Autumn 2019 - Optical Connections Magazine


OpenZR+ Offers Performance and Interoperability

Standardisation activities have defined a variety of interoperability modes supporting operation ranging from 100G to 400G. Here, five experts review the benefits of combining elements from 400ZR and OpenROADM, two existing standards, to define and deliver OpenZR+; an open, flexible and interoperable coherent solution in a small form factor pluggable module.

I nitially adopted in long-haul applications, coherent optical interfaces have traditionally been based on proprietary implementations that required equipment from the same vendor on both sides of the optical link. This approach is commonly referred to as a “book-ended” solution. In these long- haul applications, small differences in performance can differentiate one vendor from another by reducing the number of costly regeneration elements in the network. In recent years, coherent technology has been adopted for applications that have different priorities when making trade-offs between performance, power dissipation and interoperability. THE CASE FOR STANDARDISED COHERENT INTERCONNECTS Standardization has a long history in optical transport networks going back to the SONET/SDH standards that were defined in the 1980s. When coherent technology was initially introduced, it created new challenges for interoperability. Coherent interconnects often utilised special framing, as well as proprietary signal processing and Forward Error Correction (FEC) algorithms. Initial standardization efforts at 100G demonstrated that standardisation was possible, but adoption was limited due to issues with timing and performance. As network operators looked toward 400G,

several industry trends drove greater interest in standardised interfaces. With each new product generation, coherent technology has been adopted for applications with shorter reach. Moving from long-haul to metro and now to edge and access, volumes tend to increase, and standardisation takes on a higher priority for network operators. Looking toward data rates beyond 400G, coherent interconnects are likely to be utilized for applications inside data centres, where interoperability is considered mandatory. Increasingly, pluggable module form factors are being utilised for coherent interfaces. The first pluggable coherent modules utilised the CFP form factor, followed later by the CFP2 form factor. While these solutions offered many benefits of pluggability, such as a pay- as-you-grow commercial model, they were still primarily deployed in transport equipment because these form factors were larger than the QSFP28 modules that were being utilised for 100G client optic interfaces. In 2017, a few vendors and network operators identified 400G as an intersection point for the industry to support coherent optics in the same form factors as emerging high-volume client optics, such as QSFP-DD and OSFP.

Driven by service providers in the Open ROADM MSA and ITU, an interoperable standard was defined that could be supported by multiple vendors, while leaving opportunity for differentiated proprietary implementations with higher performance. In particular, the standard defined a hard-decision FEC and differential encoding. Hard-decision FEC offers less coding gain than the soft- decision FEC algorithms widely used in proprietary implementations. Differential encoding simplifies interoperability but also introduces a performance penalty compared to non-differential encoding. Though supported by multiple equipment vendors, this 100G standard has not been widely deployed by network operators to date. There are two primary reasons for this: Widespread deployment of standardised architectures requires a complete ecosystem beyond the optical interconnect. Other elements of this ecosystem, such as the management software, have been under development in parallel. The efforts to define a standard began materialise. In this time, the performance gap between the standard and proprietary implementations became quite large. 400G ZR Recognising the challenge of standardising after solutions are already after 100G solutions were already available and took several years to

100G STANDARDISATION As previously mentioned, the first efforts to standardise coherent interconnects took place at 100G.


| ISSUE 18 | Q3 2019

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