Spring 2018 Optical Connections Magazine




As pressure mounts on networks pushing them on to 400G and more, there are several key factors, approaches and challenges linked to upping the optical capacity ante, finds John Williamson

OLD AND NEW Networks at dierent stages of development and/or maturity and geographic reach have dierent optical capacity growth priorities and options. As an example, Brandon Collings, CTO of Lumentum, points out that it will be challenging for current generation systems to achieve 400G/lambda over long haul and marine distances, and this may curb the extent to which 400G will be deployed in larger, longer reach networks. “Beyond 400G, multi-carrier superchannels are generally required for metro and longer reaches,” he observes. “Technology for implementing superchannels is essentially available now, but greater integration to improve the cost of multi-carrier transceivers will improve the attractiveness of superchannel transceivers.” A number of factors are associated with increasing transmission performance over meaningful distances, one of which is baud rate. “When increasing transmission speed from 100G to 400G-600G per wavelength, the most important innovation step is to increase or double the baud rate that current technologies have,” judges Fischer. “The side eect for doubling the baud rate is that we have to double the transmission window: generally we currently use fixed 50 GHz grids, and doubling the baud rate you will have to go up to 75 GHz grids or more flexible schemes.” In this overall environment, a flex spectrum-capable network is required. “While most new networks being deployed are fully flex spectrum-capable, a majority of deployed networks are not and therefore will not support 400G interfaces,” reasons Collings. Coherent and Direct Detection In the general framework of higher data

A s has been extensively reported, escalating global demand for cloud computing, hyperscale DataCentre Interconnect, HDTV and 4K video services and Internet TV are together pushing optical networking inexorably towards 400G and above capabilities. The upcoming arrival of 5G, with its predicted increased fronthaul and backhaul requirements, only adds to the bandwidth pressure to grow to 400G and Terabit networking. NUTS AND BOLTS Non-trivial challenges are evident at the nuts-and-bolts level of things. “The optical networking roadmap for 400G and beyond is moving to higher order modulation,” points out Siddharth Sheth, SVP, Networking Interconnect at high- speed data movement specialist Inphi Corp. “PAM and coherent will require complex Digital Signal Processing chips, high speed SerDes, higher speed lasers/ optics at 28/56 Gbaud-plus, and linear amplifiers and drivers to go along with the optics.” Sheth notes that the relentless pursuit of lower power and lower cost solutions posed by the industry further intensifies the challenge of

building those components. “Building cost- eective optical platforms with these components, and making a new generation of solutions reliably available in volume every three-to-four years, will be one of the most challenging aspects of moving the roadmap forward,” he contends. The R&D costs to develop suitable components are of some concern. These costs are increasing substantially as CMOS electronics enters the realm of 7 nm and lower. “The DSPs required to service the 400G-plus roadmap moving forward are expensive to build,” acknowledges Sheth. “The drive to the lowest dollar/Gbits/s solution and a highly fragmented market with many suppliers will make it challenging to fetch an ROI on some of these investments. “ Again, increased data rates will also bring with them more stringent Optical Signal-to Noise Ratio (OSNR) requirements. “Speeds up to 600G per wave will need an OSNR-optimised optical layer to make the most of them,” adds Uwe Fischer, Coriant Executive Vice President, R&D and PLM, and CTO. “Even more important, the many choices and the programmability of the interfaces involved cannot be harnessed without a lot of inbuilt automation and performance awareness.”


| ISSUE 12 | Q1 2018


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