ITU standardization experts have accomplished a major feat of engineering in the delivery of multi-vendor interoperable 100G DWDM coherent line interfaces. The standard interfaces will accelerate industry innovation to achieve greater capacity in metro networks.
The achievement of a standard for multi-vendor interoperability represents an industry first.
“This is the first 100G coherent multi-vendor interoperable specification, in fact the first coherent interoperable optical specification of any sort,” says Pete Anslow, Ciena, Associate Rapporteur for ITU’s standardization work on ‘characteristics of optical systems for terrestrial transport networks’ (Q6/15).
‘Interoperable’ is the key word, explains Anslow.
The revised ITU standard – ITU G.698.2 “Amplified multichannel DWDM applications with single channel optical interfaces” – supports a phase-modulated signal using DP-DQPSK (Dual Polarization – Differential Quadrature Phase Shift Keying).
DP-DQPSK is a “complex modulation format,” says Anslow. “It’s a major reset on the modulation format, which gave rise to a challenging question: ‘how should we define the quality of an optical signal with the data encoded in the phase?’”
ITU-T Study Group 15 has been first to answer to this question with a metric called ‘error vector magnitude’. The metric defines the quality of a transmitter, a consideration fundamental to multi-vendor interoperability.
“The term ‘error vector magnitude’ comes from the wireless world but we have repurposed it for optics and done a considerable amount of work to show that the metric correlates properly with actual link performance,” says Anslow. “Our success in adapting this metric to optics is an industry first, which we believe will also be an industry-leading innovation in applying this specification methodology to optics.”
Other standards bodies are beginning to consider the inclusion of the metric in their specifications for coherent line interfaces.
“The race is on to provide a full multi-vendor interoperable 400G spec,” says Anslow.
ITU-T Study Group 15 has started validating the ‘error vector magnitude’ metric for 16QAM modulation with a view to standardizing 400G line interfaces.
“This validation started in the same meeting we completed the 100G version,” says Anslow. “Work is in progress, this work is gathering pace very quickly, and it won’t be very long before we do a 200G and 400G version.”
ITU-T Study Group 15 has also standardized two digital frame formats, including a forward error correcting code, to run over these new 100G line interfaces.
ITU G.709.2 “OTU4 long-reach interface” provides an OTU4 frame format capable of transporting 100G clients such as 100GBASE-R Ethernet signals, or multiple, multiplexed, lower-rate client signals.
ITU G.709.3 “Flexible OTN long-reach interfaces” provides an OTUCn frame format for a signal greater than 100G, inversely multiplexed over multiple 100G line interfaces. This provides a mechanism to transport client signals at greater than 100G over multiple 100G line interfaces, including 200GBASE-R and 400GBASE-R Ethernet signals.
The international standards developed by ITU-T Study Group 15 (Transport, access and home) detail technical specifications giving shape to global communication infrastructure. The group’s standards define technologies and architectures of optical transport networks enabling long-haul global information exchange; access networks through which subscribers connect; and home networks connecting in-premises devices and interfacing with the outside world.
Learn more about ITU-T Study Group 15 in an ITU interview with the group’s Chairman, Stephen Trowbridge.