Siemens Claims 160-Gbit/s Milestone
Siemens, together with BTexact Technologies, and the Technical University of Eindhoven, has recently demonstrated 160-Gbit/s switching on a real network in the U.K. (see Siemens, BT Demo Fast Optical Switch). This is the culmination of several years of work under the auspices of a European Commission-funded project codenamed FASHION (ultra FAst Switching in HIgh-speed OTDM Networks).
"We have made the big step from a pure laboratory technique to equipment that is capable of overcoming the various difficulties under real conditions," says Dr. Gottfried Lehmann of Siemens Corporate Technology. He reckons it should be possible to develop a real system within two or three years.
If such equipment was developed commercially, then it has the potential to provide major cost savings thanks to the reduction in the amount of DWDM equipment required: 160-Gbit/s technology would allow sixteen DWDM channels at 10 Gbit/s to be replaced by a single wavelength.
Of course, this is a rather rosy view of the situation. In practice, more research is needed on other aspects of 160-Gbit/s systems before commercial development begins, according to Lehmann. And, if no new technological gremlins pop up, there are still other issues to contend with, such as whether the technology will be needed in the foreseeable future. Forecasts of huge increases in bandwidth requirements have evaporated in recent years, and 10-Gbit/s equipment has come down in price so much that a lot of 40-Gbit/s development projects have been mothballed.
It's also worth pointing out that Siemens isn't alone in developing 160-Gbit/s transmission systems: Japan's Mitsubishi Electric Corp., Germany's Heinrich Hertz Institute (HHI), and America's own Bell Labs have also done work in this area (see Mitsubishi Looks to 160-Gbit/s Future). Bell Labs' demonstration was over four years ago (see the original press release).
But let's put skepticism aside for a moment and look at the nuts and bolts of the 160-Gbit/s system. For a start, 160 Gbit/s isn't directly comparable to 40 Gbit/s. Unlike OC768 (40-Gbit/s Sonet), which provides a clear channel for communication, 160-Gbit/s transmission relies on a multiplexing technique called OTDM (optical time division multiplexing). Individual 10-Gbit/s channels are squashed onto a single wavelength by interleaving them. The result is a data stream where every sixteenth bit belongs to the same channel.
This presents quite a challenge at switching and distribution points, says Lehmann. "Of the 160 billion bits which now hit the distribution point every second, each sixteenth belongs to the data flow which is due to be deflected," he notes. This means one individual bit must be pulled out of the data stream every 100 picoseconds (or 100 million millionths of a second).
The only way to do this is with all-optical switching -- it's too fast for anything mechanical or electronic. Siemens has developed a switching device based on a process called four-wave mixing (FWM) in Semiconductor Optical Amplifiers (SOAs) .
FWM is a non-linear effect that results in two wavelengths interacting to create a new wavelength. A control laser, which is pulsed at the right interval to pick out every sixteenth bit, is entered into the SOA, along with the data stream. When both data stream and control pulse are present, that particular data bit is reproduced on a new wavelength, which can be filtered out. Other data bits pass through the SOA without being affected. Full details of this device can be viewed in this research paper presented at ECOC 2003.
BTexact tested the technology over four 70km fiber links between the U.K. towns of Ipswitch and Newmarket. The all-optical add-drop mux was installed at the halfway point, which switched out individual 10-Gbit/s channels from the 160-Gbit/s data stream.
— Pauline Rigby, Senior Editor, Light Reading