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AT&T Researchers Set a Long-Haul Data Record

New optical technology paves the way for more efficient ocean-spanning transmissions.

Broadband data demands are surging at 30 to 40 percent a year.

Researchers at AT&T have devised a way to increase the distance that large amounts of data can travel through a fiber-optic connection. The technique should allow 400-gigabit-per-second signals to travel for a distance of 12,000 kilometers—four times the previous distance possible—and it promises faster ocean-crossing transmission without adding more equipment. The feat is like sending 170 HD movies 12,000 kilometers—half-again as far as the distance from San Francisco to Tokyo.

The advance, which is described in terms of the amount of data one wavelength can carry, comes a few months after Japan’s NTT and research partners hit another fiber milestone, demonstrating the transmission of vastly more data—a petabit (1,000 terabits) of data per second—for about 50 kilometers.

The AT&T work uses a novel approach to modulating light and new algorithms to speed up the processing of data carried in that light signal. The NTT work was more radical. It involved a change in the way individual fiber strands are arranged in optical backbones, greatly reducing signal loss, and also exploited additional properties of light—phase and polarization—to carry more data. The NTT work is described in detail here.

The advances are applicable to routers that encode light signals sent over fiber backbones that must efficiently handle traffic for millions of customers over long distances, as opposed to fiber-to-the-home lines that go a short distance within a city or city block (see “When Will the Rest of Us Get Google Fiber?”). In some parts of the world, fiber backbones still have plenty of capacity, and many have fiber-optic cables that are installed but inactive (“dark fiber” in industry parlance). Still, these advances are needed so that the anticipated surge in data use—projected at 30 to 40 percent a year—can be handled cheaply and efficiently.

“If you don’t want to light up more fiber, which is expensive, this allows you to get much more throughput,” says Muriel Medard, a professor at MIT’s Research Laboratory of Electronics (see “A Bandwidth Breakthrough”). “This allows you to extend the life of the stuff that’s already there.”

And there’s not a glut everywhere. In recent years, the surge in data use has led to the recognition that global choke points exist–and to a boom in new trans-ocean cable connections (see “New Oceans of Data”).

The speed of 400 Gbps per wavelength transmits four times more data than is conventionally sent today over continent- or ocean-spanning distances. It is expected to be the next standard capacity for optical transmission.

Other groups have achieved 400 Gbps on a single wavelength—but would have required adding more equipment to correct or amplify the signals over such long distances. The new work was led by Xiang Zhou at AT&T’s labs.

“This is the world record for the product of spectral efficiency [the amount of data carried on a wavelength] times distance, and no one else has used this technique,” says Peter Magill, a researcher at the lab in Middletown, N.J.

While growth in wired broadband is happening fast, growth in the wireless kind is happening even faster: it is estimated to surge by a factor of 18 by 2016, according to Cisco, and Bell Labs predicts it will increase by a factor of 25 in that time. The recent boosts in long-haul fiber performance are being paced by comparable ones in wireless bandwidth. Earlier this year, the Chinese handset maker ZTE and carrier China Mobile said they had achieved a peak download speed of 223 megabits per second using a new wireless standard known as LTE-Advanced (see “LTE-Advanced Is Poised to Turbocharge Smartphone Data”).

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