Bandwidth bloom

DSL providers live and breathe two terms: rate and reach. Unfortunately for them, the two are inversely related. That is, data rate declines as distance increases. The further Joe Customer lives from the central-office (CO) facility serving his neighborhood, the lower the bandwidth he can expect to enjoy. That stinks not only for Joe, but also for his service provider, which desperately needs to turn Joe and all his neighbors into happy, paying customers.

Barring alterations to the laws of physics (which ultimately dictate how far a signal can travel without degrading), the rate-versus-reach problem isn't going away. However, the DSL community is well on its way to deploying new flavors of DSL that will fling data faster and farther.

Under these emerging standards, speeds still decline over distance. But because they're so high to begin with, they remain high throughout a wider radius than with plain-old ADSL. In other words, these standards extend the umbrella of high-speed coverage over a larger customer population. And lest you think that we're discussing a far-off dream, note that these capabilities are available in chip form today.

The DSL community has generated an array of standards with names that almost seem calculated to confuse. The table summarizes the most significant ones. Of these, ADSL2+ is generating the most short-term excitement because it is very close to receiving formal approval from the ITU (International Telecommunication Union) and because it offers a dramatic improvement over conventional ADSL.

ADSL2+ increases data rates by devoting twice as much frequency spectrum in the copper wire to downstream data. Doubling the spectrum from 1.1 MHz to 2.2 MHz permits downstream data rates of around 25 Mbits/sec on local loops of 3000 feet or less and 20 Mbits/sec on lines as long as 5000 feet (though these figures vary somewhat depending on who's doing the talking).

The maximum rates aren't terribly important. The key advantage ADSL2+ offers service providers is a wider customer base for high-speed service. To be exact, ADSL2+ provides a 66 percent increase in the area a provider can reliably reach with a 12-Mbit/sec service, according to Todd DeBonis, vice president of sales and marketing for chipmaker Centillium Communications.

Centillium this week introduced its Maximus chipset. This two-chip, 12-port product features the company's eXtremeDSL MAX feature set, which delivers standard-compliant ADSL2+ through a technology called MAX-DS (Double Spectrum). The chipset also offers a quad-spectrum implementation, dubbed MAX-QS, and other technologies aimed at tweaking rate and reach to the service provider's advantage.

MAX-QS promises a top downstream rate of 50 Mbits/sec over short local loops. Perhaps more significantly, it extends the area that can receive a 25-Mbit/sec service out to a 3500-foot radius, compared with a 500-foot radius for ADSL2+. For service providers, that spells premium revenue from business customers. In addition, such bandwidth provides ample room for TV services and other advanced offerings.

The menu of features on the Maximus chipset also includes MAX-EU (Extended Upstream), which increases upstream rates to 3 Mbits/sec; MAX-HBL (High Bit-Loading), which boosts bandwidth by filling each DSL "bin" with more bits; and MAX-LR (Long Reach), which extends ADSL reach to as far as 22,000 feet at modest data rates. The latter feature is Centillium's implementation of READSL (Reach-Extended DSL), another standard wending its way toward ITU approval.

"It's important to note that all of these features are selectable and provisionable on a per-port basis," says Afshin Shaybani, Centillium's product marketing manager for CO products. Customers have already designed the Maximus chipset into CO gear and are heading into production, he adds.

Although its offering appears to be the most robust at the moment, Centillium is not alone in supporting emerging DSL flavors. For example, intellectual-property provider Aware recently announced the availability of an ADSL2+ core, the StratiPHY2+. Analog Devices, STMicroelectronics, Texas Instruments, and GlobespanVirata have all announced or will introduce ADSL2+ support as well.

In fact, Texas Instruments is already offering silicon support for ADSL2+ on the CPE (customer-premise equipment) side of the DSL link. The company's AR7, billed as a router on a chip, supports both ADSL2+ and READSL. Centillium also offers a double-spectrum CPE chip, the Palladia 210. (For more on these devices, see "Don't go away," 4/28/2003, and "Broader band," 2/12/2003).