As end-to-end transport becomes the mantra of ever more carriers, long-haul optical technology is revolutionizing approaches to overall network design with promises of improvements in capacity, flexibility and cost- efficiency at the regional and metro levels as well as dramatic gains in signal reach over great distances.
The expanding impact of techniques typically associated with ultra-long-haul (ULH) performance ties directly into the fact that as optical wavelength switching gains traction among carriers that operate end-to-end networks, network designers will have to plan for how each aspect of the optical architecture affects every other aspect across traditional topological boundaries. In fact, say some vendors, the driving force behind the roll out of optical platforms employing innovations such as solitons, Raman amplifiers and so-called "super" forward error correction (FEC) has at least as much to do with this perspective on network design as it does with any demand for systems that can deliver a terabit payload in a straight shot over a single fiber for a distance of 3,000 kilometers, which is the performance level now being reached by some ULH systems.
"When you look at it from the end-to-end perspective, there's something to be said for operating wavelengths over the backbone at the same rate as you operate the metro segments if you can pass those circuits from one point to the next without touching them electronically," notes Tim Krause, vice president for global marketing, terrestrial networks, at Alcatel "So the interest in ultra-long-haul technology is not just about economies of scale on the backbone."
Heroic Efforts
Earlier this year, Alcatel performed "hero" tests to demonstrate what might be done eventually with ULH technologies. In one instance, Alcatel scientists demonstrated transmission of 300 10gbps wavelengths, or a total capacity of three terabits per second, over a distance of 7,300 km. In another, the company broke the 10tbps barrier with a transmission of 256 40gbps wavelength channels at a distance of 100 km, representing an aggregate usable throughput of 10.2tbps.
Developments on the commercial horizon aren't far from hitting the distance and capacity marks set by the Alcatel tests. Already Nortel Networks Ltd. is offering the OPTera 4000 system with a purported ability to deliver up to 112 10gbps wavelength channels a distance of 4,000 km. And Corvis Corp., now rolling out its CorWave LR system with the capability of delivering 3.2tbps a distance of up to 800 km or 1.6tbps a distance of 2,000 km, has plans to expand the system in the year ahead to where it will deliver up to 2.8tbps a distance of 3,200 km.
The importance of the flexibility that results from combining ULH and optical cross-connect technologies has been a recurring theme in the public appearances of Broadwing Inc. CEO Richard Ellenberger. Ellenberger gambled his company could take the leap to all-optical networking as it moved forward with plans to light two more fibers in its nationwide grid of cables earlier this year. Working with Corvis, Broadwing met its goal on time and under budget, Ellenberger says. In the process, it became the first carrier to deliver services across a nationwide backbone without having to resort to electrical regeneration of signals at the core switching points.
"We've been able to install and turn on OC-192s (10gbps wavelength services) in less than 45 days, which would have taken one to three years to do in the past," Ellenberger notes. "Now the challenge is to get all the way to the customer premises over an all-optical platform at the metro level, which is something I'm sure Corvis is working on."
Offering Ethernet services on an end-to-end, plug-and-play basis "across the country" is an especially attractive opportunity that comes with the flexibility afforded by next-generation optics, he adds.
Give and Take
So far, as Alcatel's Krause and others make clear, precisely how the new long-haul technologies will affect network planning in the end-to-end context is far from certain, which makes it difficult to be precise about the evolutionary path of new UHL systems beyond their immediate applications in the traditional backbone mode.
For example, Krause notes, the techniques that open the way for seamless extension, add/dropping and switching of 10 gigabit wavelength circuits across long-haul and metro boundaries from one end of the nation to the other also support high densities and long distance transmissions of 40 gigabit wavelength channels, but at approximately half the density and distance levels attainable for 10 gig wavelengths using the same techniques.
"Is pushing 40 gig circuits as far as possible, with electro-optical conversion at major PoPs [points of presence], the best approach?" Krause asks. "Or is it smarter to back off to 10 gig and use ULH to create an all-optical end-to-end operating environment?
"You have to ask the question in the context of the fundamental cost in terms of dollars per kilobits per kilometer," he says. "But you also have to ask whether there's a reason why you'd do one or the other based on what your service requirements are."
Nortel is betting that some carriers will want it both ways, where they would be willing to sacrifice some of the benefits accruing to dense spacing of 10 gig wavelengths in the interest of being able to add 40-gig wavelengths in the future.
