Cable's pursuit of interoperable 200-Gig coherent optics presses ahead

Cable's potential use of 200-Gig coherent optical signals on the access network took a step forward following a recent interop at CableLabs that involved a handful of tech suppliers.

CableLabs confirmed that equipment and silicon from those players – Acacia (now part of Cisco), Ciena, Fujitsu Optical Components, Lumentum and Marvell – were found to interoperate with the organization's Point-to-Point Coherent Optics (P2PCO) 2.0 specs.

Those 2.0 specs doubled the operating capacity – from 100 Gbit/s per wavelength in the 1.0 specs, to 200 Gbit/s.

Figure 1: (Source: the lightwriter/Alamy Stock Photo)

The CableLabs interop focused on 200-Gig gear follows the 100-Gig interops held in 2018 and 2019. Those efforts tie into a broader initiative to use coherent optics technologies, typically used for long-haul, metro and submarine networks, to expand the capacity of fiber that's already deployed on the hybrid fiber/coax (HFC) access network.

The CableLabs specs also describe a new gizmo called the Coherent Termination Device (CTD), which can be deployed in an outdoor aggregation node.

New CTD device is the outdoorsy type

Matt Schmitt, a principal architect at CableLabs, said the scope of CableLabs' interoperability efforts focus on the modules on the optical end – basically describing how a transceiver works at the physical layer.

And to help fit the cable network environment, the end of the network using the CTD is made to reside outdoors, rather than inside a facility.

"Almost every other application of coherent that you see, both ends of the link are in facilities," such as a data center interconnect where many links are densely packed with racks and modules, Schmitt explained. The cable access application of coherent optics might involve one end that does sit at a facility, such as a hub site, with the other end involving the aforementioned field-deployed CTD.

"Those field boxes didn't really exist when we started this," he said.

The broader concept is to help cable operators improve the performance of their access network fibers situated between headends and hubs and fiber nodes for a range of use cases, and to do so without getting locked into one supplier.

CableLabs and its partners originally thought this 200-Gig interop would be completed sooner, but it was delayed a bit during the pandemic when travelling and in-person gatherings were limited or non-existent.

But Schmitt said the plus side of that intervening period meant that the interop ended up with wider supplier participation, particularly at the DSP (digital signal processor) level, than it might have otherwise.

Driving the need for speed

Beyond raw capacity, the 200-Gig capability should help to support the new distributed access architecture (DAA), supported by multiple remote PHY or remote MACPHY devices, and the cable's industry's broader pursuit of delivering symmetrical 10Gbit/s performance to customers on the access network.

Schmitt said 200-Gig technology gets particularly interesting when operators look to support large, high-density areas that are being split into smaller service groups. It might also factor in as operators explore services beyond high-speed data over cable, such as mobile XHaul.

The use of CTDs with pluggable optics is also designed to support a relatively easy upgrade path. If an operator starts with 100-Gig, those modules can be swapped out for 200-Gig modules later.

This point-to-point P2P use case is just one aspect of coherent optics being explored by CableLabs. A separate-but-related coherent PON initiative still uses coherent signaling, but is focused on point-to-multipoint links.

For now, Schmitt said CableLabs doesn't plan to hold another interop for P2PCO v2.0 products. "It really just worked so well. I'm not sure what more there will be to do in a follow-up interop," he said.

CableLabs would be open to doing qualification testing for these new P2P coherent products if the market demands it. "Thus far, I haven't been hearing of a big push for that," Schmitt said. "I think people have been comfortable with what we're getting from the interops and doing their own testing to see how it works."

As for next steps, this latest batch of handiwork will be showcased at the 10G Lab at CableLabs, Schmitt said.

Meanwhile, future commercial deployments will be determined by the availability of CPDs and interest form cable operators.

Among suppliers involved in the recent interop, Ciena confirmed that it currently has interoperable, CableLabs-compliant 200G coherent pluggables available as part of the supplier's WaveLogic 5 Nano coherent pluggable portfolio.

Figure 2: Ciena says it already has 200-Gig coherent pluggables that comply with the CableLabs specs.
(Source: Ciena)

Update: Acacia tells Light Reading that the company has coherent pluggable modules available today that support the CableLabs 200G Point-to-Point Coherent Optics specs for use in CTDs.

Solving the cost conundrum

Another factor for adoption will be costs compared to the 10-Gig DWDM tech that's in use today. Schmitt acknowledges that the first endpoint is going to be greater with coherent technology, since it involves putting a switch or router in the field.

"Where it gets interesting is every time you need to add another device that's sharing that same fiber run," Schmitt said.

"With coherent, you have a higher upfront cost, but you're going to have a much lower slope, because as you add more devices, all you have to do is add a pair of gray (standard) optics modules – very low cost … Where's that crossover point in terms of number of endpoints? To me, is going to be one of the big deciders on when and how widespread the deployment of this technology gets."

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— Jeff Baumgartner, Senior Editor, Light Reading