CableLabs released the first pair of coherent passive optical network (CPON) specifications targeted at advancing the development of commercial networking systems that can support increased link budgets and higher speeds.

The newly released 1.0 specifications are for the physical media dependent (PMD) layer and transmission convergence (TC) layer.

The PMD layer specification “defines how CPON operates at the physical layer over the fiber. This ensures that compliant optical line terminals (OLTs) at operator hubs or central offices can interoperate seamlessly with optical network units (ONUs) in customer homes.”

The TC layer specification “defines how the PMD layer interacts with other network layers to enable the transport of data.” This specification is based on work from international technology standards group ITU, which should help with integration into current telecom network topologies.

“Together, these specifications provide the core definition for how a CPON system will operate,” CableLabs noted.

CableLabs’ CPON progress

The releases build on long-standing industry work toward defining CPON standards.

CableLabs unveiled one of the first architectural specifications for CPON back in 2023, showing how it could be applied to point-to-multipoint access networks. The group’s goal was to enable 100 Gb/s services over a single wavelength to multiple endpoints.

Matt Schmitt, a distinguished technologist at CableLabs, previously downplayed that speed target in an interview with SDxCentral.

"To me, what I think is exciting about [CPON], is less to do with the capacity," Schmitt said. “When you go to coherent, you’re increasing your link budget significantly, you can do much higher split ratios or serve much more distant customers ... you don’t have to put OLTs out in the field anymore.”

Such a shift would have some significant architectural implications. This includes allowing operators to collapse access and metro layers, remove active components from the field, and centralize operations, all work that could help lower capital and operating costs compared to more traditional PON technologies.

“Skipping over 50G might look kind of interesting, because you're getting that extended reach, the higher split ratios, and more capacity, but you're not necessarily paying double the cost for it,” Schmitt said. “Will it happen? I don't know. It's really going to come down to what [hardware developers] are able to do. Design wise, it's amazing how coherent technology has matured.

Schmitt explained that when CableLabs was doing the point-to-point specifications, "the assumption was you needed these big modules. Now we have companies putting out 100G coherent point-to-point on a QSFP28. And with that, the costs have come down dramatically.”

What is CPON?

CPON is lined up to be the next generation of passive optical networks, able to support both upstream and downstream bandwidth for demanding applications like 5G backhaul and high-speed internet.

Unlike traditional PON, which uses intensity modulation-direct detection (IM-DD) – where the intensity of a light beam is varied to represent data, and then directly detected by a receiver – CPON uses more advanced signal modulation and detection methods to transmit far larger amounts of data over much longer distances.

What makes CPON potentially disruptive is its ability to overcome the fundamental distance-versus-density trade-off in legacy PON systems. Traditional deployments forced operators to balance reach to serve far-flung customers or densify to increase capacity closer to the edge.

CPON has the potential to dramatically improve optical sensitivity and spectral efficiency, and to support high-capacity links to greatly expand that last-mile network, with less power, and without the need for amplification.