In weighing whether to deploy Nortel's OPTera 5000, slated for introduction next year, "you have to decide whether you want to be 40-gig ready or whether you want to optimize for 10-gig," says Phillipe Morin, vice president of next generation optical Internet at Nortel.
Using OPTera 5000, the carrier could set link budgets of 2,500 to 3,000 km for the 10 gig wavelengths and then provide for add/drop points at half those distances to accommodate the need to regenerate the 40 gig signals in the future, Morin explains.
But, no matter whether the hybrid or all-10-gig approach is taken, to make the cross-boundary, all-optical design work requires UHL technology and optical cross-connects, Morin notes.
It also requires that DWDM be integrated into the grooming devices that multiplex multiple lower bit-rate streams such as OC-3 (155mbps) or DS3 (51mbps) into the 10 gig wavelength streams, and that those devices be equipped with the optical layer intelligence to transport those streams as far as and in whatever direction they need to go.
As mesh configurations in the all-optical networking domain replace rings, UHL technology provides support for rerouting of signals over longer paths in instances where the shortest path is blocked without having to worry whether such rerouting will require that the signals be regenerated. But taking UHL to its ultimate limits in support of all such contingencies on a nationwide network could be prohibitively expensive, Morin says.
Consequently, many of Nortel's customers are asking for hybrid design options that would provide for optical-to-electrical-to-optical (OEO) conversion at certain key PoPs to allow for regeneration of signals, while other PoPs would be all-optical.
"This also allows you to design the network to link budgets for adding 40 gig later on," Morin says.
Fortunately, he notes that the demand for ULH systems for traditional long-haul applications alone is strong enough to allow vendors to proceed with development of gear that holds the potential for designing the integrated all-optical networks of the future.
"The first applications of this technology are being driven by the installation of big routers that can direct high volumes of traffic between places like Los Angeles and Chicago," Morin says.
Other vendors see the same thing.
"I'm astounded at how much demand there is for our 160 wavelength ultra-long-haul system," says Nick Doran, chief technology officer for soliton projects at of Marconi Corp. plc. "The cost justification for this technology applies to many different scenarios."
Marconi's UPLx 160-wavelength system, due to be available commercially this quarter, is designed to operate over a distance of up to 3,000 km without electrical regeneration, but it can be cost competitive with conventional high-capacity options at any distance where at least one opto-electronic conversion point would be required, Doran says.
"Something over 50 percent of all optical links carrying Internet traffic are over 800 km in length," he adds, noting that such distances require regeneration in traditional DWDM systems.
A Strong Pulse
Marconi, like most other providers of ULH systems, is using soliton technology as the foundation for achieving such distances. Solitons are concentrations of multiple light pulses that capitalize on the fact that when such pulses are in precise phase with each other they combine into a single, high-energy pulse that is more resistant to degradation as it travels through fiber than any of the original pulses would be. The magic occurs at the receiver end, where an interfering, out-of-phase lightwave projected into the soliton breaks the concentrated pulse into its original constituent parts.
While the soliton potential has been a focus of R&D since fiber optics became a telecommunications technology, it wasn't until the late '90s that soliton transmissions became commercially feasible, starting with undersea applications. And in 1999, they expanded to terrestrial systems with a product release from French startup Algety Telecom, which Corvis acquired last year. Now that the technology is a mainstay of several vendors' ULH systems, the battle is over whose implementation of solitons is most cost-effective, which has a lot to do with how well the vendor's system can manage the subtle changes in dispersion effects as the soliton pulses move through the fiber.
"The pulses exist in a periodic environment where the width is not constant, so you want to allow the pulses to breathe naturally," Doran says. "That means you need to have a slight degree of dispersion management in sync with the periodic changes so that the soliton isn't either suffocating or hyper-ventilating."
That means dispersion must be managed in accord with individual link lengths, which can range all the way to 150 km between optical amplifiers if Raman amplification is used in conjunction with erbium-doped fiber amplifiers (EDFAs).
"Solitons are really good if they're properly managed, but I can show you situations where they don't give you any benefit at all," Doran explains. "Everybody does dispersion management now, but not everybody does it right."
The use of Raman amplifiers is another key element to the new flexibility ULH systems provide. Operating in the 1450 nanometer wavelength region, Raman pump lasers deliver a stream of photons into the fiber core or into a Raman-optimized EDFA where, as they travel through the fiber and lose energy from collisions with atoms in the glass, they drop to lower frequencies and assume the wavelength and signal pattern of the primary lightwave in the 1550 nm waveband, resulting in a great boost in the signal power without the addition of spurious signals or noise.
