using ADSL 2 Plus' 24 Mb/s, carriers would have enough capacity to offer at least one HDTV channel and one other video stream along with high-speed data and voice. With VDSL, though, carriers would have sufficient capacity for multiple HDTV streams.

“Let's say HDTV becomes table stakes,” Bolton said. “In that environment, VDSL certainly could play.”

Barring the widespread adoption of HDTV, though, most believe VDSL will be limited to niche status in the U.S. market for the moment, in part because that's the way the largest carriers will deploy it. “ADSL still really is the standard, and ADSL 2 Plus is where the thrust is,” said Blakley. “It's coming from the RBOCs because it fits in well with their designs.”