FEATURE TERABIT NETWORKS
The battle of the modulation schemes
Adam Carter and Dan Tauber examine which modulation schemes – DMT, PAM4 or coherent – are likely to prevail in the race to higher speed transmission for more cost-sensitive links
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any industry experts are calling data the ‘new oil’ and just as oil once created amazing economic opportunities, it also produced
many challenges in drilling for and distributing the oil. Te same is true today with data, except the challenge is making the entire pipeline faster, while also ensuring that networks become easier and more cost-effective to deploy. To meet these bandwidth and network
demands, the industry has been undergoing a major transition to higher speed networks that can deliver 100G, 200G and even 400G speeds. Tis is why we’ve been seeing the rapid success of core components such as coherent 100G optical modules – both CPF2-ACO (analogue coherent optics) and the newer DCO (digital coherent optics) – that have enabled the first 100G network deployments. Tese pluggable WDM optical modules offer the lower power dissipation, lower install costs, and higher port density needed to deliver these higher data rates, particularly in the long-haul market over distances of more than 80km. Due to this rapid adoption of coherent
systems and components, the coherent WDM 16 FIBRE SYSTEMS Issue 15 • Spring 2017
market has been growing very quickly. In fact, Andrew Schmitt, of market research firm Cignal AI, is projecting that this market will continue achieving 40 per cent compound annual growth rate between 2017 and 2020. But, with the onset of more cost-sensitive
applications such as data centre interconnect (DCI), and the number of links less than 80km, the industry is starting to think that coherent is too expensive from a ‘dollar per bit-kilometre’ perspective. Tis is driving the emergence of competing approaches based on different modulations formats such as four-level pulse amplitude modulation (PAM4) where the
emphasis is more on the silicon than the optical technology. Tis article will look at the various modulation schemes (see Table 1), discuss pros and cons and where each may win or co-exist with one another.
DMT modulation Discrete multi-tone (DMT) is a modulation format – widely used in digital subscriber line (DSL) systems – that splits the available bandwidth on the optical carrier, or wavelength, into a large number of radio-frequency (RF) modulated sub-carriers. For 100Gb/s transmission, 256 individual sub-carriers are
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