IGCSE Computer Science Paper 1 Definitions Worksheet: 2.2 Methods of Error Detection (Modules 168–178)
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Key terms
Write what each term means, in your own words.
1
checksum
2
checksum = sum(bytes) % 256
3
recalculate and compare
4
echo check
5
echo check limit
6
check digit
7
weighted check digit
8
ISBN-13 check digit
Answer Key
IGCSE Computer Science Paper 1 · Modules 168–178 · Student wording will vary for definitions
1checksum — A value calculated from a block of data by a set rule and sent with it. The receiver repeats the calculation and compares the results. If they differ, an error occurred.
2checksum = sum(bytes) % 256 — Add all the byte values in the block, then keep the remainder after dividing by 256 so the result fits in one byte. Example: 200+100+50=350, 350 mod 256 = 94.
3recalculate and compare — The receiver recalculates the checksum from the received data and compares it with the one sent. Match means no error detected; mismatch means error, so resend is requested.
4echo check — The receiver sends a copy of the received data back to the sender, who compares it with the original. A match means no error was detected; a mismatch means the data is sent again.
5echo check limit — An echo check cannot tell if the error happened on the way there or back, and it can miss errors if both trips are damaged the same way.
6check digit — An extra digit added to the end of a number, calculated from its other digits. It is recalculated on entry and compared, to detect data entry mistakes.
7weighted check digit — Multiply each digit by its weight, add them up, divide by 11 and use the remainder (often 11 minus it) as the check digit. A 10 is written X in ISBN-10.
8ISBN-13 check digit — The last digit of an ISBN-13, found by weighting the first 12 digits 1 and 3 alternately, adding them, and choosing the digit that makes the total a multiple of 10.