This 64-bit counter tallies frames arriving off the ring fiber that failed a standard CRC/frame-check-sequence validation, over the current, still-running 15-minute window. This is the classic signature of line-level noise or fiber degradation, so tracking it over successive intervals is how an admin distinguishes a one-off blip from a fiber segment or connector that is steadily getting worse. A CRC error count that doubles interval over interval for several hours in a row would justify inspecting or cleaning that segment's fiber connectors before the link degrades further.