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Computer Science IGCSEPDF 15
142 pagesrevision 46930
PDF 15
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· Secondly, the intersection of row 8 and column 5 identifies the
position of the incorrect bit that caused the error. The 1-bit at this
position should actually be a 0-bit. Therefore, byte 8 should correctly
read as:
0 0 0 1 0 0 1 0
· This correction also restores even vertical parity in column 5
(which now contains four 1-bits). The byte can either be
automatically corrected as shown above, or an error message may
be sent back to the sender requesting re-transmission of the entire
data block.
Checksum
● Sender calculates checksum using an agreed algorithm → attaches to data.
● Receiver recalculates checksum from received data.
● If mismatch → error → request resend.
Echo Check
● Data sent to receiver → receiver sends it back.
● Sender compares returned data with original.
● Limitation: Cannot tell if error occurred during sending or returning.
Check Digits
● Last digit in a code, calculated from the rest (using ISBN-13, Modulo-11
etc.).
● Used in barcodes, ISBNs, VINs.
● Detects human entry errors (e.g., mistyping, mis-scanning).