CableLabs has taken the first step in making its vision of coherent PON (CPON) a reality. The organization has released “Coherent Passive Optical Networks 100 Gbps Single-Wavelength PON – Coherent PON Architecture Specification” (CPON-SP-ARCH-I01-230503), which describes the architecture, use cases, and operational needs that will guide technology development of CPON point-to-multipoint optical transceivers.
Coexistence with legacy PON technology as well as point-to-point coherent transmissions is a key requirement for CPON within the specification, according to Curtis Knittle, vice president, wired technologies research and development at CableLabs. The document references a Coexistence Element that will enable CPON transmissions to share the same fiber as current PON networks, but not how the element would work. That said, Knittle adds that CableLabs has performed coexistence testing of a coherent PON transmission with such traffic types. Among the keys to success, he says, will be avoiding such hazards as crosstalk.
The new specification foresees a tradeoff between split ratio and reach. Scenarios described in the document include a 512:1 split ratio at 20 km, 1:64 at 50 km, and 1:16 at 80 km. This last distance should cover at least 95% of subscribers using current optical distribution networks (ODNs) as well as handle most rural deployment requirements, the document states. The specification adds a link budget of 35 dB should support such requirements “and should be achievable at reasonable cost.”
Use cases spelled out in the specification include:
- Residential fiber to the unit in multiple dwelling units
- Residential fiber to the home for single-family units
- The supply of aggregation connectivity for remote optical line terminals (OLTs) and for Remote PHY devices (RPDs) and Remote MAC-PHY devices (RMDs) in distributed Converged Cable Access Platform (CCAP) architectures
- Mobile xHaul
- Mixed use cases that may combine some or all of the above
- A variety of more specialized use cases, including Network as a Platform, WiFi backhaul, edge computing, fixed wireless backhaul, data center interconnect, passive optical LAN, and IoT-based smart city applications.
Knittle speculates that the aggregation connectivity application will see the most initial traction.
Other elements covered within the specification include various ODN scenarios, fiber characteristics, a 100-Gbps symmetrical line rate, support of QoS and network slicing, power consumption, security, and protection and resiliency. The document does not address cost targets. Knittle acknowledges that CPON technology, particularly at the subscriber end, will prove more expensive than current PON units. He says that operators would be best served by focusing on the total cost of ownership (TCO) benefits that CPON will enable, particularly in its ability to enable a passive network over as much as 80 km.
Next steps
With the initial version of the architecture specification released, the focus now turns to establishing technical specifications. Knittle envisions at least two such releases, one each for the physical and MAC layers. A third that would cover Operations Support Systems Interface (OSSI) requirements is possible as well, he says.
In an interview late last year, Knittle said that such work wasn’t as far along as he initially hoped (see “CableLabs moving toward decisions on Coherent PON”). He now says that additional vendor contributors have come aboard since then but that these technical specifications are still a year or two away from being completed. He adds the goal is to see deployments of CPON over the next four to five years.
