How receiver detects error
The frame is 10011101. The generator is 1001. The message after appending the three zeros is 10011101000. The reminder on dividing 10011101000 by 1001 is 100. So, actual bit string transmitted is 10011101100. The arriving bit stream with an error in third bit from the left is 10111101100.Dividing this by 1001 produces remainder 100 that is different from zero. Therefore, the receiver detects error and can ask for a retransmission.