DWDM: The Long (Haul) and Short (Haul) of It

Posted: 12/1997

DWDM: The Long (Haul) and Short (Haul) of It

By Peter Meade

Dense wavelength division multiplexing (DWDM), already the darling of long distance carriers for its ability to efficiently add impressive capacity to their existing fiber optic networks, is becoming a growing attraction for the local loop.

Long- and short-haul carriers share a mutual interest in this technology because both are seeking remedies for fiber network congestion. DWDM systems are available today from such vendors as Alcatel Network Systems Inc., Cambrian Systems Corp., Ciena Corp., Fujitsu Network Communications Inc., Lucent Technologies Inc., Nortel and Pirelli. They have shown the technology can be used to multiply the capacity of an existing single optical fiber--which typically runs at 2.4 Gbps--by 16 times or more and support bandwidth of 40 Gbps or more in each direction over a fiber pair. Moreover, tomorrow's 40-channel systems promise 100 Gbps, the equivalent of 10 OC-192 transmitters.

"No one talked about bandwidth like this a year ago," says Pawan Jaggi, senior product planner, bandwidth transport planning for Richardson, Texas-based Fujitsu Network Com-munications. "And recent technological advancements have ensured the economic attractiveness of using DWDM, because it comes down to price in key enabling technology."

Two Factors

In addition to the growing demand for bandwidth, DWDM's increasing popularity can be attributed to two other factors: Prices for WDM components are dropping, and the technology continues to improve.

Mark Lutkowitz, president of Trans-Formation Inc., a Birmingham, Ala.-based market research house, says component prices have come down as much as 20 percent to 25 percent in the past six to eight months.

"Whatever has been done with an electrical signal can be done with optical, and it can be done cheaper at a higher level," says Tim Krause, senior director for business development and optical networks at Richardson, Texas-based Alcatel Network Systems Inc.

Time-division multiplexing (TDM) was commonplace when WDM first appeared in the 1980s. WDM started out in dual-channel wideband form, in 1310 nm or 1550 nm wavelength bands.

In the early 1990s systems emerged that could multiplex four to eight wavelengths, resulting in a notable gain in market momentum. The big gain came last year, when DWDM debuted, bringing with it network management capabilities and add-drop multiplexer functions. Now a selection of currently available WDM systems can handle eight to 16 wavelengths, which in many applications help interexchange carriers squeeze more capacity from their OC-48 (2.4 Gbps) backbones.

According to Trans-Formation Inc., the domestic market for WDM will grow from last year's $80 million to about $330 million by 2000.

The most common form of DWDM uses half of a fiber pair for transmission and half for reception. There also are systems in which a single fiber carries bi-directional traffic, but this approach reportedly sacrifices capacity and amplifier performance due to their configurations, according to several market players. In contrast to wideband WDM, also called interband or non-dense, all the optical carriers are within the same window and run within only a few nanometers of each other. Due to signal amplification traits of today's new optical amplifiers, this window is around 1550 nm.

This new brand of amplification, performed by erbium doped fiber amplifiers (EDFAs), can amplify all the wavelengths at the same time, helping DWDM systems achieve widespread commercial deployment. EDFAs differ from traditional regenerative or electro-optic repeaters in that the light does not require conversion to an electrical signal, amplification and then reconversion to light, explains Alcatel's Krause. EDFAs also can lengthen transmission distances to more than 300 km without regeneration, saving both time and money.

Long-haul carriers, such as AT&T, Bell Atlantic Corp., BellSouth, MCI Communications Corp., Sprint Corp. and WorldCom Inc., have invested heavily in the technology for long-haul applications. According to Kevin Slocum, managing director at Stamford, Conn.-based SoundView Financial Corp., Sprint is the most aggressive deployer of DWDM. "But we are just scratching the surface of fiber in the network," he adds. "The lower the cost, the closer the fiber will get to the home."

Fiber Relief

Fiber relief is the main requirement in designing for long haul, says Solomon Wong, director of product management for Ottawa-based Cambrian Systems, which--along with Linthicum, Md.-based Ciena Corp.--is leading the charge to the local loop. However, Wong notes, short-haul applications require systems that offer greater flexibility, scalability and operational ease.

Moreover, explosive Internet, intranet and extranet growth has changed the economic model for building these networks. For example, data traffic in the public switched network in the San Francisco Bay area doubles every 10 months, says Joachim Vobis, a Santa Rosa, Calif.-based product manager, optical spectrum analysis, at Hewlett-Packard. With that kind of growth, megabits become gigabits very quickly, he says. DWDM offers a promising solution.

