Coherent Optics lays out new ITU standard for submarine networks

submarine networks
Since the launch of the first Atlantic cable in 1858, submarine cables have provided the backbone for telecom networks globally. Thanks to the relentless efforts of the undersea cable ecosystem providers, we now boast a robust communications network of 1,000,000 km of submarine cable exchanging millions of Terabits of Internet data every day across the globe.

The evolution of cables from the gutta-percha copper wire to the modern fiber powered by coherent optics technology was so swift that it outpaced the existing ITU-T standard that was used to measure the performance of undersea cables.

Obviously, this resulted in several inconsistencies in the methods used in quantifying the performance of subsea cable network. As noted in Ciena’s corporate blog, Power Budget Table (PBT) was one such dilemma that called for the industry’s attention towards codifying an industry standard that fitted into the fast evolving coherent optic era in subsea networks.

Power Budget Table or PBT is a term used by cable system industry to quantify the performance of a subsea optical system. The PBT algorithm was introduced several years ago — much before the coherent technology was introduced in undersea cable network. For submarine cable operators, PBT was a way to ensure that the equipment met all the performance criteria and any failure on its part to deliver the results could bring significant financial penalties to the equipment vendor.

The coherent revolution in the submarine transport industry challenged the mathematics involved in PBT. Coherent transponders appeared in the market in around 2010 as an upgrade to 40 Gbit/s per wavelength of old systems running legacy 10 Gbit/s. The PBT calculation did not fit into the new systems with higher bit rates per channel. As bit rates per channel increase, fiber impairments that can severely degrade transmission performance and signal quality also increase. These systems are also likely to face higher chromatic dispersion, polarization mode dispersion (PMD) and optical noise.

Priyanth Mehta, a key strategist in Ciena’s submarine systems development team and one of the people leading the effort to update ITU-T G.977 standard, says PBT is an anachronism in the coherent transmission era because it was based on the mathematical assumptions pertaining to the traditional “on/off” optical modulation called intensity-modulated direct-detection or IMDD. The coherent technology does not have the same properties for transponder noise or impairments as IMDD, Mehta said.

As Mehta points out, some significant manipulation was required to make the numbers fit the existing ITU-T G.977 standard. Industry standards like ITU-T G.977 were created to facilitate a level-playing field where all parties including service providers and equipment vendors could compete in, and so they need to evolve to keep pace with the industry requirements, Mehta said.

With submarine cable networks now driving the need for 100Gbit/s line speeds, the industry is facing serious challenges; a conflict arising from applying PBT in modern cable systems results in significant inconsistencies in the PBT methodology from different vendors, making it impossible to compare solutions on an even playing field, says Elizabeth Rivera Hartling, a Ciena field systems engineer who has been an active participant in the PBT initiative.

Several industry initiatives were launched to address the PBT dilemma, but without any results until a consortium led by Ciena, Verizon and Orange Labs published a conference paper at SubOptic 2013 with a proposal for an entirely new PBT compatible with all transponder technologies. As an immediate consequence, the ITU began the process of updating the PBT with the new suggestions from the consortium.

As the industry witnessed greater adoption of 100Gbits/s networks in 2013, the demand for the new “coherent PBT” became even higher. By late-2014, the ITU agreed on a set of changes to the ITU-T G.977 standard that included a modified Power Budget Table designed to meet the requirements of coherent optical transport.ITU has now approved the draft standard, and the new standard is expected to be released in a few months.

Ryan Perera, Country Head, Ciena img1

By Ryan Perera, India Country Manager, Ciena

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