What Is a “Legacy Network” in 2026?

by Josh Seawell, director of product management, Sumitomo Electric Lightwave

That answer depends on the operator.

As 2025 draws to an end, what it means to be a “legacy network” is rapidly evolving right along with everything else in the industry. It is not so much defined by the network’s age but on the technology it’s built on and the quality of its services. Hallmarks of a legacy network now include costly outages, significant maintenance, and unnecessary inefficiencies. Even the presence of mature technologies can signal a legacy network, no matter how old the facility is.

Not to knock legacy networks; they have accomplished much and brought us where we are today, with eye-popping AI adoption rates and surging infrastructure investments. It is a hard truth, though, that legacy networks are unable to compete with modern fiber optic networks when it comes to performance, reliability, and sustainability.

Right now, the legacy landscape can be described in two groups. On one hand, there are network operators pouring time and money into patching their early 2000s infrastructure. On the other hand, major construction projects are underway using dated technologies, products, and processes.

The result: there are many data centers running on “legacy networks,” all too often from Day One of turning up services.

Greenfield or brownfield, it’s a fair question for network operators to ask themselves whether their spending would deliver better returns if it went into network modernization.

That answer is going to be unique depending on the operator and their specific goals for the next five-plus years. And with co-packaged optics on the horizon, the “right” answer might be a unique, strategic migration path as opposed to a wholesale upgrade.

As network operators head into 2026 planning for the next three to five years (and beyond), some of the considerations for them to weigh include:

Scalability and Performance

Modern networks are up to the challenge of meeting escalating network demands – brought on by AI, 5G/6G adoption, and cloud-based services. Whereas rigid, legacy systems are hindered by bandwidth limitations and prone to bottlenecks and inefficiencies, AI-ready network infrastructure delivers the performance and scalability that organizations need if they are going to be competitive moving toward 2030.

Security and Reliability

The total amount of data created, captured, copied, and consumed globally is forecast to more than double between now and 2028. Therefore, it is more critical than ever for thriving businesses to invest in secure, reliable systems. Legacy network constraints cause outages and alert fatigue. In stark contrast, modern network designs enable automated real-time threat detection and activation thanks to AI.

Installed Cost

Another way that network operators can make AI work for them is by leveraging next-gen automated processes for predictive maintenance. These enhanced applications alone drive efficiency improvements and reduce costly downtime. Legacy networks cannot do that, and in the meantime, operators are spending up to 80% of their IT budgets on short-term resolutions to challenges stemming from dated infrastructure and technology.

Sustainability

Next-Generation Thinking™ fiber optic solutions offer a modular, strategic approach to prepare networks for what’s next. Sustainable design and manufacturing principles are built-in from concept all the way through product lifecycles. High-quality components are precision-assembled to ensure reliable long-term performance, and Sumitomo Electric Lightwave’s compact, recyclable packaging aligns with environmental responsibility and corporate sustainability goals.

Connect with us to discuss your network build or upgrade plans. We can help evaluate the ROI of the various options available for your unique scenario.

About the author: Josh Seawell, director of product management, oversees the development and distribution of Sumitomo Electric Lightwave’s optical fiber and connectivity portfolio in North America and CALA.