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Metro 100G and the coherent brick wall


The future never works out quite how you planned, especially in metro optical networks, writes Coriant’s Dr Maxim Kuschnerov


proprietary coherent interfaces for the metro sector, enabled by falling component costs and higher-order modulation schemes that decrease the cost per bit of the physical interfaces. Tese higher-order modulation schemes


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Product design engineers now lust for a progression of proprietary coherent interfaces for the metro sector


include 200G 16-QAM technology, which has already been announced by a number of industry suppliers. Te natural evolution of this idea would take the industry to 64-QAM and beyond – although there is a trade-off between modulation complexity and reach that could lead to some novel diversions. But is it really this easy to predict the path


the industry will take? In a word, no. Conquering metro networks will not be this simple. New technology paradigms usually arise in the low-cost metro first and then spread into core networks. A move of coherent core technology into the metro segment would be an exception to the rule, rather than a logical consequence. First, sophisticated coherent technology


based on flexible-rate interfaces simply cannot compete with classical 10G optics in terms of costs. Since most metro networks are relatively simple and fibre-rich, introducing a proprietary (read: expensive) technology is a direction only selected Tier 1 carriers are likely to take. It would also represent a complete about-face from the XFPs and SFP+ pluggable optical modules of today. Second, the higher price-per-bit of coherent


technology is inevitably tied to its greater power dissipation and physical footprint. Optical coherent interfaces are going through a similar development curve as did 10G, with


12 FIBRE SYSTEMS Issue 7 • Spring 2015


ſter introducing 100G coherent technology into core and submarine networks, product design engineers now lust for a progression of


Dr Maxim Kuschnerov


a move from discrete components to pluggable modules such as the CFP2-ACO introduced by Oclaro and others. Coherent digital signal processing (DSP) chips – the brains behind the complicated unravelling of information at the receiver – are also steadily becoming smaller and more efficient. But is it enough? Will the resulting coherent


module power, footprint and prices be competitive against 10G inside the next year or so? Te answer again is no – it is too soon. Instead of the huge 100G coherent metro surge in the next two years, it is more likely that we will see a more controlled market introduction of coherent technology. Good enough to make money? Yes. But living up to the promise of transforming an industry? A far cry from it.


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