CABLE NEXT-GEN EUROPE DIGITAL SYMPOSIUM – CableLabs has recently published its first set of specifications for a new device that enables operators to take advantage of coherent optics technologies, typically used for long-haul, metro and submarine networks, in their fiber-limited access networks.

The CableLabs specs for that gizmo, called the Coherent Termination Device (CTD), were published June 9 (PDF). A primary aim of the CTD is to provide cable operators with a fresh strategy to beef up capacity on the optical access network as data demands increase, but do so using the limited number of fibers situated between their headends and hubs and the fiber nodes used to deliver data and video services to customers.

CableLabs reckons in the specs that cable operators often have one or two fiber strands available to service groups of fiber nodes. But rather than adding more fiber, a costly and time-consuming proposition, the addition of the CTD would enable operators to use their existing fiber assets more efficiently when coherent optics technologies are teamed with wavelength-division multiplexing (WDM) in the optical access network.

Figure 1: Click here for a larger version of this image.
(Source: CableLabs)

The new specs, part of the Point-to-Point Coherent Optics suite of specs developed by CableLabs, define a CTD that resides in an aggregation node (AN) in the field that combines multiple fiber endpoints onto a single fiber or fiber pair. The CTD itself includes one or more coherent transceivers to terminate coherent optics links.

The specs also identify several potential use cases: for use in DOCSIS DAA deployments, PON deployments, business Ethernet using P2P Ethernet links, mobile backhaul for LTE and 5G deployments, and mobile fronthaul for 5G deployments. The specs also spell out several network architectures, though a direct link architecture, whereby each CTD connects directly to a hub or headend using P2P coherent optics, is anticipated to be the architecture of choice for most cable operators.

The general approach effectively lets cable operators squeeze more life out of the fiber they already have in the ground, Curtis Knittle, VP, wired technologies, CableLabs, explained here Tuesday on a panel focused on cable's activities around fiber and PON technologies.

He said the CTD turns a current fiber node in an aggregation point that terminates 100 Gbit/s coherent links and breaks them into 10 Gbit/s Ethernet links to remote OLTs (optical line terminals). He said the CTD more or less completes that coherent link in a distributed access architecture in a way that suits the outside plant of a hybrid fiber/coax (HFC) network.

Tied in, CableLabs, Knittle said, has just launched a 100-Gig coherent PON working group. Illustrating a new way of operating at the organization, the working group will welcome non-CableLabs members to participate in an attached advisory group, Knittle said. That approach, a big change in the more intra-industry way CableLabs has historically gone about specifications development, is also evident in CableLabs' new initiatives focused on fixed mobile convergence.

Sizing up cable's PON past, present and future

All of that work is emerging as a potential option for brownfield and greenfield scenarios. It's also coming about as the deployment and use of fiber and PON technologies become increasingly prevalent for cable operators, particularly in greenfields.

Vodafone Spain is keeping constant tabs on future traffic trends as it weighs network decisions, but also keeps a close eye on capital requirements as it looks to roll out fiber and extend fiber deeper into its DOCSIS network, Angel Campos, group engineer, network manager, NGA evolution, at the operator, noted during the panel.

However, Campos did allow later that the operational costs of a passive optical fiber network will be significantly lower than those for an HFC network.

Telenet is also using multiple network options, according to Bart Acke, VP of network build and field services at Telenet. However, it is starting to take a closer look at 10G-capable XGS-PON technology as a future-proofing move. That, he added, is also viewed as a technology that will be easier to implement than RF-over-Glass (RFoG), an SCTE standard that enables cable operators to deploy fiber-to-the-premises but continue to utilize legacy OSS and provisioning systems, along with RF-based customer premises equipment, such as set-tops and DOCSIS modems and gateways.

Most cable operators, particularly CableLabs members around the globe, "are thinking about 10-Gig PON" as they move on from first-generation PON and start to augment those already-deployed systems, Knittle agreed.

Deploying PON in greenfields "has been a relatively easy decision for them. The more challenging one is brownfield deployments … It's opportunistic in the brownfield deployments," he added.

Although the CableLabs DOCSIS Provisioning of EPON (DPoE) specs focus on EPON networks, both 10G-EPON and XGS-PON are being deployed by CableLabs members, Knittle noted.

In China, big operators have already begun to transfer to 10-Gig PON technology, Michael Song, GM of fixed and multimedia product, and overseas market VP, at ZTE, said. "It has already changed our lives in China," he said. "I think 10-Gig PON is ready for the transformation of cable networks."

But deploying PON in support of all services presents special challenges for cable operators, particularly around video. RFoG does allow support for legacy QAM-based video tech and equipment, but to take full advantage of PON as a unified, all-service platform, they'll have to shift to IP-based video. That requirement can prolong any thoughts about transitioning from RFoG to 10-Gig PON technology.

And the issue also extends to the underlying IT systems. Migrating or bridging those IT pieces from the HFC network to PON will take time. Vodafone Spain's Campos said deploying a PON network might take nine to 12 months, but getting the full support of the provisioning could take 18 to 24 months.

"That's something you need to think of in advance to get ready before you sell your first PON customer," he said.

The good news is that cable's pursuit of PON no longer appears to be centered on picking EPON over GPON, or vice versa. While EPON worked well for cable business services delivered via Ethernet, that importance is waning, as the whole point becomes getting fiber to the customer, Lewis Hansen, systems engineer at Precision Optical, said.

Knittle agreed, noting that what matters is that the operator has Ethernet going in and Ethernet going out, and that the commonality between the optics and silicon for 10G-EPON and XGS-PON is making the point moot.

It's also not clear yet when or if cable operators will need 25G or 50G PON technology, at least for residential services.

"I think PON, going forward, is not about speed. It's about other characteristics of the PON, like distance and split ratio, as well as speed and latency," Knittle said.

And Knittle tossed in another nod to the activity percolating around coherent PON technology. "I think it's the technology that will carry the industry for the next 20 to 40 years, instead of just the next five to ten years," he predicted.

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— Jeff Baumgartner, Senior Editor, Light Reading