DSL decision time

DSL standards have a way of piling up. Between 1999 and late 2003, no less than six different ADSL-related specifications and one VDSL specification made their way through the industry standards bodies such as the American National Standards Institute, the European Telecom Standards Institute and the International Telecommunications Union. These specifications included not only ADSL and ADSL2, but also ADSL2+ and Reach Extended ADSL2 or RE/ADSL2.

As is typically the case with standards, their long incubation and approval process at the ANSI, ETSI and ITU has been followed by intensive interoperability testing, and as a result, the enhanced versions — ADSL2, ADSL2+ and ADSL2/RE — have yet to make their market debut. VDSL is still another DSL technology option for telcos in waiting. All of these technology flavors have been undergoing carrier trials for some time, but they are first expected to see broad commercial usage in an upcoming round of telco upgrades likely to play out over the next few years.

“ADSL2 and ADSL2+ are still in their earliest stages of market development,” said Jay Fausch, senior director of marketing for Alcatel. “The vast majority of lines in service right now are still garden-variety DSL. From standardization, it takes a while to do all the certification and interoperability testing, but these are the things that make it viable for mass-market deployment.”

ADSL2 and ADSL2+ interoperability tests are ongoing at the renowned University of New Hampshire InterOperability Laboratory, and a chipset “plugfest” capping the lab tests is imminent.

“Interoperable silicon is just now starting to be a reality for ADSL2 and 2+,” Fausch said.

However, Kevin Walsh, vice president of marketing at Calix, said his company already is shipping ADSL2+ product.

“The greater concern for carriers than interoperability is whether the technology is living up to its rate and reach promise,” he said. “They need it now to support their video plans.”

The long path that DSL specifications take to broad market deployment may surprise no one. Some forms of DSL technology were developed as early as the late 1980s, and ADSL itself first appeared in 1993. At that time, ADSL was being targeted at potential telco video applications, but the technology, lacking a clear standard, expensive and problematic to deploy, proved to be as immature as the telco video market was at the time. While the last five years or so have seen increasing DSL deployment in the broadband service race versus cable TV modem technology, telcos have taken their time in the last year making decisions about expanding and improving their DSL services, according to some vendors. In the case of the former Bell companies, they were waiting for the regulatory relief that arrived from the FCC during the second half of 2004.

“There was a bit of a slowdown in the DSL market in 2004,” Fausch said. “But telcos were trying to come to grips with all of these choices coming their way, and they were waiting for regulatory decisions. Paradigm shifts can sometimes cause hiccups.”

Any market hiccups were relieved more recently by the news that ADSL technologies would figure prominently in the broadband upgrade plans of two major telcos — SBC Communications and BellSouth (see Telephony, Dec. 13, page 8). Both large ILECs plan to bring ADSL2 into homes, rather than running fiber all the way, as Verizon Communications is doing. BellSouth recently shed further light on its ADSL2 deployment plans, saying it is testing DSL bonding solutions that are capable of vastly improving bandwidth. BellSouth officials said last month that they expected to be testing ADSL2 and ADSL2 with bonding in the early part of this year.

ADSL2 was designed with the same kind of video applications in mind that ADSL originally was developed for, but with several features that improved on the original standard. Enhanced signal processing algorithms, a reduction in framing overhead and other modulation efficiencies allowed for better DSL performance in deployments over longer distances. Overall, ADSL2 increased downstream bandwidth of ADSL by about 50 kb/s and 600 feet, translating to a total coverage area increase of about 2.5 square miles, according to a standards white paper from ADSL technology developer Aware. The peak data rate for the standard is about 12 Mb/s, depending on distance and other factors.

The ADSL2 standard also allows for enhancements that provide more efficient power consumption. While earlier ADSL technologies only operated in a single, full power mode, ADSL2 provides for two lower power modes. The first mode shifts to low power operation when lesser amounts of traffic are traveling over the ADSL connection. The second mode is a sleep mode automatically initiated when the line is out of use for extended periods of time.

Another technical benefit to be gleaned from ADSL2 is seamless rate adaptation. To avoid the problem of signal crosstalk on ADSL lines, the data rate can be adjusted while the line is in service, without affecting the transmission. The standard also allows for the creation of voice channels to further reduce the potential for errors when both data and voice are running over ADSL.

Aside from performance, ADSL2 also improves the ability of telcos to offer service level agreements by assigning a small amount of bandwidth to be used for diagnostic purposes.

“Double-ended line testing for troubleshooting can help you determine and maintain performance on a given line,” said Alcatel's Fausch.

Other benefits of the ADSL2 standard include a fast initialization mode, reducing system synchronization time to 3 seconds, about 7 seconds faster than outlined in the original ADSL standard, said Fausch.

While ADSL2 offers a variety of advantages, ADSL2+ offers many of these advantages, while only improving overall performance. Though it was approved in 2003, three years after ADSL2 received standards approval, ADSL2+ has matured quickly to become supported in a wide range of vendor equipment.

