Checksum checker know that the received data unit is undamaged?

How does the checksum checker know that the received data unit is undamaged?

Checksum Checker or generator:
The sender subdivides data units into equal segments of ‘n’ bits(16 bits)

1. These segments are added together using one’s complement.
2. The total (sum) is then complemented and appended to the end of the original data unit as redundancy its called CHECKSUM.
3. The extended data unit is transmitted across the network.
4. The receiver subdivides data unit and adds all segments together and complement the result.
5. If the intended data unit is intact, total value found by adding the data segments and the checksum field should be zero.
6. If the result is not zero, the packet contains an error & the receiver rejects it.