Data integrity and error checking
Match common transmission and entry errors with parity, block checks, echo checks, checksums, check digits and ARQ.
Section Component 1 · Computer Systems and Logic
Mapped to the Cambridge IGCSE Computer Science 0265 syllabus for exams in 2029. These original explanations and self-checks are independent study support, not Cambridge papers, official marks or grades. Practical programming is taught with worked code and trace tasks; an executable Python workspace and timed digital-exam simulation are not yet available.
Before you begin
- Use the 2029 syllabus for this course. Cambridge specifies Python 3.10 or higher for programming and digital exams for both components. The syllabus currently identifies this route for 2029 only; confirm later-year availability with Cambridge and your school.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before data integrity and error checking, which statement is the safest starting point?
What you will learn
- Explain how errors arise in storage, transmission and entry
- Describe the specified error-detection and correction methods
- Calculate parity and parity-block checks
Errors can occur at different stages
A bit can change during transmission, a storage medium can be damaged, or a person can mistype a value. Integrity controls detect or correct changes, but a detector does not necessarily identify the exact wrong bit.
A parity bit is added so the number of 1s is even or odd as required. A parity block check applies parity across rows and columns of a block, helping locate some errors. An echo check sends entered data back for comparison; a checksum combines data values so a receiver can check for inconsistency.
Check digits and ARQ serve different roles
A check digit is calculated from other digits and appended to a number. It can detect common input errors in ISBNs, bar codes, airline tickets or bank account numbers; it is not a secret and does not prove a transaction is authorised. The syllabus does not require calculating check digits or checksums.
Automatic repeat reQuest (ARQ) uses acknowledgements. A positive acknowledgement indicates that data arrived acceptably; a negative acknowledgement or timeout can cause retransmission. The sender and receiver coordinate so lost or damaged data can be sent again.
Pause and explain
Which habit best checks your understanding of data integrity and error checking?
Worked example
Find the even-parity bit to append to 1011001.
Show the worked solution
- Count the existing 1s: there are four.
- Four is already even.
- Append 0 so the total remains even.
Answer Parity bit 0
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Add an odd-parity bit to 110010. State the resulting bit count.
Give me a hint
Count the 1s and choose a bit that makes the total odd.
Compare my reasoning
- There are three 1s in 110010.
- Three is already odd, so append 0.
- The resulting data still contains three 1s.
Append 0; the total number of 1s is 3 (odd).
Look for these in your work
- Count correct
- Parity chosen
- Odd result verified
A learner types a long account number and the system displays it back before saving. Name the method and explain its purpose.
Give me a hint
Identify the human input step and the comparison performed.
Compare my reasoning
- The entered value is echoed back to the learner.
- The learner compares the displayed value with the source.
- This is an echo check, used to detect some data-entry mistakes.
Echo check; it lets the user compare the entered value with the original.
Look for these in your work
- Method named
- Comparison described
- Purpose stated
A file is split into packets. The receiver detects that one packet failed its check and sends a negative acknowledgement. Explain what should happen next.
Give me a hint
Use the ARQ sequence of error indication, resend and acknowledgement.
Compare my reasoning
- The negative acknowledgement tells the sender the packet was not accepted.
- The sender retransmits that packet.
- The receiver checks the resent packet and can acknowledge successful receipt.
Retransmit the failed packet, then check and acknowledge it when received correctly.
Look for these in your work
- Negative acknowledgement interpreted
- Resend action
- Successful receipt confirmed
Common mistakes
- Thinking a single parity bit can correct every error.
- Using even parity when the question specifies odd parity.
- Confusing error detection with proof that data is authentic.
What can you explain now?
What does a negative acknowledgement usually tell the sender?
Compare with the explanation
The data was not received correctly and may need retransmission
ARQ uses acknowledgements to decide whether to resend data.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which methods detect errors, which can prompt correction, and what does parity actually guarantee?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.