Understanding the condition of the PCS lanes is crucial to troubleshooting 100G links during network deployments.
At the most basic level understanding that there are errors at the PCS indicates that the root cause of the fault lies with the physical signalling and not at the MAC layer or any of the high network layers and that there is a very low layer fault.
Beyond this, understanding how the PCS lanes are mapped to the CAUI can also help identify the root cause of the fault.
As an example; if only two PCS lanes are display error or defect conditions and those two lanes are known to be multiplexed together into the same CAUI lane then this would suggest that the fault actually lies within the CAUI and that troubleshooting should focus on areas such as the CFP connector.
Consider the case of using an LR4 CFP for the optical transceiver; each of the 4 wavelengths used on the link will be carrying 5 PCS lanes. In the case of 5 of the PCS lanes being in errors this may indicate the errors being specific to that wavelength and so areas of investigation should include the individual transmitters and receivers within the CFP.
If a 10 lane CFP (SR10 or LR10) is being used then each wavelength (in the case of the LR10) or fibre (in the case of the SR10) would each be carrying two PCS lanes. In this case then if two PCS lanes within the same CAUI lane are found to contain errors or defects then as well as investigating the CAUI the two lanes would also be carried on the same wavelength or fibre. In this case once again the optical components should be investigated at both ends of the link. In the case of an SR10 based link the multi-fibre cable should also be checked as it may be possible that one of the individual fibres has been damaged within the cable.
In summary understanding the number of PCS lanes that have errors and how those lanes are mapped into the CAUI can be critical in narrowing down the root cause of a problem when testing and troubleshooting 100Gb/s Ethernet links.