CableLabs has a new plan to increase capacity in broadband access networks. Borrowing from technology used in long-haul optical transport, CableLabs is bringing techniques for improving optical efficiency to the part of the network that connects cable headends to fiber endpoints.

It's the second innovation project, after Full Duplex DOCSIS, that CableLabs has advanced to a research and development phase. Cable engineers believe it has the potential to increase bandwidth in the access network by as much as three orders of magnitude above current rates.

The technology CableLabs is working on is called coherent optics, and so far the organization has achieved speeds of 256 gigabits per second over a single wavelength while testing coherent optics in a lab environment. Engineers have also combined eight wavelengths using coherent optics, resulting in top speeds of more than 2 terabits per second in the lab. As the technology develops further, researchers predict that it could deliver up to a thousand times more capacity than the analog optics used in many cable networks today.

So how does it work? In a non-coherent optical system, information is transmitted by switching a laser source on or off. A coherent optical system, however, uses four dimensions to transmit information: amplitude and phase modulation, as well as separation across two polarizations. This significantly improves optical efficiency and subsequently overall bandwidth capacity.

The technology behind coherent optics isn't new, but CableLabs is adapting it for the access network. Transmission distances are much shorter in the access network than in long-haul links, and operators can push fiber deep enough so that no amplifiers are needed to boost signals. The combination of shorter distances and no amplification means less signal dispersion, non-linear distortion and general noise. That, in turn, makes implementation of coherent optics in the access network significantly less complicated, and should ultimately make it less expensive than applying the technology in long-haul transport connections as well.

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The good news with coherent optics is that it doesn't require any changes to fiber already deployed. The only change needed is at an optical endpoint, like an optical node or base station. The technology is also well suited for distributed access architectures (DAA) and node-plus-zero (N+0) deployments. The more capacity operators can deliver to that last node, the more options they have for extending high-bandwidth services to a diversity of targets leading off of that node. (See Cable's Upgrade Moment – Part IV and Comcast Goes N+0 in Gigabit Markets.)

CableLabs Distinguished Technologist Alberto Campos also emphasizes that coherent optics is not limited to network type. "This technology is not just for cable. It's for access [networks], any type of access," says Campos. Although the cable industry is where the initial work is beginning, it will also be shared with the broader telecom community.

CableLabs VP of Wired Technologies Dr. Curtis Knittle adds that coherent optics technology for the access network is being designed with software defined networking (SDN) and network functions virtualization (NFV) in mind.

"As operators start to deploy this, and as they're deploying SDN and NFV solutions, we intend to make it amenable to those new SDN/NFV architectures," says Knittle.

Now that CableLabs has moved coherent optics on to the research and development phase, the next step is to set up a working group within the organization; something that is likely to take place in May or June. Following the R&D phase, CableLabs expects to start developing formal specifications.

— Mari Silbey, Senior Editor, Cable/Video, Light Reading