4.5.5.3 Error checking and correction Quiz: AQA Computer Science, Unit 5

The 20 questions

1. What is a parity bit?

   • A bit that marks the end of a file, so that a reader knows when no more data follows in the stream
   • An extra bit added so that the total number of 1s is always even or always odd
   • A bit that stores the sign of an integer, with a one meaning that the stored value is negative
   • A bit that counts the characters in a message, which is then compared with the stored length

2. What is majority voting used for?

   • Adding a checksum to each block of data, so the receiver can recalculate and compare the total
   • Converting analogue signals to digital form by taking many samples and rounding each to a level
   • Deciding a bit's value by taking the value that appears most often among repeated copies
   • Choosing the shortest route for data through a network of connected routers and switches

3. What is a checksum?

   • A list of every character in a file, stored in order so that each one can be looked up later
   • A value calculated from a block of data, sent with it, and recalculated to check for errors
   • The number of bytes in a file, which is stored at the start so the reader knows the size
   • A value that corrects every error without retransmission, so the original data is always recovered

4. What is a check digit?

   • A digit that stores the sign of a number, with zero for positive values and one for negative
   • A digit that is always zero in a valid code, so any other digit shows that the code is corrupt
   • A digit that counts the number of bits in a byte so that the byte length can be checked later
   • A digit calculated from other digits so that typing or transmission errors can be detected

5. Which error checking method adds a single bit to each byte?

   • Check digit
   • Checksum
   • Majority voting
   • Parity checking

6. What is the main limitation of a single parity bit?

   • It detects every error, including two flipped bits in the same byte, so it is fully reliable
   • It always corrects the error automatically, so the receiver never needs to ask for a resend
   • It cannot detect any errors at all, so it is useful only as a label for the data block
   • It detects an odd number of flipped bits but cannot tell which bit is wrong

7. Which method can correct errors without retransmission when several copies are sent?

   • Majority voting across several copies of the data, which can correct a faulty copy
   • A checksum on its own, which detects that a block changed but cannot correct any bit
   • A single parity bit added to each byte, which only reveals that an error has occurred
   • A check digit calculated from the other digits, which is used to detect typing mistakes

8. A byte 1011001 is sent with an even parity bit. What is the parity bit?

   • 10
   • 0
   • 1
   • Parity cannot be applied to a 7-bit value

9. A 7-bit value 1010101 is sent with an odd parity bit. What is the parity bit?

   • 11
   • 0
   • 1
   • Parity cannot be applied to a 7-bit value

10. Three copies of a bit arrive as 1, 0 and 1. What value does majority voting give?

    • 0
    • Unknown, because a vote cannot be decided
    • 1
    • An error, because the copies disagree

11. Data bytes 10, 20 and 30 are summed modulo 256 to form a checksum. What is the checksum?

    • 70
    • 600
    • 60
    • 50

12. A block sent with a single even parity bit arrives with exactly two bits flipped. Can the parity check detect this?

    • No, because parity checks only work with odd-length messages of at least sixteen bits
    • Yes, the receiver can correct both bits, since the parity bit records which bits were flipped
    • No, two flipped bits leave the parity unchanged, so the error goes undetected
    • Yes, the parity will always show the error, because any change in the block alters the parity

13. Five copies of a bit arrive as 1, 0, 1, 1 and 0. What does majority voting give?

    • An error, because five copies cannot agree
    • No majority, because the copies are split
    • 0
    • 1

14. Digits 4, 7 and 2 have a check digit equal to their sum modulo 10. What is the check digit?

    • 3
    • 2
    • 13
    • 9

15. An 8-bit checksum is the sum of bytes 200 and 100 modulo 256. What is the checksum?

    • 100
    • 56
    • 300
    • 44

16. Why is majority voting more robust than a single parity bit for correcting errors?

    • It needs only one extra bit per block of data, so it adds very little to the total size
    • It always gives the correct answer even if all copies are wrong, because the vote uses the majority
    • It can identify and correct a faulty copy when most copies are unaffected, which parity cannot do
    • It detects an even number of errors that parity misses, so it is stronger for every block

17. Three copies of a bit are received, and two of them are corrupted in the same way. What happens?

    • The parity bit identifies the wrong copy, so the receiver discards it and keeps the rest
    • The error is detected and corrected to the true value, because the vote identifies the wrong copies
    • The wrong value is accepted, because a majority of the copies is wrong
    • The vote is a tie, so the error is reported to the sender and the block is requested again

18. Why can a checksum miss some errors?

    • Different data blocks can produce the same checksum, so some corruption leaves it unchanged
    • Checksums are computed from the length only, so content changes are ignored
    • Checksums only apply to single bits, so block errors are never checked
    • Checksums store the original data, so they cannot compare it

19. A block of 8 data bits is sent with one even parity bit. Which corruption is always detected?

    • Any single bit flipped in the block
    • Any error that changes the parity bit and one data bit
    • Two bits swapped in position
    • Two bits flipped in the same byte

20. Which error does a check digit equal to the sum of the digits modulo 10 fail to detect?

    • Changing one digit by exactly 1, which alters the sum by one and so changes the check digit
    • Changing a single digit to a different value, which changes the sum that the check digit uses
    • Adding an extra digit to the end of the number, which changes the length as well as the sum
    • Swapping two digits, since the sum of the digits is unchanged

Related quizzes

• Natural, rational, irrational and real numbers Quiz · 4.5.1.1 · 20 questions
• Ordinal numbers, counting and measurement Quiz · 4.5.1.6 · 20 questions
• Number bases Quiz · 4.5.2.1 · 20 questions
• Bits and bytes Quiz · 4.5.3.1 · 20 questions
• Unsigned binary Quiz · 4.5.4.1 · 20 questions
• Unsigned binary arithmetic Quiz · 4.5.4.2 · 20 questions
• Signed binary using two's complement Quiz · 4.5.4.3 · 20 questions
• Numbers with a fractional part Quiz · 4.5.4.4 · 20 questions
• Rounding errors Quiz · 4.5.4.5 · 20 questions
• Range, precision and normalisation of floating point Quiz · 4.5.4.7 · 20 questions