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Full Duplex, Coherent Optics Specs Advance

Alan Breznick
8/24/2017

Even as cable operators steadily accelerate their rollouts of the new, multi-gigabit-enabling DOCSIS 3.1 spec, leading industry engineers are pressing forward with two next-gen technologies that promise to boost broadband speeds even further.

In an interview with Light Reading earlier this month, two senior technologists from CableLabs said the industry R&D group is wrapping up work on the forthcoming Full Duplex DOCSIS spec while at the same time ramping up activity on the proposed coherent optics spec. If all goes according to plan, the Full Duplex spec will be completed before the end of the year while the main parts of the coherent optics spec will be completed by next summer. As a result, both technologies could be available for equipment vendors to start product development in the next year, with Full Duplex going first.

Full Duplex in home stretch
Considered a next-gen version of the DOCSIS 3.1 spec, Full Duplex (FDX) will enable cable operators to leverage the same spectrum to carry both upstream and downstream traffic over the last mile of coaxial plant running to subscribers. Using echo cancellation technology, cable fiber nodes will send and receive traffic at the same time by filtering out transmission noise so the devices can simultaneously "hear" incoming signals while sending out their own messages. As a result, operators will be able to offer symmetrical 10-gigabit speeds without all the expense and time needed to replace every bit of coax cable in their networks with fiber. (See How Cable Plans Symmetrical Gigabit via FDX, Full Steam Ahead for Full Duplex DOCSIS and Full Duplex Is a Go; Cable Aims for 10 Gig.)

Approved for spec development last summer, FDX is heading into the final stages of crafting as this summer draws to a close, according to CableLabs officials. Emphasizing that the spec is "on track" and has reached "a mature level," they say they expect it to be publicly available before the end of the year. They also expect vendors to begin developing products for it this fall.

"We're very confident that what we've specced out will work really well in the field," said Belal Hamzeh, VP of wireless technologies at CableLabs. "The spec stands ready for vendors to start designing products."

In a key sign of vendor preparation for the coming FDX era, Cisco Systems Inc. (Nasdaq: CSCO) staged a proof-of-concept demo of the technology at the ANGACOM show in Cologne, Germany in late May. Although the display didn't show true FDX performance because today's cable modems can't support it, the demo did showcase a critical enabling component -- the FDX echo canceler. Previously, Nokia Corp. (NYSE: NOK) staged its own PoC demo of FDX's potential to deliver symmetrical 10-gig speeds over a coaxial connection. (See Cisco Demos Full Duplex and Nokia Demos 10-Gig Over HFC.)

For the DOCSIS 3.1 spec, CableLabs overhauled its product certification and qualification process to speed up testing and approval of products and make the process more flexible as well. While they declined to say exactly what they will do for FDX, organization officials indicated that they'll take similar steps this time around. "CableLabs will do whatever is needed to accelerate the process," Hamzeh said.

If the FDX spec continues to proceed as planned, CableLabs officials expect to see cable operators and vendors conduct the first field trials of FDX modems, nodes and other devices sometime next year. In turn, that could lead to the first commercials deployments of FDX products in late 2018 and early 2019.

Coherent optics lighting up
In a plan that surfaced early this year, CableLabs is adapting an existing technology for long-haul optical transport to boost optical efficiency significantly in the part of the cable access network that connects headends to fiber endpoints. That technology, known as coherent optics, works by using four dimensions, rather than just one, to transmit much more information over the fiber part of the cable HFC network. In fact, CableLabs engineers believe that coherent optics can increase bandwidth in the access network by as much as three orders of magnitude over today's rates.

With testing proceeding apace, engineers have achieved speeds of up to 256 Gbit/s on a single wavelength of fiber optics in a lab environment. And, by combining up to eight wavelengths, engineers have produced top speeds of more than 2 terabits per second in the lab. As a result, researchers predict that coherent optics could deliver up to 1,000 times more capacity than the analog optics used in many cable networks today. (See New CableLabs Optical Tech Promises Big Bandwidth Boost.)

Despite the obvious potential of coherent optics for cable, some senior MSO technologists have privately questioned the need for it. They have also expressed concern about the cost of aggregating many digital nodes in the access network to support the technology.


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Nevertheless, CableLabs executives are quite bullish about coherent optics' prospects. They argue that operators can push fiber deep enough in their networks so that no amplifiers are needed to boost signals, resulting in less signal dispersion, non-linear distortion and general noise, as well as less network complexity and lower operating costs. They also note that coherent optics doesn't require any changes to fiber already deployed, just to the optical endpoints. In addition, they point out that the technology is well suited for distributed access architectures (DAA) and node-plus-zero (N+0) deployments.

As a result, CableLabs officials have set up a working group of cable operators and vendors to hammer out the proposed coherent optics spec, just as they did with FDX and previous cable specs. Established in June, the working group is meeting monthly to craft the details of the spec, with frequent conference calls taking place in between the face-to-face meetings. The next meeting is slated for early September at CableLabs headquarters in Louisville, Colo.

"It has been very well received by the operator community and the vendor community," said Alberto Campos, a distinguished technologist at CableLabs. "All the results are extremely promising. It's very encouraging."

That's not to say that the new working group hasn't already encountered some technical challenges. Chief among them is how to keep the coherent optics gear in the access network from overheating under the broiling sun in many locales. Researchers are now exploring ways to keep the equipment from getting too hot.

"One unique thing about this access flavor is that one of the components will be in an outdoor location," Campos said. "So you have to make sure the temperature range will be suitable... You have to figure out how to cool the device."

CableLabs officials said their first goal in writing the coherent optics spec is to define how the point-to-point coherent link in the PHY layer will work. Viewing that as the most important part of the spec, they aim to complete that portion by the end of next March. They then plan to nail down the management portion of the spec by the end of June 2018.

Once the spec is completed, CableLabs execs expect to see vendors develop products over the following 12 to 18 months. In turn, that means operators could start deploying the technology commercially by the end of 2019 if all goes well.

— Alan Breznick, Cable/Video Practice Leader, Light Reading

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