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Bellcore delves into HFC amid support for FITL

Sat, 09/30/1995 - 8:00pm
Fred Dawson, Contributing Editor
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Bell Communications Research, in an effort to expand understanding of hybrid fiber/coaxial network technology, is generating data which appears to underscore long-standing in-house preferences for more traditional telephone industry topologies.

The RBOC-owned organization, now in limbo as the Bell companies decide how to go about finding it new owners, has added a wide range of programs devoted to testing and developing specifications for HFC, responding in part to telco interest in the technology, and in part to a perceived opportunity to broaden its customer base in the cable industry. But, despite confirmation of many claims made by manufacturers for the flexibility and expandability of HFC, leading staff members remain ambivalent about the long-term advantages of the cable-derived network owing to the noise sensitivities of digital modulation techniques, ingress in sub-band upstream communications and the vulnerability of system electronics to power surges.

"Not all the problems are obvious, and we're certainly not the only ones pointing them out," said Paul Shumate, executive director for broadband local access and premises networks, who served as guide to CED representatives in a recent tour of three Bellcore sites in New Jersey. "For example, CableLabs discovered things nobody realized about coax," he said. "It turns out the least little imperfection in the electronic components generates impulse noise. This hasn't been a problem in the downstream, where the frequencies are relatively noise-free, but it is a serious phenomenon in terms of impact on the upstream signal."

There are problems for downstream digital signals as well. In one of several labs devoted to testing HFC, Tom Chapuran, a staff technician, offered demonstrations of the impact of microreflections on downstream signals, showing how failure to use 75 ohm terminators at end points in household wiring can defeat digital signals. For example, in one instance the reflected impulse from a wall-plate terminated branch of coax knocked out a 16 QAM-modulated digital signal delivered in the downstream channel at 180 MHz.

"It's ridiculous not to sell 75 ohm terminators as part of a multiport device," Shumate said. "It wasn't seen as sufficiently important in the analog environment to require it, but, with digital, it's a different story."

All the problems shown in the demonstration could be cured through adaptive equalization, Chapuran said. But at 64 and 256 QAM, the problems are more acute, and the going also gets tougher in the upstream environment, he noted. "You can't solve all the possible problems with electronics," he commented.

Adaptive equalization also carries a price beyond basic costs, Chapuran noted. "The equalization changes at different frequencies," he said. "It can take time to acquire the equalization state when you're switching channels, which can be frustrating to the viewer."

It's too early to say what the typical range of premise wiring conditions is, or what their impacts are on the various approaches to distributing signals in the home, Chapuran added. "That's what we're finding out here," he said.

In all their testing, officials were cautious when it came to generalizing about real-world conditions, saying that much data remains to be collected before a full range of specifications and recommendations can be developed.

But the yellow flag is up, even when it comes to using coax at the end of fiber-to-the-curb networks.

"As soon as you put coax in for broadband distribution, even at a curbside ONU (optical network unit), you have new concerns about reliability," Shumate said.

Bellcore is looking at a variety of alternatives to coax drops in the star/star fiber environment, including twisted pair copper, fiber and wireless. Its longest experience, of course, is with twisted pair, and here, the findings continue to support the long-held Bellcore thesis that there is much still to be mined from the telcos' copper plant, though maybe not as much as some telcos would like.

"We believe ADSL (asymmetrical digital subscriber line) is much more amenable to interactive services than HFC," said David Waring, director of business development. ADSL, which originated several years ago at Bellcore, delivers video and data at up to 8 megabits per second in the downstream, together with bi-directional ISDN (integrated services digital network) and POTS (plain old telephone service) over standard telephone lines.

A lot of testing now is focused on the technical variant of ADSL known as VDSL or BDSL (very high speed or broadband digital subscriber line), which uses similar but lower-level modulation techniques, such as 16 QAM versus 64 QAM, to generate much higher bit rates over shorter strands of distribution copper. The technology, supporting broadband digital service distribution to approximately 24 single dwelling households from an ONU, vastly reduces the number of ONUs in the FTTC design while making use of existing distribution plant.

But there's a potential problem which Bellcore officials decline to be very specific about. Known as "far end crosstalk" or "FEXT," the interference occurs across lines over the first 100 feet or so in the transmission path when too many VDSL transmitters are put together in a single bank at the ONU. There are three ways to solve the problem, said Craig Valenti, another technical staff member: shorten the range, which can cut the number of households served per ONU by a substantial margin; use coax or shielded twisted pair at the drop end, which keeps the signal-to-noise ratio up by reducing attenuation, or "shape" the spectrum using discrete multitone or wavelet modulation.

Valenti declined to discuss details of performance of VDSL, including what the distance limits are for 24 gauge shielded twisted pair at 51 megabits per second, which is the ATM-compatible speed many telco engineers appear to favor. But with regard to the impact of FEXT on overall advantages of VDSL, he said, "It could be serious."

The upshot of the findings concerning use of coax or copper at the end of fiber links, whether in FTTC or HFC constructs, is that Bellcore experts continue to see fiber to the home as a desirable option, especially if the cost curve continues to fall relative to the other options.

"When we looked at FTTH costs a couple of years ago with respect to comformity to our TR 909 (fiber in the loop) specifications, we found a difference of $330 per household over FTTC for narrowband, and about $500 for broadband," Shumate said. "Today, there's been so much progress in loop lasers and other areas that the difference now looks more like $460 for broadband and $220 for narrowband."

