Not all 400G is created equal… 400GE FEC measurement:
The IEEE standard body implemented Forward Error Correction or FEC as mandatory for 400G transmission. The reason why Forward Error Correction is mandatory is because compared to 100G NRZ transmission where it supports error-free transmission, 400GE such as FR4 and DR4 optics always has some errors in the transmission and requires FEC to correct those errors. The graph below is a simple visual to explain the differences with and without Forward Error Correction.
You see the 100GE transmission without FEC is purple, whereas the errors increase in the transmission due to aging, fiber bending or longer distance, there is a gradual dropoff in throughput once the FEC threshold is past. However, 400GE transmission throughput drops off significantly once the FEC threshold as the bit errors accumulate. So the challenge here is determining how many errors are being corrected with FEC and if you are within your FEC budget. You may well be within the correctable FEC limit or at the edge.
The goal here is to verify if the 400G optics, cables and equipment are within the Forward Error Correction threshold limit. However, the results can be deceiving depending on how the data is presented. For example, I’m displaying the results from two different 400 devices. On the top row are the initial 400G FEC measurement for two different optical device, where the device on the left has a better margin of corrected errors than the device on the top right.
How can you tell? The device on the top right is corrected 11 to 12 symbol errors per codeword, highlighted in yellow. What happens when both devices go through the same optical attenuation? The one on the left stays within the FEC tolerance limit, as seen in the bottom left image. However, the device on the right exceeds its FEC tolerance limit and begins receiving uncorrected errors, as seen in red on the table. This is an example of although both 400G devices began with the same FEC performance, they reacted differently when attenuated. This is a common measurement taken by vendors to verify their devices FEC performance.
Is My 400GE link good or Bad?
Just to highlight the last point in a little more detail, I’ve labeled the 400G FEC distribution to get a better view of the test results from 400G Data Center Interconnect installation at an Edge network hub. The point here is the symbol Error rate count in the graph changes over time based on multiple factors within an Edge deployment, such as heat, temperature and use. The higher the symbol error rate per codeword, the higher the risk of failure. We color code it so it makes it easier to see. Green is a low symbol error rate per codeword, only 1 to 3 errors. Yellow is warning as you approach a higher number of symbol errors at 10 to 12 errors per codeword. Red means you are receiving more errors than the FEC can keep up with and ultimately impacting your service performance and throughput, which we are trying to avoid.
