SAN DIEGO -- OFC/NFOEC 2009 -- Just when you thought you knew all the modulation schemes out there, ADVA Optical Networking comes up with DPSK-3ASK.

It's a proprietary scheme, being announced today, for 100-Gbit/s optical transport in metro spans.

Why go through the trouble, and why use so many letters to do it?

Well, the second part has to do with whoever named "three-amplitude shift keying" (3ASK). As for the first part, ADVA says it's found a way to create a metro link using 40-Gbit/s components -- meaning it can be done now.

The scheme is proprietary, but that's OK for line-side connections (the links that traverse long distances between nodes in a network). Because a carrier usually owns both ends of such connections, a line-side link can be proprietary without causing any interoperability fuss.

"If you look at 40 Gbit/s, it's never really embaced standards on the transport side," says Jim Theodoras, ADVA director of technical marketing.

ADVA has talked about this idea for a while and presented some simulation results last fall. This week, the company plans to announce it's gotten the concept working in the lab, producing results that match the simulation.

DPSK-3ASK is a spin on the dual phase-shift keying (DPSK) that's more common in optical modulation. It sends a 40-Gbit/s optical signal but transmits five bits of data for every two symbol transmissions -- which multiplies out to a 100-Gbit/s data rate.

By using this format, a carrier could send the data using off-the-shelf 40-Gbit/s equipment and components, rather than having to wait for every missing piece needed to make pure 100-Gbit/s transport work.

"We're not having to wait for coherent detectors. We're not having to wait for new DSPs [digital signal processors] or a polarization tracker," Theodoras says.

Other examples of new parts needed at 100 Gbit/s include a Serializer/Deserializer (SerDes), something Sierra Monolithics Inc. and CoreOptics Inc. are working on separately.

A new type of analog-to-digital coverter (ADC) chip, capable of running 56 billion samples per second, could also be a requirement; Fujitsu Microelectronics Europe (FME) is the only company so far claiming such a device. (See Sierra Strikes Forth for 100G and Fujitsu Micro Tackles 100G.)

ADVA is targeting the metro because it sees a no man's land there between standards.

Short-reach connections -- 10 km and under -- are being covered by the Institute of Electrical and Electronics Engineers Inc. (IEEE) 802.3ba standard for 40 and 100 Gbit/s Ethernet, which is expected to be ratified in mid-2010. The Optical Internetworking Forum (OIF) , meanwhile, is looking at standards for long links, the kind reaching up to 2,000 km.

"The need is real. I think what's been holding it up is the massive investment required to go 2,000 km," which has pushed much of the standards attention to those longer spans, Theodoras says. Because that represents a "limited number of links," ADVA feels some more immediate work should be done to bring 100-Gbit/s transport to the metro network.

— Craig Matsumoto, West Coast Editor, Light Reading