Nortel takes novel approach to 40G, 100G

Using RF encoding techniques, Nortel promises smooth 40G, 100G migration

Nortel Networks introduced new equipment today allowing carriers to migrate from 10 Gb/s links to 40 Gb/s and eventually 100 Gb/s. A key aspect of the new gear is its ability to enable these migrations while maintaining the characteristics of the existing network—something Nortel achieved through a novel approach that applies wireless technology to optical networks.

New cards for the OME 6500, available at the end of April, offer 40 Gb/s interfaces that are designed to match key characteristics of the existing network, such as 50 GHz channel spacing and use of the C band, in order to ease the migration. The gear offers a similarly non-disruptive path to 100 Gb/s in the future, Nortel said, predicting trials of a 100G version this year and commercial availability in late 2009.

Retaining those characteristics throughout a 100 Gb/s migration was one of the priorities Verizon Business voiced when it trialed 100 Gb/s gear in Florida last year, and one purpose of the trial was to show vendors what it wanted, the company has said.

“It’s a seamless transition from 10G to 40 gig to 100G without the need for ripping up your infrastructure, deploying new fiber, changing network designs, spacing for regenerators or reducing the number of flexibility points you have in the network,” saidScott McFeely, vice president of product line management for Nortel’s metro Ethernet products.

Upping network speeds from 10G to 40G typically entails trade-offs such as shortening the reach of each network segment or increasing chromatic dispersion or polarization mode dispersion (PMD)—the “spreading out” and weakening of optical signals as they travel. To remedy this, carriers would normally need to add amplifiers or regenerators, settle for shorter reaches or add dispersion compensation equipment and, in some cases, better quality fiber.

That’s not necessary, Nortel said, because it has integrated dispersion compensators into the ASIC chips of its new gear. And new fiber isn’t needed because the OME performs better at 40G than 10G, with better tolerance for dispersion.

The 40G performance increase came in large part from Nortel’s use of an advanced signal modulation technology gaining attention across the industry called dual-polarization quadrature phase shift keying (DPQPSK).

Though DPQPSK is widely known as just one technique proposed for 40G, Nortel applied its own unique approach to DPQPSK, leveraging technology from its wireless products. Nortel’s internally developed “coherent receiver,” borrowed from RF designs, helps decode the signals on the receiving end, using algorithms to untangle the complexity of DPQPSK.

“Nortel's coherent receiver technology decodes the signals at 4 bits per baud on 10G as opposed to competing modulation schemes that pack 1 bit per baud onto 40G or 2 bits per baud onto 20G,” said Michael Howard, principal analyst and cofounder of Infonetics Research. “It allows the bit rate to stay at 10G while pumping 40G worth of bits down a single wavelength. This reduces a number of the usual impairments.”

As McFeely puts it, “This allows us get 40 gig worth of signal down a fiber and still have it look like a 10 gig signal.”

In a research paper at last month’s OFC NFOEC trade show, AT&T reported a pair of successful tests of Nortel’s approach.

“The Nortel announcement is very interesting and will catch a lot of carrier attention,” Howard said.