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The long wait for 50G PON: a view to realistic deployments

A moving staircase towards a big window

Fiber access networks are revolutionizing the industry, enabling seamless connections for everything: consumers, businesses, smart cities and more.  

The evolution of PON technology has brought increased capacity, better performance, lower latency, automation, and energy efficiency. While 25G PON is already deployed and proving its value, the anticipation for 50G PON grows. However, the deployment timeline and missed opportunities during the wait, are crucial factors to consider.  

Let's explore the maturity of 50G PON technology and market demand. History tells us that there are two challenges for every new PON technology: making the technology work and making it optimized to be deployed at scale. 50G PON demos have been successfully shown, but achieving commercial readiness for real deployments is a different story. 

While 25G PON is a simple extension of XGS-PON, 50G PON requires an unprecedented complexity in the evolution of TDM PON, and optical components will be stretched to their limits.  These are solvable technical problems, but equipment and operational costs will be inherently higher, and more time will be required before maturity is reached.  The causes are several.   

First, higher speed always causes a receiver penalty which must be overcome.  In the case of 50G PON, the burden will primarily land on the OLT and ONU transmitters, which must launch higher power to compensate.  As a result, even for the lowest N1 loss budget, lasers will be pushed to their limits. 

Second, higher speed components are required.  There is always an associated cost premium, but in the case of 50G, a conventional PON receiver is just not feasible.  Nokia played a key role in solving this problem: the solution is to use a 25G PON receiver followed by digital equalization.  More specifically, this requires an ADC (analog-to-digital converter) and DSP (digital signal processing).  While these components have been introduced into ultra-high speed Ethernet transceivers used in data centers, this will be their first appearance in PON.  The ADC/DSP introduces complexity, cost and higher power dissipation. 

Third, the system penalty caused by dispersion (the spreading out of a signal as it travels along the optical fiber) is higher at 50G than at 25G.  Compensation for this effect requires additional complexity in the DSP. 

Finally, it was necessary to create a new performance metric, PON TDEC (transmitter and dispersion eye closure).  This will require more complex and expensive testing to validate, and at this time there is only a single test equipment vendor that supports it. 

We’re working hard as an industry to solve these problems, but more time is needed to optimize the technology. Since the ecosystem for 50G is still immature, the first prototypes are based on field programmable gate arrays (FPGA). This is good for trials and technology validation but it’s not so good for commercial deployments. And since 50G is not planned to scale for residential use any time soon (even in China), costs will probably not come down before the next decade.  

There’s also another important consideration. The way the 50G PON developments are shaping up, the first commercial solutions will undoubtedly be asymmetrical (50G down, 25G up), with full symmetry following a few years later. Businesses – the main market for multi-Gigabit services – require symmetrical speeds. So, the first iteration of 50G PON will not provide any advantage over 25G PON, though it will be more costly. What’s more, asymmetrical and symmetrical 50G PON use the same wavelength windows. This conflict means there’s no easy upgrade path from one to the other; any asymmetrical 50G PON ONTs would need to be replaced. If this symmetry issue sounds familiar, it’s because we’ve seen it before. Most operators skipped asymmetrical XG-PON and preferred to wait for symmetrical XGS-PON.  

History also tells us that every big leap in technology needs around eight years to mature. 25G is well on the way: it is based on the existing technologies, mature, proven, and already delivering commercial 10G services. Until end of the decade, 25G PON will be the simplest, most cost-efficient PON technology, capable of efficiently addressing demand, and kickstarting PON’s life as a single infrastructure capable of connecting everything and everybody. 50G PON should be right on time for the step after: 20G and beyond. 

Choosing between waiting for 50G PON and investing in 25G PON today boils down to the business case. Factors such as cost, competition, market demand, timing, and business priorities should be weighed to determine the optimal return on investment. Fortunately, operators have the flexibility to adopt 25G PON now and seamlessly transition to 50G PON later, ensuring a gradual and practical evolution. 

Maurizio Severi

About Maurizio Severi

Maurizio Severi leads the fiber business line at Nokia Fixed Networks with responsibility for our fiber access portfolio strategy and roadmap. Working closely with our customers and with our researchers to develop industry-leading innovations is both an art and a science. So it’s just as well Maurizio complements his extensive technical expertise with a love of reading, painting and photography.

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