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Taking aim at cable modems

Fri, 04/30/1999 - 8:00pm
Roger Brown, Editor-in-Chief

Consolidation within the cable television industry has clouded the issue a bit, but most people still consider Denver and the Rocky Mountain West to be the capital of cable TV. It is perhaps appropriate, then, that US West has targeted the area for its massive roll-out of digital subscriber line-based services.

Like the old-lore gunfights of the past, this battle that's shaping up between DSL and cable modems may be more rooted in fiction and hyperbole than in tangible evidence, but one thing is clear: no one's ready to give any ground.

When US West's !nterprise Networking group began its wide roll-out of high-speed data service, US West executives said it was part of the company's vision to provide "Webtone" to customers throughout its 14-state territory. The goal of Webtone is to offer high-speed access to the Internet—or company intranets—from anywhere in US West's territory, over virtually any line.

But implementing the "MegaBit" service, as it's been dubbed, isn't as simple as turning on the data spigot and waiting for customers to line up in droves. There are myriad technical and logistical issues that have to be overcome if the Webtone vision is to become a reality.

Perhaps the biggest obstacle is US West's infrastructure. The rate-adaptive DSL equipment the company uses doesn't work over every line. For example, it won't work at homes and businesses that are beyond 15,000 feet from the central office. It also won't work on lines that are serviced by a digital loop carrier. Loading coils, which were commonly placed on copper lines, destroy DSL signals, as do some splices and bridge taps.

Today, roughly 36 percent of the loops in the wire centers where DSL has been deployed actually qualify for service, according to Larry Yokell, a former CableLabs employee who is now director of product development for MegaBit Services. The goal is to increase the number of qualified loops to 50 percent of the lines in deployed wire centers by the end of 1999. Ultimately, US West is aiming to serve between 90 percent and 95 percent of the lines in major Western cities.

To get to that level, US West will have to use a combination of technologies, including DSL, ISDN and others that are being closely guarded at the moment, says Yokell.

A key limitation is digital loop carriers (DLCs), which serve approximately 20 percent of US West's lines. Because it's a relatively new technology, US West has deployed DLCs primarily where it hurts it most-in new-growth areas, which is also where many of the most desirable customers reside. "We've got to get to those customers," Yokell notes.

Fortunately, most of those customers are served by DLCs manufactured by either Lucent or RELTEC, says Yokell. Therefore, he really only has to find two solutions.

His options include: placing the DSLAM (Digital Subscriber Line Access Multiplexer, a rack of ADSL line cards with data multiplexed into a backbone network interface/connection) in a vault near the DLC and wiring around the DLC as a sort of "bypass;" working with the vendors to replace the DLC's backplane and to install new software as a card-based solution; or pursue a "hybrid" solution whereby the DLC is made smaller and the DSLAM is added "on top." From Yokell's point-of-view, the first option is expensive and inelegant, the second solution is already under development and the last idea is already being tested.

Despite the limitations and relatively low loop qualification levels, US West has deployed the Cisco Systems-based DSL equipment in 213 wire centers in 13 states. By the time the year is out, it should be deployed in up to 265 wire centers, according to Yokell.

As of the time of this interview, US West had approximately 22,000 residential and business subscribers signed up for one of its suite of services.

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Figure 1: Multimedia applications and their bandwidth requirements.

To give its subscribers choice, US West is working with nearly 300 Internet service providers. In areas where the service exists, customers often have a choice of as many as a dozen ISPs, including US West itself.

Getting service

Those who do qualify for MegaBit are finding that the ordering and provisioning process is orders of magnitude easier than the nightmare many experienced with ISDN (Integrated Services Digital Network). Anyone with access to the Internet can visit www.megaspeed.com to find out if they reside in a MegaBit-served area, and whether their telephone line will support the service.

This Web-ordering tool, or MegaWOT, as US West has named it, has had a significant impact on the company, according to Yokell. Roughly 10 percent of MegaBit customers have ordered the service via the Web site, using a process that takes, on average, seven minutes to complete.

