How does forward error correction (FEC) allow the receiver to correct data errors?
Why Forward error correction works by sending extra, mathematically derived redundant bits along with the message data. The receiver uses that redundancy to detect and actually reconstruct bits that were corrupted in transit, without asking for a retransmission. That is why FEC modes like FT8 and some PSK/MFSK modes can decode cleanly on weak or noisy paths where an ARQ system would have to keep repeating itself.
Watch out A parity bit is tempting because it also adds a bit, but parity only flags that an error occurred, it cannot locate or repair it. Varicode is just the variable-length character encoding used by PSK31, unrelated to error correction.
Forward = fix it here, no asking back. The fix costs redundant bits.