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April 2001 Issue
WDM Technology: Which Way Do You Turn?
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Dense wavelength division multiplexing (DWDM) is all the rage at the trade shows these days, but how do you decide if it’s worth it?

As the cable industry moves from an analog-based video delivery business to an all-digital, fiber-optic interactive communications service, it is under great pressure to adopt the latest and greatest technologies.

When it comes to optical transmission, the latest and greatest is dense wavelength division multiplexing (DWDM). The ability to transmit multiple wavelengths along a single fiber greatly enhances the amount of data carried over the network and the number of customers that can be served simultaneously.

It’s no surprise cable operators are excited about DWDM designs aimed at providing eight, 16 or even 32 wavelengths per fiber.

But there are a number of voices calling for a reassessment of that strategy. Perhaps, some argue, jumping into DWDM right now is putting the cart before the horse. Maybe it’s better to go with a simpler approach now and upgrade to DWDM later when customer demand warrants network improvement.

"If you make a big investment in DWDM, you’re betting that demand will be high in those areas," says Ham Matthews, director of marketing at ADC’s cable systems division. "But if you have the capacity for a lot of channels but you’re not using them, then you’ve over-invested."

To Matthews and other industry players, a more sensible approach might be to go with wavelength division multiplexing (WDM), meaning a single 1550 nanometers (nm) wavelength (channel) upstream and a 1310 nm downstream. Once revenue from new users starts coming in, operators can better afford to scale up to a four-channel coarse WDM (CWDM) system or a DWDM setup of eight or more channels.

Putting off the decision now also offers the potential of lower prices in the future. Telcos and other network providers will continue to install the technology for their existing voice and data services, causing prices to drop as more units are shipped. Of course, those telcos also are your competitors, so it wouldn’t be prudent to wait too long.

Vendors report that about half of new orders are for CWDM or DWDM, with the other half for WDM. Some observers think this may be caused partly by worries that a slower economy may hamper the desire for advanced cable services. Still, others say it’s simply a desire on the part of many operators to go slow when it comes to adding network capacity.

There is very little difference in the underlying technology behind WDM, CWDM and DWDM. The idea is to split the light wavelengths in the fiber so each is able to carry a discrete data stream. But while CWDM and DWDM require specialized multiplexers and filtering technology to ensure there is no crossover between wavelengths, WDM requires no special technology. A pair of lasers—set for 1310 nm and 1550 nm—a high-low bandpass filter and a receiver that distinguishes between the two wavelengths is all that is needed.

"DWDM is significantly more expensive on the equipment side," says Eric Schweitzer, senior director of product management at Harmonic. "The lasers need to be thermally stabilized. The optical passives are expensive and difficult to build."

But for those arguing that full-scale DWDM is the way to go, the higher cost is one of the prime motivations for installing it now. After all, why invest in a limited solution like WDM when you know you’re going to have to expand in the future, possibly before the existing WDM units have depreciated?

"The WDM approach is more of a Band-Aid than a real solution," says Curtiss Smith, manager of advanced systems architectures at Motorola’s transmission network systems business unit. "Do I want to use something that I will have to rip out in six months and redo with a higher order system? Or do I bite the bullet and just add wavelengths where and when I need them?"

But unlike most things in life, where the more you buy the cheaper the unit cost, the expense of DWDM increases as the channel count goes up.

"The ability of optical technology to support more channels at a given cost-level has been increasing," says Paul Connolly, vice president of marketing and network architectures at Scientific-Atlanta. "We’ve been migrating from eight to 16 to 24 channels, provided we can keep the cost flat at about $1,000 per channel."

The main reason for this is that channel separation becomes narrower as wavelengths are added, which requires more sophisticated filtering technology. A four-channel CWDM system, for example, typically allows 20 nm spacing in the 1550 nm domain. That’s 10 nm on each side of the selected wavelength to play with. In contrast, an eight-channel DWDM system affords only 1.6 nm between wavelengths—barely eight-tenths on each side. Stray outside those parameters, and the passive filter will block the signal. All of this places an enormous amount of pressure on the laser transmitter, which requires highly advanced cooling circuitry to maintain those tolerances.

As in most decisions regarding network architectures, the answer to this dilemma depends largely on circumstance. Owners of large metropolitan systems will need to ramp up with DWDM quickly because of the large number of potential users. Rural system operators can probably afford the slower approach. And those caught in the middle? The decision will depend on the market.

"If you’re just doing residential service with cable modems and set-tops, you’re probably okay with WDM for a few years," ADC’s Matthews says. "But if your hybrid fiber/coax (HFC) plant passes a number of small business out there who are getting starved because they can’t afford the [Regional Bell Operating Company] RBOC’s telephone rates, you could put DWDM in that region and go after the small business market. That would provide a much better return on investment than the residential market."

Of course, WDM and it’s many varieties are not the only ways to add channel capacity to your system. Frequency multiplexing can be mixed and matched to other transmission techniques, such as time domain multiplexing (TDM). Under this approach, data flowing over segmented channels on a single fiber is further segmented by time, so that a receiver allocates data from perhaps every other nanosecond into a discrete channel. Again, the cost of this approach should be weighed carefully against customer requirements, network architecture and other factors.

No matter which option you choose, it is likely to be cheaper and easier to install a few muxes and filters than to pursue the only other capacity-building option: laying more fiber. Because of the high demand for fiber from the telcos, many operators have been looking at a year-long wait for new orders. Because of the looming economic downturn, it appears that shortage will ease later this year as telcos scale back rollout plans.

Even if the price drops considerably and availability increases, installation costs alone are likely to limit fiber to new rollouts, rather than increasing capacity of existing systems.

"I’ve seen a couple of systems where the operators put in up to 144 fibers on a ring," Harmonic’s Schweitzer says. "Splicing 144 fibers is very expensive. If they were to use DWDM technology, they could have probably dropped that to 20 fibers and had capacity to spare."

Most cable operators installed their fiber backbones within the past five years, so glass quality is such that just about any transmission technology is possible. Older fiber, however, might not be clean enough to handle numerous wavelengths.

"If you have good, relatively modern fiber in place, you could probably get 40 channels out of DWDM," Matthews says.

Inevitably, cable operators considering expansion into digital services embrace some type of signal multiplexing technology. For those on the cutting edge, it probably makes sense to push ahead into the more advanced DWDM approaches, but only in those areas where you are certain to draw substantial numbers of new customers.

For the rest, there is no shame in a slower approach. After all, digital services are reaching only a small fraction of the overall cable universe. Only about four percent of customers are connected to high-speed data, while only one percent has telephony. If you think you have the capacity to handle demand, then perhaps WDM is right for you. But if you see take-up rates higher than you anticipated, be prepared to upgrade—and fast.

Arthur Cole is a contributing editor for Communications Technology. He may be reached at .

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