In just the last 20 years, the bitrates achieved on fiber broadband networks have increased by more than a factor of 100: from less than a gigabit in the early 2000s to now being able to achieve 100 Gb/s over a single wavelength in 2022. This ability to support ever-increasing bandwidths is one of the strongest attributes of fiber, and every new generation of fiber PON technology unlocks more of fiber's potential. This makes it possible to deploy a fiber network once and use it for the next 50+ years. However, to deliver on its promise, fiber networks must guarantee a graceful evolution by leveraging existing assets to the maximum.

Certainly, the history of fiber broadband shows us that evolution doesn't happen overnight. In fact, it can take years, so we should expect old and new technologies to coexist in networks for a considerable time. Therefore, new PON technologies must be backward compatible to ensure a smooth evolution from generation to generation. This allows operators to easily and iteratively upgrade their network capacity in the most cost-efficient way.

Without the coexistence of old and new PON technologies, operators would be faced with heavy-lift migrations, complicated and lengthy procedures, possible changes in the outside plant and long service outages affecting users.

Backward compatibility is achieved through the assignment of the wavelengths on which the technologies operate. Every PON technology uses specific wavelength pairs for upstream and downstream traffic, which are defined by the relevant standard. The access nodes (OLT) and customer modems (ONT) at either end of the fiber use the same wavelengths so they can talk to each other and transmit and receive data. For two different technologies to coexist on the same fiber, they need to be assigned to different wavelengths. Otherwise, the signals will interfere, and there will be penalties on the performance. Introducing a new technology on the same fiber simply means adding a new wavelength.

In fact, backward compatibility was one of the key considerations for defining 25G PON technology. Wavelength plans chosen by the standardization body enable 25G PON to coexist on the same fiber with GPON and XGS-PON. This is important because GPON is still the most widely deployed PON in use today, but it is being upgraded to XGS-PON. Many operators are likely to have both GPON and XGS-PON in their networks for many years. Adding 25G PON, though, is straightforward because backward compatibility with both GPON and XGS-PON is assured.

In contrast, 50G PON can coexist with GPON or XGS-PON, but not both together. Operators considering 50G PON would need to carefully consider their migration strategy. For example, they can completely decommission GPON from their network (i.e., move all their GPON customers to XGS-PON) before moving to 50G. Or they would have to skip XGS-PON and move from GPON directly to 50G PON, which is quite a big step. In both cases, the migration requires long-term thinking and does not leave the possibility of efficiently reacting to new market opportunities or competitive challenges.

Moreover, fiber networks are becoming a unified infrastructure to connect consumers, businesses, smart cities and 5G cell sites. Operators are seeing new revenue opportunities beyond the consumer segment, which has shifted the way we think about fiber networks. The shift from fiber-to-the-home to fiber-to-everything is enabled by higher-speed technologies and their abilities to coexist with previous ones, so that every service can be delivered at the right speed and cost. These opportunities all need reliable, high-speed connectivity that often exceeds XGS-PON's capabilities (due to overheads, XGS-PON can deliver 8.5Gb/s). The possibility to easily introduce 25G or 50G PON helps get more from the fiber network, create more revenues, differentiate from competitors and provide the most sustainable way to connect everything and everybody.

In the world of fiber broadband, looking backward is by far the best way to move forward.