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FEATURE : NEXT GENERATION OPTICAL NETWORKS


OPTICAL NETWORK EQUIPMENT MANUFACTURERS HAVE A STRONG ROLE IN MAKING NETWORKS OF THE FUTURE A REALITY


other, resulting in inter-symbol-interference (ISI).’ At bandwidths of 10Gb/s, Clarkson said


chromatic dispersion can limit network links to approximately 80km. ‘Past 10Gb/s, at bandwidth rates of 25Gb/s and beyond,’ he said, ‘chromatic dispersion begins to limit link distance to 15 to 20km. Tis does not meet the distance requirements of most access networks today. Tough dispersion compensating modules (DCMs) exist, they add higher levels of system complexity.’ As detailed previously, coherent and PAM4


optics are atractive solutions for evolving access networks for higher speeds and bandwidth while addressing these challenges. However, Clarkson believes that each technology also has its own limitations. Looking at PAM4, he explained the standard


employs four amplitude channels, each with two bits, which doubles the data rate, making it twice as efficient as legacy binary models. ‘Compared to the space and power requirements of coherent optics,’ said Clarkson, ‘PAM4 leverages components that can fit in smaller, more common form factors such as the QSFP28 model. As such, it delivers significant cost savings advantages to network operators that leverage it. However, going past 100Gb/s at an optical link reach of 5km requires amplification, dispersion compensation and/or forward error correction (FEC) on the optical line. Tis means the installation of active equipment and greater power expenditure, potentially mitigating PAM4’s low-cost advantage.’


Cost factor Although access-centric CFP, CFP2 and QSFP56- DD/OSFP coherent optics, said Clarkson, ‘enable a long link reach of greater than 100km (up to thousands of kilometres for CFP and CFP2) at data rates as high as 400G, they have higher space and power requirements than other types of transceivers used in an operator’s access network. As a result, leveraging coherent optics potentially raises total operator costs, which poses a challenge to network operators with constrained budgets.’


20 FiBRE SYSTEMS n Issue 30 n WINTER 2021


Despite these perceived drawbacks, said


Clarkson, both coherent optics and PAM4 technology will play a strong role in propelling access and other networks to 800G and beyond. ‘What is missing from the discussion, however, is a solution that can solve for the longer reach and costlier nature of coherent optics and the cost-effective yet constrained reach of available PAM4 solutions.’ As Williams highlighted, there are many


network applications, each of which have their own applications. Modern access networks can serve many different types of structures, from cell towers, enterprise buildings and multi- dwelling units, to data centres, aggregation sites, schools and sports stadiums. ‘Te new trend in access networking,’ said Clarkson, ‘is a focus on data transmission across shorter distances, oſten less than 40km. Extremely low latency requirements are also adding a new and interesting dimension into the access network, especially as 5G rollouts continue.’ For Clarkson, access network operators


require a solution that can achieve three key requirements. One that allows for 100G DWDM, to ensure the speed and capacity needed to meet current and future end-user requirements; one that leverages a common transceiver form factor, like QSFP28, for seamless interoperability and cost savings, and one that enables data transmission at a reasonable distance of 40km without requiring any external transport equipment.


Trend seter Te trend here, said Clarkson, is a positive one. ‘In fact, a new dawn for access networks


is already emerging,’ he enthused. ‘We are now part of an advanced engineering group that is developing a commercial 100G DWDM QSFP28 switch pluggable product, which does not require any DCM or other equipment for transmission up to 40km. Tis new transceiver is designed to meet the needs that other high- bandwidth, high-speed solutions cannot.’ Te product is 4TB of maximum capacity with


bidirectional capabilities, all in the common and cost-effective QSFP28 form factor. ‘Tis is the vision I see for the future of


European access networks,’ said Clarkson, ‘solutions that will make it easier for network operators to upgrade their access networks without breaking the bank.’ As we launch into 2021, Clarkson believes


that one thing is clear, ‘the future of the optical access network in Europe is bright.’ With governments continuing to invest in 5G rollouts and individual network operators jockeying for position, the trend toward convergence and the need for higher speeds and bandwidth is only heating up. ‘Optical network equipment manufacturers


have a strong role to play in making the networks of the future a reality,’ said Clarkson. ‘Tough existing coherent and PAM4


solutions help operators overcome the challenges of upgrading bandwidth to a certain extent, there is still room for improvement. Fortunately, that time has come, and European network operators can look forward to a bold future.’ Williams concurs, stating that whatever the


path may be, ‘this industry has always been very good at coming up with the right solution.’n


www.fibre-systems.com @fibresystemsmag


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