Lucent DWDM goes the distance: Experiment delivers terabit per second over 400 kilometers using new amplifiers
Dense wavelength division multiplexing was the center of much attention at OFC '98, with vendors touting new systems that deliver increasingly higher channel densities. Ciena Corp. and Pirelli showed off 40- and 64-wavelength systems, respectively, and Lucent Technologies' Optical Networking group touted its 80-channel WaveStar product, due out later this year.
But as the show wound down, Lucent topped its own booth display by announcing it had achieved terabit-per-second transmission over a single fiber. In a lab, Lucent used five ultra-wideband optical amplifiers to push 100 10 Gb/s wavelengths with 100 GHz spacing over 400 kilometers.
Atul Srivistava, a researcher in Bell Labs' Photonic Networks Research department and co-leader of the experiment, said delivering terabit rates over such a distance is significant. "This has demonstrated optical amplifiers can make a national [terabit] system possible," Srivistava said. "Previous experiments didn't have the distance."
The rapid evolution of DWDM is making it difficult to keep up with the latest developments. It seems that no sooner do vendors introduce a new threshold for channel capacity than another is announced. Lucent certainly isn't the only vendor testing a 100-plus channel system, and those systems don't approach the natural limit of fiber. Srivistava estimated that a single fiber could carry as many as 250 channels. The only obstacles to developing such systems are the cost of laser diodes to generate the channels and optical amplifiers to recondition the channels and keep them intact over long distances, he said.
Does the snowballing channel capacity mean that new fiber being laid by companies such as Qwest and Williams is redundant? Not according to Mathew Steinberg, optical networking director for Ryan Hankin Kent.
"The question is, how do you best use the capacity that's available?" Steinberg said. "Right now, you've got not much [bandwidth] supply and a lot of demand."
The DWDM systems that are in use today were expected to have infinite lives when they were developed, said Yan Sun, co-leader of the Lucent team.
"It's hard to predict the future," Sun said of the need for maximum-capacity DWDM systems. "Years ago, people thought a few fibers would be enough, and [when the first DWDM systems were introduced] people said eight channels would be used forever."
The possibilities for capacity on fiber are enormous because the newer systems can be applied even to older single-mode fiber. The Lucent experiment used Lucent's own TrueWave fiber-one of the newer types that combats dispersion and allows longer spans between optical amplifiers. The system will work on any fiber, although it may require more amplifiers and, therefore, cost more to implement, Srivistasva said.
Lucent has not set a commercial target date for the new system. However, there will be demand when such systems start to hit the market, even though carriers aren't likely to use the full capacity immediately, Steinberg predicted. Even now, with 16- and 32-channel systems available, most carriers are using no more than eight channels per fiber in their DWDM programs. The important factor with the larger systems is ease of migration from lower capacities.
"How do you get from here to there in the most seamless way possible?" he asked. "Do you have to do [the upgrade] at 2 a.m.? Do you have to take down traffic? How much [equipment] can you reuse?
"If you look at the systems out there, it's really a migration path," Steinberg said.
SETTING DSL STANDARDS? Only three months after it acquired Amati, Texas Instruments is taking a prominent position among ADSL vendors. TI, Alcatel and Analog Devices announced that they are working to develop interoperable silicon solutions for ADSL. The three companies hope that their cooperation will help advance the development of ADSL standards and that other companies will participate in their interoperability efforts.
CABLE & WIRELESS PICKS QWEST Qwest Communications snagged a large commercial customer when Cable & Wireless Inc. signed a five-year, $107 million contract to use Qwest's network to deliver its services, which target businesses exclusively. With the signing, Cable & Wireless also became the first participant in a new co-marketing program called QwestLinked.
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