Let There Be (Network) Light

Growth in optical switching and dense wavelength division multiplexing promises to eliminate some of the bottlenecks between corporate networks.

Sending corporate network traffic over Category 5 Ethernet cable at 1G bit/sec. - which is now possible via LANs - may sound fast. But that's snail-like compared with what's possible over backbone networks using optical data transmission that can accommodate 80 different data streams, each running at 2.5G bit/sec. New optical networking technologies, especially those involving switching and multiplexing running on backbones, promise to eliminate bandwidth bottlenecks between corporate networks.

Big networking names such as Cisco Systems Inc. in San Jose, Calif., Lucent Technologies Inc. in Murray Hill, N.J., and Nortel Networks Corp. in Brampton, Ontario, see opportunities aplenty. They've made optical development a priority, as have companies focusing only on the optical market such as Sycamore Networks Inc. in Chelmsford, Mass., and Ciena Corp. in Linthicum, Md.

For corporate users, optical networks could mean more robust connections to public and private backbones and faster connections between buildings, across metropolitan areas and to data centers maintained by network outsourcers.

With site-to-site optical switching over fiber-optic cable, remote servers can respond as quickly as if they were local.

Laser Ride

Data sent over fiber rides on a laser. And unlike the beam from a flashlight that dissipates into the night sky, a laser concentrates light so it can streak through the fiber for 500 kilometers or more before it has to be amplified or regenerated. Convert a digital electrical signal to laser pulses, and optical becomes an extraordinarily fast and high-volume way to transmit data.

Optical networks are already in place inside many large corporations. Here, fiber is often used in backbones that serve LANs. The prospect of all-optical connections from the wiring closet to the Internet backbone has network folks sensing major change.

Think about it: a seamless optical connection that hurls text, voice and video at 186,000 miles per second across cities or countries. Mouse-click in Boston, and a server in Seattle responds without a wait. That's already prompting outsourcers to use fiber to link scattered offices.

Optical transport for long-haul telecommunications isn't new, either. As early as the 1980s, telecommunications companies were combining multiple voice signals on a single fiber-optic cable by sending different signals over the line at very precise intervals. The technique is called time division multiplexing.

And through an optical carrier standard called the Synchronous Optical Network, or Sonet, companies were able to achieve speeds of 2.5G bit/sec. over optical fiber, according to Kathy Szelag, vice president of the business optical group at Lucent. That, she says, is equivalent to 33,000 simultaneous phone calls per fiber and was just fine up until about 1995, when the Internet began gobbling bandwidth.

One solution has been to bury more fiber-optic cable. But that often means digging trenches, which can be expensive, especially in urban areas. Now there's another option, at least where there's already fiber in the ground: Dense wavelength division multiplexing, in use since the mid-1990s, combines different wavelengths of light, each carrying a different data stream, into a single beam that's sent over a single fiber.

Szelag explains: "You take several lasers running at 2.5G bit/sec., each running at a slightly different color, a slightly different wavelength. They're closely spaced in terms of frequency. You feed these different colors into a prism. The prism combines the waves into a single beam. At the other end of the fiber, there's another prism, which separates the single beam back into the original colors."

Carriers can now send up to 80 separate wavelengths over a given fiber, Szelag says. That equates to 2.6 million simultaneous phone calls. And consider this: More than 150 fibers can be bundled in a single fiber-optic cable.

Traffic Cops for Light

Still, telling information-laden light waves where to go and how to get there has been a challenge. Until recently, it was necessary to convert optical signals to electrical signals and back again before network traffic could be switched (from one circuit to another) or routed (directed) to the appropriate destination based on the addressing information carried by the signal.

The complexity of the current hybrid optical/electrical digital cross-connect systems that control traffic on most of the large public and long-haul private networks makes maintenance and change costly and slow.

Reconfiguring incoming and outgoing traffic at a major carrier cross-connect can take weeks or even months, says Mike Coghill, head of network engineering at Global Crossing Ltd., a broadband information provider and data center outsourcer.

Global Crossing will be the first to deploy an all-optical switch, Lucent's WaveStar LambdaRouter. Announced in November, the LambdaRouter performs many of the functions of a digital cross-connect, but it does so optically - there's no optical-to-electrical-and-back-to-optical conversion.

Nortel is also in the all-optical switching game. It's purchasing Xros Inc. in Sunnyvale, Calif., to get the Xros X-1000 cross-connect, which is still in the works. Like Lucent's technology, it's an all-optical switch that analysts say may go into trial later this year.

Both Lucent and Nortel use a micromirror technology. Tiny mirrors, one for each wavelength, catch the wavelengths and then reflect (switch) them to the appropriate fiber, based on the settings programmed into the switch.

For example, an incoming wavelength destined for the Southeast that's coming into an optical switch in Chicago could be sent to Atlanta by "shining" the signal to the appropriate outbound fiber.

Coghill says Hamilton, Bermuda-based Global Crossing is testing three Lucent optical switches and plans to begin deploying them later this year. Neither Lucent nor Nortel has been forthcoming about prices for the new switches. But Coghill suggests that a single Lucent switch will run somewhere between $3 million and $5 million, depending on how many in-and-out ports the switch accommodates.

Lucent's current product can switch 256 incoming by 256 outgoing wavelengths. Lucent says the switch will eventually scale to 1,024 by 1,024 wavelengths. The Xros switch, according to Nortel, will be 1,152 by 1,152 wavelengths.

Why are Coghill and others so fixated on these developments?

More for Less

"The core (of the network) today is electrical," Coghill says. "You have to convert optical signals to electrical signals before you can do any switching." And with the Internet's requirements for "bandwidth doubling every four to six months, conventional (switching involving electrical to optical conversion) simply cannot be scaled fast enough," he adds.

"The goal," says Coghill, "is to replicate everything in electrical to optical."

And while few are willing to predict when - or whether - there will be an all-optical equivalent to the sophisticated content routing now possible through electrical routers, Coghill says he believes that in the next couple of years optical networking will drive down cost and increase throughput.

"In the optical domain," he says, "throughput-per-bit for an optical device is halving every six to nine months. Put another way: You're getting twice the throughput for the same price."

Copyright © 2000 IDG Communications, Inc.

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