IGCSE Computer Science Paper 1 Word Scramble: 2.2 Methods of Error Detection (Modules 168–178)
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Word scramble
Unscramble the letters and write each key term on the line.
1
HSEUMKCC
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.
2
EMSUKCCH = TMS(BYEUS) % 625
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.
3
OCEH HCEKC
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.
4
OEHC EKCCH TMILI
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.
5
HKCEC TIIDG
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.
6
DEGEIWHT KHCEC IGIDT
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.
7
3NI1-SB HKCCE ITGDI
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.
Answer Key
IGCSE Computer Science Paper 1 · Modules 168–178
1HSEUMKCC → checksum — 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.
2EMSUKCCH = TMS(BYEUS) % 625 → checksum = 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.
3OCEH HCEKC → echo 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.
4OEHC EKCCH TMILI → echo 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.
5HKCEC TIIDG → check 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.
6DEGEIWHT KHCEC IGIDT → weighted 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.
73NI1-SB HKCCE ITGDI → ISBN-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.