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The New Datacom Imperative: Next Generation Optical

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The New Datacom Imperative: Next-Generation Optical…. BY MAURY WOOD, AFL W hile structured cabling using multi-fiber connectors such as MPO/ MTP® have been in use in enterprise data centers for many years, the prevalence of this connector type continues to steeply increase. This is due to the confluence of commercial dynamics (including the apparently insatiable consumer demand for broadband data services) and technical dynam- ics (including the need for parallel full-duplex lanes of transceiver optics for perfor- mance purposes). Concurrently, the relentless drive for high optical network oper- ating efficiency, and minimal lost productivity, is leading to an expanding desire, particularly by hyperscale network operators, for 100% microscopic inspection of their infrastructure connectivity. AFL estimates there are more than 10 million MPO/MTP connectors in use across the world today, with more than 1 million forecasted to be fielded in 2019. An MPO connector market compound annual growth rate (CAGR) of at least 10% is expected to sustain for the next five years. As all communications engineers and technicians are aware, nearly all link fail- ures occur at points of connection, and very rarely across unbroken network spans. At the same time, the signal modulation technology that underlies the new short- reach 200G/400G optical Ethernet standard reduces the available link budget in high-bandwidth datacom applications – making the operational need for immacu- late connector endfaces all the more imperative. Next-Generation Optical Ethernet Transport Standards (200G/400G) in the Data Center Data centers typically rapidly adopt the latest networking technologies as their oper- ators seek a competitive performance edge. The new IEEE 802.3bs 200G/400G Ethernet standard specifies the use of PAM4 modulation, a departure from the older non-return to zero (NRZ) modulation method. PAM4 provides higher spectral effi- ciency, but because it encodes two bits (four states) into the same carrier signal dynamic range as NRZ (which encodes one bit or two states), PAM4 requires about 9.6 dB more link optical signal-to-noise ratio than NRZ to maintain the same sym- bol error rate statistics (see Figure 1). While this physical layer transmission technology change may initially seem unrelated to optical connector cleanliness, there is a distinct and important link. In 10G, 40G, and 100G systems using NRZ modulation, a contaminated connec- tor endface may cause optical losses that can be largely ignored on the short- reach cabling common in data centers. However, in 200G and 400G systems using PAM4 modulation, the same level of endface contamination will erode a greatly reduced link budget margin, driving optical network technologists to demand pristine connector endfaces. Best-practice connector cleaning and inspection procedures will become essential to maintain the highest levels of performance and reliability. A simple real-world analogy might paint a picture here. To a family car driving along at 30 miles per hour, road debris is a mere annoyance. To a sports car rac- ing along at 120 miles per hour, the same road debris is a big risk that may even cause a fatal crash. The light-carrying core of a single-mode MPO fiber has a diameter of 9 microns or an endface surface area (πr2) of about 64 square microns. A 2-micron-diame- ter speck of dust has a surface area of about 3 square microns, or about 5% of the endface surface area. A 5% reduction in laser power is about -0.2 dB. In an envi- ronment in which link budgets are narrowing and transmit laser power is a significant contributor to overall data center power consumption (there are tens of thousands of semiconductor lasers in a typical modern data center), it is quite easy to understand the opex-driven desire of network technologists for completely clean transport optics. Moving into 2020, hyperscale data centers are expected to become even larger, leading to structured MPO cabling that is naturally longer in some spans. Unlike the signal losses (attenuation) due to cable-reach physics (about 0.4 dB per kilo- meter at 1550 nm on single-mode fiber), connector contamination losses can be identified using proper microscopic inspection techniques and fully mitigated using proper endface cleaning methods. Data Center Cable Infrastructure – An Increasingly Valuable Asset It is possible to quickly model the asset value of a 400G link for a hypothetical broad- band internet service provider. The major players in this global market are chasing the goal of providing the majority of their residential and business subscribers with 1-Gbps downstream service by 2020. Today in the United States, a typical consumer pays about $100 per month or $1200 per year for fiber-to-the-home internet service. With no statistical oversubscription, a 400G link serves 400 customers, and thus places the asset value of each 400G MPO terminated cable (eight fibers at 100 Gbps per fiber full duplex) at $480,000 per year. With a conservative 2:1 oversubscription ratio, REPRINTED WITH REVISIONS TO FORMAT, FROM THE MARCH 19TH, 2019 EDITION OF LIGHTWAVE ONLINE. COPYRIGHT 2019 BY ENDEAVOR BUSINESS MEDIA. FIGURE 1. Example PAM4 eye diagram showing the four encoding states. (Photo courtesy of Keysight Technologies)

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