Optical Connections Magazine Spring 2024



W hile the industry focus on a high- volume metro point-to-point data center interconnect use case can be credited for early 400G coherent pluggable market success, 400G market expansion can be attributed to additional 400G standardized variants that enabled applications beyond metro data center interconnect. After the Optical Internetworking Forum (OIF) released the 400ZR implementation agreement to address the metro data center interconnect use case, a variant commonly referred to as 400ZR+ included a higher performing forward error correction code, called oFEC, that resulted in additional optical signal- to-noise (OSNR) margin compared to 400ZR. The improved OSNR translated to a longer reach capability, especially over an amplified line-system. With the industry now transitioning toward 800G coherent pluggable applications,

coherent standardization efforts are also including interoperable features to enable a host of network operator applications beyond metro point-to- point data center interconnects. In this article, we will review how the path to 800ZR/ZR+ has similarities and differences compared to the 400ZR/ZR+ journey, and also touch on 1600ZR/ZR+ efforts. OFEC IN 800ZR PROVIDES ROBUST BASELINE CAPABILITY FOR HYPERSCALERS Compared to 400G, there are two noteworthy industry agreements that are different at 800G. The first is the incorporation of a high-performance forward error correction specification adopted by OIF for 800G. When OIF introduced 400ZR, the effort was laser focused in addressing pressing hyperscale network operator needs for a 400 gigabit Ethernet coherent optical interconnect capable of achieving up to a 120km amplified reach. In the OpenROADM MSA, service providers wanted to leverage this

400G pluggable technology, but required higher performance than 400ZR and the inclusion of Optical Transport Network (OTN) protocol support. Adoption of oFEC in OpenROADM enabled module suppliers to offer improved receiver performance to address more challenging and longer reach links, including networks containing multiple reconfigurable optical add-drop multiplexers (ROADMs). The OpenZR+ Multi Source Agreement (MSA) formalized the use of oFEC with Ethernet optimized framing, which removed the extra OTN overhead, allowing for slightly lower baud rate transmission. OpenZR+ stood alongside the OIF 400ZR and OpenROADM to offer a full range of capabilities to network operators for 400G optical interconnects. When OIF efforts moved to 800ZR, a decision was made to stay with 16QAM transmission with double the baud rate of 400ZR. To close the gap in performance at twice the baud rate, a decision was made in OIF to adopt oFEC as part of the solution to address the same network applications as 400ZR.

Figure 1. How the current approach at 800G to enhance OSNR performance differs from the previous approach at 400G


| ISSUE 36 | Q1 2024


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