Recent research from IHS Markit indicates that data center interconnections are the fastest growing segment for coherent transceivers. Currently, metro and long-haul DWDM equipment is used to connect data centers around the world, but metro and long-haul DWDM solutions are overkill to interconnect data centers in campus and urban metro areas. Metro-access DCI are point-to-point connections with fiber reach between 40 and 120 km, making it possible to use unamplified links at least to 100 km, well beyond the capabilities of direct detect datacom transceivers. Coherent telecom transceivers can be used, but they have higher performance than required, are more complex, and are generally offered at a higher price point. They also use proprietary SD-FEC modes that are not interoperable. In response to this, the Optical Interconnect Forum (OIF) has developed the 400G ZR MSA to define specifications for 400GbE transceivers optimized for 80–100 km unamplified links using a grey channel in the center of C band. There is also strong interest from telecom and cable service providers for a DWDM version of the 400G ZR, so the OIF developed a colored version of the 400G ZR specification.