FEATURE: NEXT GENERATION OPTICAL NETWORKS


rates at very useable distances. Te higher data rate per wave, the less cost per bit. Tis is useful for everyone from internet service providers, content providers, and so on. Classic customers that require the ability to drive down cost per bit. It’s really quite amazing, the performance that can be achieved with fiſth generation technology. It’s really difficult to point to one piece of coherent innovation and say “that’s the thing”.’ Acacia’s marketing vice president Tom


Williams agreed that network operators are under pressure to build and deploy multi- generational architectures comprising terminal equipment and optical line-systems to support both current and future data rates in an efficient, scalable and cost-effective way. ‘Traditionally,’ he said, ‘coherent optical


transmission was primarily used for longer reach applications, but traffic paterns are changing in ways that are leading to the need for high- capacity interconnects that target shorter reaches in a power-efficient, cost-effective manner.’ Tis next-generation optical technology,


he feels, has additional benefits to operators, because it can be used in a more bespoke way. ‘We’re taking the pluggable solutions and making them a litle bit more tailored to the application,’ he said. ‘As pluggable coherent expands, it’s no longer a “one-size-fits-all” approach, we’re tailoring solutions for the individual applications.’ Indeed, both organisations have introduced


their own solutions. Infinera’s ICE6, with features such as Nyquist subcarriers, probabilistic constellation shaping (PCS), and a high modem signal-to-noise ratio (SNR) featured in a number of successful trials last year. Likewise, Acacia’s pluggable modules were used late last year as part of a successful module-level interoperability test between DSPs in 400ZR and OpenZR+ operating modes. On the competing solutions available,


Williams added: ‘It’s good for the market that we have these different types of solutions and it doesn’t have to be an all or nothing. Tey’re providing solutions about different types of optical network applications. When you get coherent into the right form factor so that it’s simpler to deploy, that’s when you start to address the broader market.’


bandwidth as you can, so expanding the C band and L band is a way to do this before you have to start deploying more fibre.’ Tat, of course, requires the development of


a host of technology ecosystems to support the enhanced spectrum range.


Constant improvement Also on the panel discussion was Geoff Bennet, Infinera’s director for solutions and technology. He discussed how some of the next, or ‘fiſth’, generation coherent optics play their role: ‘Every generation of coherent offers beter performance at lower data rates than the previous generation at that data rate,’ he said. ‘So, they’re allowing us to go to even higher data


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All about choice Like coherent, PAM4 optics can be used to put more bits into the same amount of time on a serial channel to double the density of the data and achieve a higher bandwidth for the same data path. So, for a given channel, twice the data could be pushed through for the same frequency and channel bandwidth. Anthony Clarkson, technical director EMEA


at Precision OT, believes that there are a number of technical challenges of moving beyond 10Gb/s in the access network. ‘Distinct from backbone and metro networks, access networks have two key features,’ he said. Te first is a ‘wide variety of link lengths that


IT’S REALLY QUITE AMAZING, THE PERFORMANCE THAT CAN BE ACHIEVED WITH FIFTH-GENERATION TECHNOLOGY


connect multiple locations using outside plant infrastructure. Tough an access network can have link lengths that span 10 to 120km, the majority of links span 40 to 60km.’ Secondly, access networks have a large


number of links that leverage diverse protocols and data rates, which adds to the complexity of the overall network. ‘As if operating access networks was not already complex,’ said Clarkson, ‘upgrading bandwidth above 10Gb/s brings its own set of issues.’ One is the optical link budget. ‘When


network operators push past 10G per lambda,’ said Clarkson, ‘optical link budget considerations become paramount. Overall, the amount of optical power at the link receiver must be above the receiver’s sensitivity to successfully gain a link. As access networks span longer distances, accounting for link budgets becomes increasingly complex. Tough one can always add optical gain to a system with amplifiers like EDFAs, additional amplification always brings an increase in noise.’


Go the distance Optical signal-to-noise ratio (OSNR) is also an issue. Just because a receiver sees light above sensitivity, does not mean, says Clarkson, that data is being transmited successfully. ‘Te OSNR is the ratio of the actual signal level to the level of noise in the system. Te smaller this number, the more likely there will be bit errors in the transmission. As links move to higher order modulation schemes to get more bits per symbol (such as PAM4 or QAM), the beter the OSNR needed to distinguish the signal levels. Digital signal processors (DSPs) and forward error correction (FEC) can be used to compensate for bit errors, but they also add complexity, power, latency and general interoperability concerns.’ Tirdly, said Clarkson, is chromatic dispersion.


‘For networks with bandwidths of 10Gb/s and above, chromatic dispersion has a negative impact on optical transceiver receiver sensitivity. In slower signaling systems, ample time between symbols means that spreading does not create an issue. However, as the baud rate increases, the


pulse spreading causes symbols to overlap each Issue 30 n Winter 2021 n FiBRE SYSTEMS 19


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