In EDFAs, where noise is a limiting factor, amplification results as electrons, pushed into excited states from the collision of the pumped light with the erbium atoms in the doped fiber, release photons at the wavelength of the primary signal stream.
But Raman amplification and soliton modulation are expensive components of ULH, which is why some strategists prefer to work with FEC--the third major technique in the ULH hierarchy--as the basis for extending network parameters.
Synchronizing Streams
FEC serves to synchronize the reading of the bit stream in accord with how the stream has been distorted as it travels over the network, resulting in a net code gain of several decibels once the coding penalty associated with synchronization is subtracted from the amount of recovered code.
Multilink, for example, supplies FEC integrated circuits to various systems manufacturers that achieve net code gain (NCG) of up to 10 dB, notes Dave Huff, vice president of marketing at Multilink.
"With 9 or 10 dB of NCG the effect is that you more than double the number of wavelengths you can deliver over any given type of fiber, or you can double the distance," Huff says.
There's another great advantage to FEC as well, notes Jim Sauer, director of product marketing for the photonics group at Cisco Systems Inc. "FEC also gives you the ability to look at how the span is operating and to detect errors and things like degradation of the fiber or amps before you have a breakdown that stops transmission," he says.
Cisco has put its energy behind FEC for what it terms "extended long-haul" applications rather than focusing on ULH on grounds that the real demand for added distance has to do with the opportunity to use add/drop multiplexing over links that run in the range of 1,600 to 2,000 km.
"The goal is to bring down the cost per bit per DS3 mile, and, if you can't do that with these other technologies, than using them doesn't matter," says Jeff Santos, director of marketing at the Cisco photonics group. He and Sauer decline to discuss what Cisco might be doing with regard to future implementations of other technologies.
Dave Schaeffer, CEO of Cogent Communications Inc., strongly supports the Cisco view after having looked at ULH techniques as possible options in his firm's efforts to build out a nationwide network of long-haul and metro links to 20 major cities. With 150 buildings connected and metro rings in place in 15 cities, Cogent is getting services under way to some of the customers in those buildings with a unique service strategy that calls for delivering 100mbps Ethernet services at a flat rate of $1,000 per month.
"In some cases we use FEC on spans to be able to add more [optical] amps between regeneration points. In other spans we're using the technology to increase the number of wavelengths," Schaeffer says, noting that the network overall is designed to support up to 128 wavelengths of OC-192, starting with the lighting of eight wavelengths in the first phase of deployment. "Each span is optimized to come up with the lowest cost per bit-mile."
Clearly, with a brand new network to start with, Cogent's decision to go with an all-Cisco approach to the buildout says something about the current state of optimization for delivering a next-generation service like Fast Ethernet. How long it will be before someone seeking to do a similar service on an end-to-end basis over a new network will have reason to consider other approaches to improving on the bit-per-mile costs is uncertain. But, given the pace of ULH technology rollouts and the emergence of optical cross-connects, it's probably not too soon to be thinking about the implications of these developments for network designs in the years ahead.
| Long-Haul Technologies Being Applied to End-to-End Optical Networks |
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Forward Error Correction (FEC) -- serves to synchronize the reading of the bit stream in accord with how the stream has been distorted as it travels over the network, resulting in a net code gain of several decibels once the coding penalty associated with synchronization is subtracted from the amount of recovered code. Raman pump lasers -- deliver a stream of photons into the fiber core or into a Raman-optimized erbium-doped fiber amplifiers where, as they travel through the fiber and lose energy from collisions with atoms in the glass, they drop to lower frequencies and assume the wavelength and signal pattern of the primary lightwave in the 1550 nm waveband, resulting in a great boost in the signal power without the addition of spurious signals or noise. Solitons -- Concentrations of multiple light pulses that capitalize on the fact that when such pulses are in precise phase with each other they combine into a single, high-energy pulse that is more resistant to degradation as it travels through fiber than any of the original pulses would be. |
| The Links |
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Alcatel www.alcatel.com Broadwing Inc. www.broadwing.com Cisco Systems Inc. www.cisco.com Cogent Communications Inc. www.cogentco.com Corvis Corp. www.corvis.com Lucent Technologies Inc. www.lucent.com Marconi Corp. plc www.marconi.com Multilink Technology Corp. www.mltc.com Nortel Networks Ltd. www.nortelnetworks.com |
