Evolving Your PON: Leveraging PON Investments to Expand Service Offerings

Advancements in fiber technology shouldn’t force providers to undertake expensive construction.

It is quite astonishing to think how bandwidth in broadband networks has increased over recent years, transforming how we all connect to the world from our homes for entertainment purposes. For some of us, it has also enabled our ability to work from home. I’ve actually worked from my home office for over 20 years.

Key to the rapid expansion of last-mile access speeds into homes is the rapid rollout of passive optical network (PON) technology. My experience working from home started in the dial-up era, which was painful at times, before moving to ADSL, which made things workable, and finally to PON. I now have 900 Mb/s PON-based fiber-to-the-home, which is great.

However, not all communities have access to PON in the last mile yet, which creates a digital divide for both recreation and business—from a work-from-home perspective but also for connectivity requirements for any business in the local area. Governments across the globe recognize this issue and are regulating, or even providing stimulus funding, to close this digital divide.

Natural market growth, boosted by additional stimulus funding, means the PON equipment market currently has considerable momentum. Julie Kunstler, Chief Analyst, Broadband Access Intelligence Service at industry analyst firm Omdia, produced a blog for the OFC 2024 event outlining Omdia’s view on the PON market. This blog stated, “PON equipment revenues are forecast to exceed $21 billion in 2028, representing a CAGR of 11.7% from 2022.” Good news for residential and small business customers with bandwidth requirements that are well served by PON technology.

Substantial PON Investments

The huge investment in fiber-based access networks extends well beyond the PON hardware itself. Data from network operators suggests that up to 90% of network deployment costs are actually for construction and trenching fiber ducts and for the fiber cables. With such a big investment in the fiber access network, service providers need to leverage this investment to deliver a broad range of services, including high-speed services beyond today’s residential PON focus.

This creates a very interesting challenge for DWDM vendors that typically operate deeper in the network in aggregation, metro, and long-haul networks. DWDM networks operate over two fibers, one for each direction of data transmission. PON networks, on the other hand, are based on single-fiber infrastructure with bidirectional traffic to minimize installation costs. This creates a bit of a dilemma for DWDM networking vendors and those deploying DWDM networks: can this single-fiber domain be used when dedicated N x 25G high-capacity services are required right out to the edge of the network?

This would all be fine if capacity in DWDM networks wasn’t also ramping up at the same time. Older 10G DWDM can easily operate over single-fiber infrastructure. But the latest 25G/50G PON technology can support 10G services natively today, and 10G DWDM is also rapidly being replaced in the DWDM access network by higher-speed coherent optics that generally don’t work over single fiber. Coherent optics have been used for over a decade in long-haul core networks, but as network capacities continue to grow, long-haul networks have moved to higher 800G, and soon 1.2T, speeds, and lower speed 100G, 200G, and 400G coherent optics have pushed ever closer to the edge of the optical network.

So, we have the optical network splitting into a dual-fiber DWDM transport network domain and a single-fiber edge/access network domain, at the same time as DWDM optics are moving to higher speeds requiring coherent technology that doesn’t support single fiber!

Of course, residential users don’t need N x 25G services. And most small and medium-sized businesses don’t either. But there are many use cases that increasingly do need this capacity, including 5G cell towers, larger businesses/enterprises, business parks, or even backhaul for PON optical line terminals (OLTs) deeper in the single-fiber access network.

How do network operators address this challenge today? Simply put, if they need higher-speed coherent optics to support higher-capacity N x 25G services, then they need to make sure dual fiber exists all the way to the end customer.

They typically can’t leverage the substantial investment they have already made in the PON network to also support these services. If, as is often the case, the dual-fiber infrastructure isn’t in place, then they have to invest significant time and money in digging new ducts and/or pulling new fibers to support the new service. These costs can be very high, at up to $60,000 per trenched mile of fiber or more, according to the Fiber Deployment Annual Report 2023¹ from the Fiber Broadband Association. These high costs may even make the service unviable economically.

Optical Innovation

Help is at hand with one of the newest innovations from the optical networking industry. The recent innovation of subcarrier-based point-to-multipoint optics offers a solution to the “coherent DWDM over single fiber” dilemma for network operators. This new class of “XR optics” pluggables is based on the concept of digital subcarriers, where a single laser is modulated into multiple subcarriers enabling a single higher-speed hub optic to communicate with multiple lower-speed leaf optics, as shown in Figure 1.

One additional benefit of this new class of XR optics is that the underlying subcarrier technology enables us to fix the problem of higher-speed coherent optics over single-fiber networks. We can use some of the subcarriers in one direction and others in the opposite direction, turning, for example, a conventional 400G dual-fiber coherent optic with 16 x 25G subcarriers into a 200G single-fiber bidirectional, or bi-di, optic with 8 subcarriers per direction. We can use this subcarrier-based XR architecture for both point-to-point and point-to-multipoint deployments over single-fiber networks like PON.

Leveraging Existing PON Deployments

We now have a solution to the challenge of higher-speed coherent optics over single-fiber infrastructure to deliver higher-capacity N x 25G services that exceed the capabilities of the underlying PON network. This means that network operators can leverage the significant investments they have made in single-fiber PON access networks for a much broader range of services and are able to support the push of higher-speed coherent DWDM optics right to the edge when needed.

We briefly mentioned earlier some services that might require capacity that exceeds the capabilities of the PON network. Let’s look at these PON overlay applications (see Figure 2) in a little more detail:

• Edge data center connectivity – Network operators who own fiber can now push their multi-access edge compute (MEC) data centers closer to end users and deeper into access networks utilizing the single-fiber domain.
• Similarly, data center owners who rent fibers can now cut fiber rental costs by renting just a single fiber for data center interconnect networks.
• High-capacity N x 25G services for 5G xHaul and demanding business customers can now quickly, and cost effectively, be deployed over what was a residential PON.
• To support local communities, high-capacity services can now also be offered to anchor institutions in the community, such as schools or libraries, where bandwidth demands might outstrip a residential service level.
• PON network owners can push their own aggregation devices that require higher-capacity backhaul deeper into their single-fiber access plant to support remote PON OLTs or new PON buildouts that weren’t initially anticipated in the original design, e.g., to support new housing developments.

A further consideration beyond the economics of reusing the existing single-fiber infrastructure rather than creating new dual-fiber infrastructure is the speed of rollout of these new services and the associated time to revenue for the services. Reusing the existing fiber network means that new services can be deployed in minutes once the hardware is shipped to the customer site. The alternative option of a new fiber build, or even a short extension to existing fiber, can take weeks or even months to survey, plan, dig/pull, and install.

New subcarrier-based XR optics, such as Infinera’s ICE-X intelligent coherent pluggables, can help us address a major challenge that has occurred due to the rapid capacity expansion in both PON-based access networks and DWDM-based transport networks. As PON and DWDM technology were both evolving and diverging, the new XR optics architecture based on digital subcarriers is enabling us to close the gap and bring these two domains back together again harmoniously.

REFERENCE

1. Fiber Broadband Association, https://fiberbroadband.org/wp-content/uploads/2024/01/Fiber-Deployment-Annual-Report-2023_FBA-and-Cartesian.pdf