cyclic redundancy check in DLL

Question

A bit stream 11100110 is to be transmitted using CRC method. The generator polynomial is X4+ X3 + 1.

1. What is the actual bit transmitted ?
2. Suppose the third bit from the left is inverted during the transmission. How the error is detected.
3. How the generator polynomial is already known to sender side as well as receiver side, please make this clear.

Solution :

Here, FCS will be 0110 since n = 4. So actual bit transmitted is >> 11100110 0110

I am confused with the problem 2, 3. please reply my 2, 3 questions. Thank You!

Answer 1

2. If you know how to generate the 0110, then invert the bit and generate a new CRC. You will see that it's different. On the other end when you compute the CRC of the eight bits sent, it will not match the four bit CRC sent.

3. The two sides agree a priori on a protocol, that includes the definition of the CRC to be used.