Increasingly, US West doesn't even have to visit the consumer's home to provision the service, either. To date, 89 percent of the customers have self-installed the modem and the NIC, according to Yokell, instead of incurring a $149 service installation fee. Of those, 86 percent never even call the technical support line, he boasts.

What makes this possible? Yokell says that after ordering the service, a customer is sent a package that includes the modem, an instructional videotape and some small filters that plug into an RJ-11 jack that block hum and noise on the return channel. The installation process typically takes between 25 and 90 minutes, adds Yokell.

Already, there are moves underway to improve that process. Dell has announced that it will soon sell computers with an internal Cisco 605 modem, making it "MegaBit-ready," according to Yokell.

E-mail: Roger Brown

 

What US West offers

To date, US West has roughly 22,000 residential and business customers for its MegaBit Service data offering. MegaBit is actually just a portion of a family of different available services, as outlined below. Prices are based on a one-year contract and prices may vary, depending on location. In addition to the monthly fee, there is a $110 one-time activation fee.

  • MegaLine: 256 kilobits per second (kbps), bi-directional. Price: $40
  • MegaOffice: 512 kbps, bi-directional. Price: $62.40
  • MegaBusiness: 768 kbps, bi-directional. Price: $76.80
  • MegaBit: 1 megabit per second (Mbps), symmetrical. Price: $120
  • MegaBit: 4 Mbps downstream/1 Mbps upstream. Price: $480
  • MegaBit: 7 Mbps downstream/1 Mbps upstream. Price: $840

MegaLine, MegaOffice, MegaBusiness and MegaBit customers will need to buy a Cisco modem or PCI card from US West for the computer. The discounted price of the modem is $95.

Thoughts on ADSL standards

There are two main approaches to ADSL technology: those that incorporate discrete multitone modulation (DMT), and those that are based on an older technique, carrierless amplitude/phase modulation (CAP).

CAP stands for Carrierless Amplitude/Phase modulation, and describes a version of QAM in which incoming data modulates a single carrier that is then transmitted down a telephone line. The carrier itself is suppressed before transmission (it contains no information, and can be reconstructed at the receiver), hence the adjective "carrierless." The primary difference between QAM and CAP concerns their implementation; QAM generates two signals and combines them in the analog domain. In contrast, in CAP, orthogonal signal modulation is executed digitally, using two digital transversal bandpass filters with equal amplitude characteristics and a difference in phase response. The signals are then combined, fed to a digital/analog convertor, and transmitted. CAP's advantage over QAM is that (digitally speaking) implementation is efficiently obtained in silicon, at some cost of flexibility.

DMT stands for Discrete Multi-Tone, and describes a version of multi-carrier modulation in which incoming data is collected and then distributed over a large number of small individual carriers, each of which uses a form of QAM modulation. DMT creates these channels using a digital technique known as Discrete Fast-Fourier Transform.

Multi-carrier modulation requires orthogonality between all the subcarriers; a Fast Fourier Transform is convenient for achieving this (although other sophisticated transforms have been developed recently). Multi-carrier techniques require much digital processing, and were not commercially feasible until recently, when IC technology could implement them economically and reliably in mass-market silicon processes.

DMT's standard specifies 256 subcarriers, each with a 4 kHz bandwidth. They can be independently modulated from zero to a maximum of 15 bits/sec./Hz. This allows up to 60 kbps per tone (some implementations support 16-giving rates of 64 kbps per tone). At low frequencies, where copper wire attenuation is low and signal-to-noise ratio is good, it's common to use a dense constellation-greater than 10 bits/Hz is typical. In unfavorable line conditions, modulation can be relaxed to accommodate lower SNR-usually 4 bits/Hz or less, and deliver the necessary noise immunity. Furthermore, as the system measures line integrity, it can avoid or compensate for cross-talk or interference. This is particularly useful, for example, when reducing modulation in a band where an AM station is causing radio frequency interference (RFI).

Today, US West's service is based on CAP technology, making it incompatible with other service providers' offerings. Yokell says that's OK for now. "We'll migrate when it makes sense," he promises. In the meantime, Cisco is developing a chassis that will support both approaches, and DMT proponents are working on making their gear interoperable.

Sources: ADSL Forum, Analog Devices whitepaper

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