Forward error correction (FEC)
Beginner
Extra check bits sent with the data so the receiver can fix a few wrong bits on its own.
Novice
Adding redundant bits to the data stream so the receiver can detect and correct errors without asking for a resend. It lets links run with a raw error rate far worse than what the application could accept.
Expert
Ethernet at 100G per lane and above uses RS(544,514) “KP4” FEC, which lets links run at pre-FEC bit-error ratios in the range. Decoding adds latency, which is why scale-up links weigh lighter FEC against retries.
Explained in Optics (Systems).