Issue link: https://www.e-digitaleditions.com/i/1327492
January/February/March 2021 I 41 FIGURE 5: Available options for the road to 400G. of light (100GBASE-FR1/100GBASE-SR1). The multi- mode side is lagging behind, but there is a solution in development that will support somewhere between 30-70 meters over a single wavelength that will be 50G, PAM4 and encoded up to 100G. That is an attractive option because other solutions available are inflexible with respect to breakout or do not line up with current cabling lane counts. A very short reach solution of 30 to 50 m could replace a lot of the copper solutions in hyperscale interconnects and possibly displace some of the active optical cable solutions. Looking at Figure 5 for higher speeds, the three axes show the available options. The vertical axis shows the lowest risk option—using lower data rates and adding lanes with 8, 16 and 32-fiber options. Currently, the 32-fiber is off the board and will not be implemented by any original equipment manufacturers (OEMs). That was released as SR16, but it has not gained any traction. The multifiber connector is lower risk because it involves taking well-known modulation rate technology and scaling it up by adding optical fibers. However, solutions greater than 12 fibers are not customer friendly because of the plug-and-play ecosystem that exists based on 12 and 8 fibers. Not many people are going to be willing to do the forklift replace for a 16-fiber solution. The 400G SR8 transceivers are really meant to be the "sweet spot" to enable eight, 50G breakouts from switch to server network interface cards (NICs). As for the third axis, encoding creates a technical challenge. The reach of the solution is reduced and the power budget becomes constrained because of the generation of more levels per clock cycle, and it is difficult to control the optical modulation amplitude. Even with this technical challenge, PAM4 is the defacto technology for 50G as it will be for 100G and potentially 200G lanes. Higher Speed (>40G) 'Toolbox' Lowers Risk >12f Not Customer Friendly Fiber per connector Fiber Capacity Most PMDs above 40G use FEC (Forward Error Correction) 8F, 16F, and 32F options Up to 8 WDM channels Baud Rate/Modulation Scheme Increased Cost No Breakout capability Technical Challenge Lowers Reach/Power Budget 25GBaud-NRZ 25GBaud-PAM4 50GBaud-NRZ 50GBaud-PAM4 Multi-core fiber has multiple cores packed inside a single 125 µm diameter fiber.