in practice, the last two variations are invariably used together. they change the transmitted crc, so must be implemented at both the transmitter and the receiver. while presetting the shift register to ones is straightforward to do at both ends, inverting affects receivers implementing the first variation, because the crc of a full codeword that already includes a crc is no longer zero. instead, it is a fixed non-zero pattern, the crc of the inversion pattern of formula_2 ones.