These sub-codes can be the same or not, Hamming code, extended Hamming code, BCH code, single parity check code , and so on.
Due to the particular relation of generator matrix and parity check matrix, it can achieve unequal error protection performance with the modified decoding algorithm.
Even-numbered errors can elude the parity check on links with high error rates.
A character parity check as well as a block check to detect errors.