UNDERSTANDING INTEROPERABLE OPENZR+ There is no industry standardisation activity currently working to define ZR+ modes, but that does not mean that the industry has no path to interoperable ZR+. Multiple vendors are developing DSPs that will support all of the standards discussed above. By simply defining a data path that includes the appropriate functionality, an interoperable ZR+ mode can be established, hereafter called OpenZR+. This mode maintains the simpler ethernet-only host interface of 400ZR, but adds support for multi-rate Ethernet and multiplexing capabilities at 100G, 200G, 300G or 400G line interfaces with higher dispersion tolerance and higher coding gain using openFEC from the OpenROADM standard (see Table 1 below). These enhanced modes will allow an OpenZR+ module in a QSFP-DD or OSFP form factor to support reaches well beyond 400ZR.
available in the market, some hyperscale network operators chose to take a different approach at 400G. In late 2016, the concept of pluggable coherent modules had been well established with CFP and CFP2 form factors. Extrapolating the trends in power dissipation and size reductions for coherent, it seemed feasible to support a low-power coherent implementation in the smaller form factors that were being defined for 400G client interfaces, QSFP-DD and OSFP. These form factors were expected to support approximately 15W power dissipation at the time. The hyperscale network operators approached the Optical Internetworking Forum (OIF) and proposed a new project targeting DCI edge applications up to 120km, with an objective to target power dissipation of less than 15W. By targeting a very specific application and starting the standardisation before vendors were well down the development path, OIF hoped to accelerate the standardisation activity. OIF was quite successful in this effort. Technical discussions proceeded quickly with power optimisation taking the highest priority in each decision. A comparison of the power trade-offs between modulation format and baud rate led to the adoption of 16QAM at approximately 60Gbaud. Several FEC proposals were debated and a concatenated (hard-decision + soft- decision) FEC, which supported low- power and sufficient coding gain for the edge application, was selected. In less than one year, OIF defined most of the technical details of the 400ZR interface, which helped to motivate increased industry investment in pluggable, interoperable coherent interfaces. OPENROADM While 400ZR benefited from a well- defined target application, some carrier applications needed features that were excluded from the 400ZR standard. For example, the scope of 400ZR is limited to a 400GbE interface to the host board. Carrier applications need the flexibilityto support multiple rates, multiplexing functionality and additional protocol types such as OTU4 DWDM. Additionally, these carrier applications have more stringent performance requirementsthan 400ZR and are not bound by theclient optic form factors, so these applications can permit multiple form factors including CFP , CFP2 ACO/DCO and line card solutions.
With these different priorities, the OpenROADM MSA defined a framing approach that supported line rates of 200G, 300G or 400G, greater host interface flexibility and chose a new high performance soft-decision FEC, called openFEC, which offered higher coding gain more comparable to proprietary implementations typically used in long- haul applications. In the same timeframe, CableLabs developed a coherent interconnect standard to address the access aggregation requirements of the cable industry. The data rates targeted for this standard were 100G and 200G. This application has its own unique set of requirements but CableLabs recognised the benefit to align with standards activity taking place in the other bodies. It was decided that alignment with the 100G and 200G standards in OpenROADM offered the best combination of benefits for this application, including the high performance openFEC for 200G. The CableLabs standard utilises a common protocol with OpenROADM, but defined optical specifications that address the specific requirements of the cable access aggregation application. ZR+ TERMINOLOGY CAUSING CONFUSION As stated earlier, the 400ZR project focused on a very well-defined target application and specific power budget restrictions. Over the last couple of years, equipment vendors have made impressive improvements in the thermal designs of their next generation systems. Leading switch vendors have indicated that they believe they will be able to support 20W in a QSFP- DD or OSFP module. 400ZR module vendors are looking to add additional features and performance to these modules to increase the addressable market for these products. The term ZR+ was loosely introduced into the vernacular to specify or capture the broader modes of operation, but it has created some confusion in the industry. The most straightforward definition of ZR+ is simply: a 400ZR module, with enhancements such as support for multiple Ethernet data rates (or multi- rate operation from 100G–400G) and/ or higher gain FEC capable of supporting longer reach DCI interconnections in a QSFP-DD or OSFP form factor.
400 ZR Open ZR+ OpenROADM
Target Application Edge DCI
Carrier Metric/ LH 100 - 400 GbE 100 - 400 OTN
Reach <100km >100km*
CFP2 or other
<15W <20W Not Defined
Leverages other standards work
Table 1: Next Generation Pluggable Coherent Module Types *Higher performance with various reaches supported based on modulation format and data rate. The OpenZR+ modes are supported by merchant DSP vendors who have exchanged test vectors to ensure the interoperability of these OpenZR+ implementations. The availability of merchant DSP solutions supporting OpenZR+ will further expand the ecosystem of module vendors supporting OpenZR+. In addition, full OpenZR+ implementation details can be made available to additional vendors interested in supporting this ecosystem. Through common sense implementation of functionality that is already well-defined in existing standards, OpenZR+ allows network operators to achieve all of the benefits of ZR+ without sacrificing interoperability.
THE AUTHORS This feature was written by the following experts:
TomWilliams, VP Marketing, Acacia Communications
Atul Srivastava, CTO, NTT Electronics America
Scott Swail, VP Business Development, Lumentum
Xiaoxia Wu, Sr. Staff Optical System Engineer, Juniper Networks
Mitsunori Hamada, Sr. Director, Marketing, Fujitsu Optical Components America, Inc.
ISSUE 18 | Q3 2019
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