What's bubbling in the labs: Industry players scurry to raise capacity bar

The never-ending demand for bandwidth increases in the optical backbone continues to sizzle in laboratories as carriers and vendors alike strive for improvements.

With developments in dense wave division multiplexing (DWDM), more wavelengths are being squeezed onto the same single fiber and higher bit-rates are being achieved on those wavelengths to help meet the capacity demands.

While trying to satisfy those demands, Pirelli Cable and Systems and AT&T Labs believe they have reached a milestone by achieving a four-channel WDM transmission over 75 kilometers of single-mode fiber at a per-channel rate of 40 Gb/s, or160 Gb/s capacity, using fiber gratings. In addition, the two companies demonstrated 320 Gb/s bandwidth by transmitting 32 10-Gb/s channels over 375 kilometers with the broadband fiber gratings.

"For increasing capacity on already installed fiber, you are dealing with so-called conventional single-mode fiber, and with that you need to take care of dispersion somehow," said Robert Tkach, division manager of lightwave networks research at AT&T Labs. There are a couple of different ways to compensate for the dispersion, Tkach said.

One method is to use dispersion compensating fiber, which coils up some of the fiber at an amplifier site to compensate for the dispersion. The other method, which is used in the Pirelli and AT&T development, uses broadband fiber gratings to handle the dispersion.

In laboratory trials, the dispersion compensating fiber has reached a record capacity of 3 Tb/s, which is far greater than the AT&T and Pirelli development. So why pursue the broadband fiber gratings that have lower capacity rates? Tkach answered that AT&T is always trying to experiment with alternate technologies. "It's a lab rule to see to it that the best technology is being deployed into the systems that we ultimately buy," said Tkach. As a caveat, Tkach said that although AT&T Labs and Pirelli are testing the technology, there is no guarantee that it will be deployed. Instead, the technology ensures that customers will be able to purchase systems that have more capacity and are less expensive. The cost savings are due in part to the length of the broadband gratings.

"The broadband fiber gratings are much shorter - about a meter long - but work in a different way," said Tkach. The fiber gratings compensate for the dispersion by reflecting different colors of light at a different point in the fiber grating, he said. By using the gratings, the delay across the optical signal can be equalized, enabling it to be accomplished over a shorter distance, thus driving costs down.

Tkach pointed to those lower costs as a reason for AT&T's involvement with the laboratory trials. "We are looking at this particular technology because we think it offers advantages in terms of costs and performance going forward," he said.

Nortel Networks' Greg Mumford doesn't share Tkach's opinion regarding the economics of fiber grating. "Dispersion compensating fiber is more economical, in our opinion," said Mumford, who is general manager of optical and Sonet networks at Nortel. According to Mumford, Nortel has achieved 1.6 Tb/s on a fiber, which is used in the OPTera 1600G optical networking system, and supports 160 wavelengths at 10 Gb/s each. "We have also shown 40 Gb/s in the lab, which is an 8-channel system, equaling 320 Gb/s capacity," he said.

But Nortel is not alone with the 10 Gb/s developments with dispersion compensating fiber.

"Alcatel has done trials over 100 kilometers with Deutsche Telekom, and Lucent [Technologies] is doing similar work with MCI [WorldCom] in the labs," said Dana Cooperson, a senior analyst with RHK. All the carriers are interested in pushing the envelope to get the most bits across their networks as fast as they can, said Cooperson. "The router vendors are pushing for this on their end, too. They need to find an efficient way to get the traffic coming out of the data device across the network," added Cooperson.

But despite all the desires for higher capacities, Cooperson pointed to management issues as a snare in getting higher capacity technologies deployed. Because the transmissions don't have overhead that allows for protection and performance monitoring, the management of the data is very critical. Traditionally carriers have been accustomed to managing everything at DS-0 and DS-1 levels, and they are just now adjusting to OC-48, and if the bar is raised to 40 Gb/s more management questions will sprout, Cooperson said.