DWDM systems, with network management gear and add-drop multiplexers, represent an attractive option because the cost of adding fiber to any network remains pricey, no matter who's talking. Costs for deploying fiber can exceed $70,000 per mile, according to several industry players. For this reason alone, as well as others--such as the limitations of current products and legacy installations--relieving fiber congestion by deploying more fiber is not always the best answer. This may be especially true for local traffic in highly congested areas, where additional fiber deployment can be extremely challenging or impossible. According to Douglas McKinley, director of engineering for Sprint's long distance division, the $70,000-per-mile price tag is "on the skinny side" of the price chart. "In the prairies, perhaps," he says. "But the sky's the limit in metropolitan areas, so we must eke every bit out of existing fiber." Jeff D. Montgomery, founder and chairman of ElectroniCast Corp., a San Mateo, Calif.-based research house, says prices may be as low as $10,000 per mile if suitable ductwork is available.

In some instances, short-haul DWDM systems actually can be simpler and less expensive to install because they do not require expensive optical amplification, says Alcatel's Krause. Moreover, simplicity, cost reduction and speed to market make short-haul DWDM applications increasingly attractive, says Cambrian Systems' Wong.

There does, however, remain the high cost of terminals, which may make some short-haul applications price-prohibitive. This is not the case when the cost of terminals is stretched over a long-haul application, explains Sprint's McKinley. In some cases, though, installing terminals remains cheaper than the cost of deploying more fiber, he says.

A universally acceptable answer is cheaper terminals. "We don't need sophisticated filters for the local loop," McKinley says. Yet his colleague Samuelson adds that some vendors remain firm in requiring them--a point of debate that may cost these vendors their share of the market.

Companies such as Cambrian Systems and Ciena, however, are delivering local boxes that do not include amplifiers, Samuelson says. If Cambrian's OPTera can handle up to 40 km, it is sufficient for 80 percent to 90 percent of Sprint's local loops, he adds.

A CLEC Delight

The growing array of short-haul DWDM systems should be of particular interest to competitive local exchange carriers (CLECs) because of their ability to deliver new services more quickly while reducing the costs associated with delivery that, in turn, allow CLECs to offer services at a more attractive (and competitive) price. "Short-haul systems will change the way CLECs as well as incumbent local exchange carriers (ILECs) deploy bandwidth," says Cambrian Systems' Wong. For example, carriers now will be able to offer services on an individual wavelength basis.

But, even with all the positive momentum, DWDM in the local loop has a way to go, according to several market watchers. "Fiber is making its way from a long haul-only application to short haul," Leon says. "But it will be a long time before fiber-to-the-house is a mass reality."

A Forward Look

Sprint currently is evaluating several 32-wavelength products and one 40-wavelength offering. Testing will begin in Sprint's lab in the fourth quarter, McKinley says.

According to ElectroniCast's Montgomery, Bell Laboratories is looking even further ahead. DWDM technology with wavelengths of 64, 72, even 100 are not inconceivable, he says. The widespread availability of these wares has become an economic --not a technological--issue, he adds.

And, Lucent reportedly has developed one laser that can replace 206 by using WDM to generate 206 separate wavelengths, each carrying 36.7 Mbps of data.

According to Alcatel's Krause, DWDM is just the beginning of tomorrow's photonic network, in which DWDM systems with open interfaces will give carriers the opportunity to deliver SONET, ATM, frame relay and other protocols on the same fiber. Achieving an open systems environment means additional costly, optical transmitters are not needed to interface with specific protocols. Instead, less expensive transmitters can do the job. "Bit-rate protocol independence gives flexibility and growth," says Cambrian Systems' Wong. "We can go from OC-3, to OC-12 to gigabit Ethernet without changing the network interface. Carriers may hate changes, but what they hate more is having to take their networks down for 16 hours for upgrades."

Sprint will be using WDM in 70 percent of its route miles by the end of this year, according to the company. And that figure is expected to surge to 98 percent by the end of year. McKinley clarifies the statistics by explaining that certain route miles already contain several WDM systems. Other route miles currently without may not carry enough traffic to be WDM-worthy. Regardless, "WDM will be ubiquitous in Sprint's network by next year," says McKinley, who predicts several of Sprint's competitors likewise will buy into WDM in a big way.

DWDM's growth from ubiquitous in the long haul to a familiar site in the local loop may slow the need for some new fiber deployment, but it is not likely to hurt the overall fiber game, says SoundView's Slocum. "After all, you cannot continue to leverage all the benefits of DWDM without having the access in the ground."

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