Nokia & Facebook Push Undersea Fiber to 32 Tbit/s

Craig Matsumoto
News Analysis
Craig Matsumoto, Editor-in-Chief, Light Reading
3/21/2017



LOS ANGELES -- OFC 2017 -- Nokia and Facebook have announced a test where they've jammed 32 Tbit/s down one undersea cable, more than double the bandwidth normally achieved today.

The technology, involving a Bell Labs invention called probabilistic constellation shaping (PCS), wouldn't be commercialized for at least three years, estimates Kyle Hollasch, director of product marketing for optical at Nokia Corp. (NYSE: NOK)

Rather, the announcement is a classic OFC post-deadline paper -- a late research result submitted in hopes of getting a Thursday evening presenting slot. Post-deadline papers often feature what are called hero experiments -- eye-popping numbers related to the next generation of speed, size, power or bandwidth.

Nokia and Facebook 's result "shows these cables have a lot of running room," Hollasch says.

Web-scale operators such as Facebook, Google, and Amazon Web Services (AWS) are running their own undersea networks. They'd been participating in consortia -- pooling resources with other companies to invest in undersea fiber -- but are now taking sole ownership of some fiber spans or even laying their own cables under the sea.

You might think they'd be better off leasing capacity, but that model doesn't work for them, said Vijay Visirikala, a network architect at Google, during yesterday's OIDA Executive Forum. Capacity isn't always available in places where Google needs it. And the pricing of leased capacity doesn't fit Google's traffic patterns, which involve unpredictable bursts of demand.

The Nokia experiment used a 5,500km fiber pair that Facebook had purchased. According to Hollasch, the project started when Stephen Grubb, Facebook's global optical network architect, was impressed by Nokia's 1Tbit/s terrestrial experiment at ECOC last fall. (See Nokia, DT Break Terabit Barrier.)

"He said, 'I have a 5,500km cable under the ocean -- how'd you like to test on that?" Hollasch says. "You can imagine how a bunch of engineers reacted to that. This was Christmas and their birthday all at once."

Typical undersea fiber capacity using 100Gbit/s wavelengths is 13 Tbit/s, Hollasch says. The Nokia team pushed that to 17 Tbit/s using the Photonic Service Engine 2 (PSE-2) chip (the star of the ECOC paper) and got up to 32 Tbit/s by applying PCS.

PCS was invented in the early 2000s but was shelved after the dotcom bust. It's a modulation method that tunes the optical signal based on the length of the fiber. Hollasch likens it to a bicycle drive train, where different gears allow better performance under different conditions -- only PCS provides a theoretically infinite number of gears.

"It lets you eke out every bit of performance," he says.

For those who want to geek out on details: PCS uses 64-QAM modulation -- that is, a signal using 64 permutations of phase and amplitude. (This is what the PSE-2 chip does.) Normal QAM assigns an equal probability to all 64 points; PCS favors the points with lower amplitude. If you graph what's happening in 3D, you'd see a small hill -- a 3-D Gaussian curve, essentially. Nokia is showing off that graph at OFC, and Hollasch assures us it looks really cool.

Craig Matsumoto, Editor-in-Chief, Light Reading

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