Protocol 2 considers noise on the channel and requires feedback from receiver.
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For CRC, if reciever finds there is a remainder, there must have been an error.
If the error rate of physical link is high, error detection and retransmission is usually more efficient
Error-detecting codes only include enough data to let the receiver determine whether the data is faulty
Hamming code (using even parity check) is used to correct 1 bit error. If the bit stream received is 001100011100, it is a correct transmission.