“Maybe I missed something, but it seems like ADSL2 came and went, and no one made a big deal about it,” said Calix's Walsh. Some vendors are playing the market by ear to figure out if ADSL2 succeeds first and potentially delays migration to ADSL2+, but Calix is focusing almost entirely on ADSL2+.

“The truth is that anything deployed will be backward compatible,” he said.

“ADSL2 is interesting,” added Alcatel's Fausch, but he admitted that ADSL2 and ADSL2+ migrations probably will not occur separately. “Most carriers probably will go directly to 2+,” said Fausch, whose company, Alcatel, is the dominant DSLAM vendor in the market and recently won a significant piece of SBC's Project Lightspeed broadband upgrade (see Telephony, Nov. 29, page 6).

ADSL2+ offers more bandwidth, but more specifically, it doubles the amount of spectrum used. This means more room for video but also for voice over IP (VoIP) and other applications, according to Walsh. Peak data rates of 25 Mb/s are possible, with averages reported anywhere between 3 Mb/s to 15 Mb/s up to about 5000 feet.

Both ADSL2 and ADSL2+ continue to evolve with improved capabilities. One such capability that significantly increases bandwidth is bonding, a sub-layer inverse multiplexing method borrowed from the ATM world. Bonding allows two or more DSL lines to be aggregated into one virtual line. Bonding ADSL2 lines can provide up to 24 Mb/s downstream bandwidth — almost as much as ADSL2+. However, bonding also can be applied to ADSL2+ lines, where it can raise the bandwidth ceiling up to around 45 Mb/s and make ADSL2+ deployment that much more attractive. Bonding is likely to see some of its first commercial applications this year.

There is much hype surrounding ADSL2 and ADSL2+, but the most recent member of the ADSL standards family is RE/ADSL2. This standard increases coverage distance up to about 22,000 feet by increasing power.

The various standard schemes for ADSL technology give telcos many choices for deployment, and ADSL2 itself might have been a long-standing goal in carriers' broadband evolution. Many of these companies might want to jump to ADSL2+ or use a mix of DSL access flavors.

Bonded ADSL2+ or VDSL could quickly become telco darlings if these companies want to make much bigger, short-term leaps in bandwidth, according to Fausch.

“ADSL2+ is where most of the telcos are focusing their energy. It will be the default access choice for many of them,” he said.

VDSL is a newer and still expensive option. By standard definition, it has a peak data rate of 52 Mb/s, with likely average rates around 25 Mb/s downstream and 3 Mb/s upstream.

Which of these DSL technologies will see the most deployment in North America remains to be seen. Globally, DSL deployment is occurring at a faster pace, and innovations such as VDSL are being deployed more quickly, according to Aviv Ronai, associate vice president of marketing for ECI Telecom. In many countries across Europe and Asia, telcos already have begun to deploy ADSL2 and ADSL2+ in large amounts. Yet each market presents its own criteria and challenges for broadband deployment.

“The deployment decision really depends on each individual operator, how deep they need to introduce fiber and when,” Ronai said. “Is it wise to introduce VDSL now or wait? What is the spectral compatibility of the line? Will they be deploying it from remote terminals or not?”

Ronai said European and Asian markets are defined by high population density in urban centers, which means shorter potential loop lengths and a better business case for VDSL. In North America, there is more of a mix of shorter loop needs in some areas and much longer loops in others.

Competitive market pressures also can play a role in the urgency of DSL deployment decisions. Ronai pointed out that two-thirds of the broadband line in the U.S. market are cable modem lines, while broadband competition from cable TV companies is not significant throughout the rest of the world.

Alcatel's Fausch agreed.

“When it comes to broadband, cable is really pushing the speed threshold.”

But more than competition, the need for more DSL bandwidth among telcos is driven primarily by interest in delivering new applications. The application most on the minds of telcos these days is video. It's the single most common and critical factor driving them to deploy ADSL2 and ADSL2+.

“Video is always the problem child. The worst-case scenario for proving the validity of any DSL technology is always video,” Fausch said. “Telcos will want enough bandwidth for video but also VoIP and for online gaming.”

Renai added that VoIP and other peer-to-peer services might be the applications most ill-suited for traditional ADSL architectures because they require significant upstream bandwidth.

“Carriers will want to put all of these services on the same platform,” Renai said. “But voice and video telephony, as well as other peer-to-peer services, whether they are legal or not, require the increased upstream bandwidth available in advanced platforms.”

Uncertainty over how and when a variety of DSL technologies available will reach mainstream deployment and market acceptance has not stopped vendors from aggressively marketing any and all of them, on modular platforms that are backward compatible and capable of delivering ADSL2 to one house and VDSL to another in the same neighborhood.

“We'll see a lot of all these standards in mixed deployments,” said Walsh.

Added Renai: “If you can produce generic line cards, than the operators will not have to try to predict what the future will be. They can have more comfort in making these deployment decisions.”