On further investigation, factoring in life cycle costs, the cost difference virtually disappears, Shumate added. For example, by moving the powering to the customer premises with backup power supplied by batteries, the FTTH life cycle savings is about $200 per household, he said.

"Maintenance and provisioning savings on metallic drops add another $200 savings over the 20-year lifespan," he noted. "The result is, there's really not much difference now, and the numbers will only get better going forward."

Chinlon Lin, director of broadband lightwave systems research, made a similar point. "When you look at the delays that have pushed deployment back by at least a year and take into account the progress on FTTH, it starts to look very good for FTTH," he said.

Meanwhile, FTTC is clearly back in the ascendancy, albeit with a delay tied to unresolved technical issues and uncertainties about broadband. With Bell Atlantic, Southwestern Bell, BellSouth, Nynex and US West signaling their preferences for FTTC, the question suppliers and engineers face is whether this position will hold before deployment begins in earnest, or whether, once again, there will be a massive shift in telco agendas.

If FTTC stays strong, and a lot will depend on the powering issue (see sidebar), the telcos' evolutionary path to FTTH would be strengthened.

"Stay tuned," Shumate said. "There's still a lot to learn."

Concern over the downsides of HFC amid the improving cost parameters for FTTH may lead to greater telco interest in all-optical networks, but first the carriers must solve the powering problem.

While the issue has slowed FTTC deployment, it appears to be more nettlesome for FTTH. In FTTC networks, as opposed to FTTH, power design alternatives to trenching copper with fiber, such as coaxial-delivered AC or DC to ONUs (optical network units), represent options that maintain the telco tradition of powering the premises telephone.

For FTTH, the obvious alternative to running copper with fiber everywhere is customer-supplied power backed up by network-supplied batteries. Tough as this might be to accept, given the many drawbacks associated with today's battery technology, Bellcore and others are working on ways to make backup support for lifeline service more palatable.

One approach now under manufacturing development in Henderson, Nev., Madison, Wis., and four other places is the lithium manganese battery technology developed and licensed by Bellcore for use in many markets, including laptop PCs and other consumer devices. "We think this technology will dominate the laptop computer battery market," said Frough Shokoohi, a Bellcore specialist in solid state chemistry research.

Shokoohi said network backup power is another potential application for the malleable, cloth-like battery material, which can be folded into compact packaging without losing its current-generating capacity. The technology meets telco needs for long-life rechargeable batteries in non-temperature controlled housings, she said, noting that the lithium manganese batteries have two-and-a-half times the longevity of nickel cadmium, and 12 times that of lead acid batteries, while generating nearly twice the energy per kilogram of nickel cadmium, and three times that of lead acid.

"We think it is the lowest cost alternative as well," she said.

Whether such performance capabilities, due in the marketplace from Bellcore-licensed manufacturers within two years, will overcome telco resistance to maintaining batteries in non-network powered FTTH systems remains to be seen. But Bellcore technical directors Paul Shumate and Chinlon Lin believe the benefits of eliminating metallic lines from the network could prove persuasive once new backup power options become available.

Meanwhile, the issue for telcos remains whether to implement broadband via HFC or via FTTC, and, here again, powering the network is a critical issue. If FTTC can't be powered to everyone's satisfaction, the platform for evolving to FTTH won't be implemented, which would strengthen the HFC option.

Coax has emerged as a strong contender for powering optical network units in FTTC systems. This evolutionary path eliminates the need to trench twisted pair copper in FTTC networks for powering purposes while supplying a pipeline over which to deliver analog TV signals during the transition to digital media.

"Coax offers an elegant powering solution, given its analog service transmission capability," Shumate said. "But it also serves as a conductor for power surges, which is the primary cause of amplifier outages." A Bellcore study of CATV networks found amplifiers go out for an average of 25 minutes per unit each year, which means telcos would have to add a lot of surge protection over what the cable industry already uses in order to prevent ONU outages, Shumate noted.

One alternative for powering ONUs now under intensive study at Bellcore and BellSouth involves use of solar power panels. Bellcore has spearheaded solar powering of curbside ONUs in narrowband FTTC arrays at a variety of sites, including two pedestals operating commercially over BellSouth Telecommunications facilities in Charleston, S.C. for the past two years.

"Since we started, we've had only one power failure, and that was because of electronics in a line card rather than lack of sunlight," said Bryant Davis, digital electronics support specialist at BellSouth's regional office in Charleston. "I'd term the trial a success."

BellSouth, with the most aggressive fiber-to-the-curb (FTTC) deployment in the U.S., is looking for a way to avoid running copper lines in the trenches with fiber, Davis said. "We're looking at AC and DC distributed powering over twisted pair and coaxial as other options," he added, noting that BellSouth just began a year-long test of coaxial powering of FTTC in conjunction with extension of a cable TV overlay.

The two solar-powered ONUs are part of a 44-ONU network segment serving the Dunes West residential subdivision. The panels supply 840 volts of reserve power to a lead-acid battery at the pedestal. Sun power in the area is rated at about 425 watts per day per square foot.

Officials said an ONU uses about 3.4 watts on average, requiring about 81 watt hours per day, which means the stored power in the battery can last through 10 days without recharging from the sun. Bryant said the trial has access to backup power from the network in the event of an outage but that, in any widescale deployment, the ONUs would be completely solar dependent.

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