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With ROADMs in Place, Next Step Is Software

Craig Matsumoto

The pieces for a flexible optical network are in place, in the sense that the hardware exists and can be gotten commercially. What's missing is the software.

"We need automation of that really fancy hardware. It would be a shame to waste it," said Glenn Wellbrock, director of optical transport planning at Verizon Communications Inc. (NYSE: VZ). He was speaking at "Driving Innovative Optical Network Solutions," an OFC workshop sponsored by the Optical Network 2020 (ON2020) initiative.

The key to the flexible optical network is the reconfigurable add-drop multiplexer (ROADM). It's an optical switching element that's been around for years, but it's only recently gained the kind of flexibility you see in packet switches. The colloquial acronym for these abilities is CDC, standing for colorless, directionless and contentionless.

Using CDC ROADMs, operators could direct any optical wavelength to any port, and could do so via software commands rather than by having humans plug cables into patch panels.

That wouldn't sound special if we were talking about packet switches, but it's been difficult to achieve in the analog world of optical networking. Development of CDC ROADMs took about seven years, and now it feels like the hardware has reached a plateau, said Heavy Reading Analyst Sterling Perrin.

"The software programmability is where I think we move for the next couple of years," Perrin said.

Wellbrock stressed the same thing. ROADMs have made Verizon's optical network more efficient, but the tradeoff has been complexity. Having 150 ROADMs in one network domain isn't unusual, and "that's going to get worse," he said.

What's really needed on the software side is automation -- which, of course, is easier said than done.

The control plane will need to talk to multiple vendors' ROADM systems, for instance. It would be nice if all vendors used the same API, so that an operator could issue the same commands to every vendor's equipment. But, realizing that's not a realistic expectation, Wellbrock is hoping to encourage a world of common models and open APIs.

"It's got to be open enough that we can write code once and reuse it," he said.

Telemetry gathering is going to be an important ability too, said Steve Grubb, the global optical architect at Facebook . That's a theme the optical network shares with the packet network and even SDN: They want data on how the network is performing so that the network can automatically improve itself.

In addition to the CDC elements, there's one more dimension of ROADM flexibility: flexible channel spacing, also called the flexible grid. This would allow a signal to eat up more spectrum than is normally allowed, redefining the boundaries between wavelengths.

That's another feature that was developed in recent years, and that Facebook's Grubb finds intriguing. "We're going to have some bizarre shapes, maybe with a 6.25GHz grid," he said, referring to a high level of granularity for optical channels.

For Wellbrock, a more important advancement would be higher port counts in ROADMs. The number of output ports is up to 16, but Verizon is finding that's not enough.

— Craig Matsumoto, Editor-in-Chief, Light Reading

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