Checksum is a calculated value that is used
to test data integrity. Errors can occur when data is transmitted or when it is
written to disk. One means of detecting such errors is use of a checksum.
Checksum is a value calculated for a given chunk of data by sequentially combining
all the bytes of data with a series of arithmetic or logical operations. After
the data is transmitted or stored, a new checksum can be calculated (using the
possibly faulty transmitted or stored data) and compared with the original one.
If the checksum do not match, an error occurred, and the data should be
transmitted or stored again. If they do match, the transmission or storage was
probably error-free. Checksums are simple validation mechanism. They cannot
detect all errors and they cannot be used to correct errors.
The parity and LRC methods are not very reliable, when more than error
occurs within a character or message. One major advantage of the CRC method is
its ability to detect multiple errors within any length of message.
Another major advantage of checksum is that it is simple to implement.
In this method, each character being transmitted is exclusive ORed with an
accumulated total of all previous characters. The final accumulated total is
the checksum character sent with the message. Once again, at the receiver
exclusive ORing all the characters and the transmitted checksum should produce
a result of zero. Any other result indicates that an error has occurred.
In comparison with CRC, checksum is more likely to experience similar
values for different messages. While the odds that enough errors would cause a
duplicate checksum to the one originally sent are reasonable high, that
possibility is still far more likely than a similar occurrence using the CRC
method.
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