CPON: The future of access network service technology

Demand for higher data rates and the rapid growth of Passive Optical Networks (PON) have set the stage for the future of access network technology—100 Gbps Coherent Passive Optical Networks or CPON. Ciena’s Jody Guthrie discusses why adopting coherent technology for the next generation of PON systems is the path forward.

Over the years, PON technologies have evolved significantly, introducing higher speeds and better capabilities to keep up with today’s market demand. Such standards as GPON, EPON, NG-PON2, 10GE PON, XG(S)-PON, 25GS-PON, and HSP PON reflect this progression, with each iteration offering improved data rates. Despite this evolution, the challenge of service speeds often falling below advertised rates has remained. Enter 100 Gb/s coherent passive optical networks, otherwise known as CPON, which aims to address this directly.

But before we dive into CPON, it’s crucial to understand PON itself and how PON technology delivers broadband services through a passive optical distribution system.

A typical setup includes an optical line terminal (OLT) connected to optical fibers and splitters which then distribute fiber connectivity to optical network terminals or units (ONTs/ONUs) in end-users’ homes. While an OLT can support multiple subscribers, actual service rates often vary from the advertised speeds due to factors like framing and forward error correction (FEC) overhead.

As adoption of PON grows in the hospitality sector, campus environments, data centers, and other enterprise verticals, so does the demand for higher rates. CPON represents the future of access network service technology, offering higher data rates, cost-effective solutions, and the ability to overlay onto existing infrastructure.

The promise of CPON

CPON leverages the cost-efficient reuse of coherent modulation technology in next-generation PON systems, delivering significant enhancements for PON networks. These include affordable ONUs capable of supporting speeds of 100+ Gb/s, seamless overlay features that maintain legacy PON services, true Ethernet-speed performance, enterprise-grade Ethernet capabilities, and the capacity to uphold dependable service agreements.

A key factor in CPON’s appeal is the narrow cost gap between 50G speeds and a single-carrier coherent 100G CPON. With the signal coding gain of coherent modulation, these systems require only a DSP, avoiding the need for a silicon optical amplifier (SOA) while still meeting the optical link budget for brownfield optical distribution networks (ODNs). This efficient design results in a smaller cost difference between 50G HSP intensity modulation with direct detection (IM-DD) and 100G coherent PON than the gap between 25GS-PON and 50G HSP. For operators, moving directly from 25GS-PON to 100G CPON, with true 100G Ethernet performance, this becomes a highly attractive option.

100G CPON not only boosts network capacity but also extends the reach of PON to an impressive 80 kilometers. Current PON technologies often fall short, struggling to exceed 25 kilometers, limiting their use to urban and suburban areas. Expanding these networks to suburban outskirts or rural regions typically demands extra infrastructure, like active optical links or fiber huts for backhaul; however, 100G CPON solves this issue by enabling a fully passive optical link across 80 kilometers. This breakthrough provides an affordable and efficient way to deliver fiber to the home (FTTH) in underserved and unserved communities.

Now that we’ve outlined the benefits of moving to coherent technology for next-generation PON applications, let's explore two vital cases where it can make a significant real-world impact.

Affordable 100 Gb/s CPON entry

What makes CPON particularly competitive is its ability to provide true Ethernet speeds, aligning with customer expectations while effectively rivaling point-to-point fiber optics.

Our first use case focuses on affordability through the development of cost-efficient ONUs for 100 Gb/s single-carrier coherent networks. With the global adoption of single-carrier coherent point-to-point technology, optical component costs are expected to drop, making CPON even more accessible. Additionally, CPON systems can be layered over brownfield ODNs, which maintains revenue streams from existing legacy PON services.

Figure 1: Relative ONU cost estimates for moving from 10G to 100G CPON

Low-cost coherent TWDM-PON overlay for 4x100G CPON

In our next scenario, operators can deploy up to four adjacent 100G CPONs within the same ODN using single-carrier coherent modulation. This approach leverages existing NG-PON2 and HSP TWDM-PON architectures, ensuring high guaranteed throughput while accommodating many ONUs. Each channel pair operates as a discrete TDM/TDMA PON domain, managed by the OLT's dynamic bandwidth assignment (DBA). The design achieves cost efficiency by utilizing single-channel-pair transceivers for each ONU.

Figure 2: CPON deployment leveraging and NG-PON2 / HSP TWDM-PON architecture

OLT ports with higher split ratios enabled by CPON allow for more efficient bandwidth allocation, leading to significant savings in space, power, and cooling at the hub. Traditional PON technologies are typically deployed either at a central office or hub, or remotely in a node or cabinet, with each OLT port supporting split ratios of 32:1 or 64:1. This means that an OLT port divides its network capacity (e.g., 10 Gb/s) among 32 or 64 end users; however, CPON takes this efficiency to the next level, offering split ratios of up to 512:1. It delivers 100 Gb/s in capacity, capable of supporting up to 512 end users on a single OLT port, all while maintaining a standard ODN reach of 20 kilometers.

Overcoming challenges in CPON deployment

Deploying CPON is not without its challenges, but as we’ve discussed, the long-term cost-efficiencies make it worth it. Adopting CPON requires addressing both technical and standardization challenges. Fortunately, the ITU-T TC/MAC architecture requires only minor adjustments to support true Ethernet rates and relocate FEC into the optical module. Additionally, the growing ecosystem of mass-market 100G/400G OpenZR+ technology provides cost-effective opportunities for integration into CPON systems.

Vertical integration also plays an essential role in advancing CPON technology. By combining the design, development, and production processes for application-specific integrated circuits (ASICs) and OLTs, companies can promote seamless collaboration between hardware and software teams. This integration enhances system performance, streamlines product development, and results in tailored customer solutions, improved reliability, and efficient supply chain management. Vertical integration not only optimizes performance but also accelerates innovation and ensures faster time-to-market for networking solutions.

In addition, standardization efforts by the ITU-T Study Group 15 Question 2 and the CableLabs CPON Working Group are helping to define the requirements and specifications for 100+ Gb/s CPON systems. Vendors and CableLabs are expected to submit detailed proposals, marking significant progress in the deployment of next-generation CPON with single-carrier coherent technology.

A look ahead

CPON is poised to revolutionize the field with faster data speeds, affordable solutions, and effortless integration into existing networks with the advancement of access network technology being driven by the collaboration of standardization bodies and industry groups. This forward leap aligns with the increasing availability and cost-efficiency of coherent single-carrier (SC) optical transport technologies commonly used in data centers and campus networks.

As the industry evolves, I look forward to sharing the latest updates with you on how single-carrier coherent technology is redefining the future of next-generation CPON. If you’re interested in where the industry is today as we look to the future, read about the path to access modernization from 25GS-PON to Coherent PON.