Check transmitted data with a checksum and echo
Compare checksum recalculation with echo comparison and identify what each method can reveal.
This sequence follows Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. It covers subject content sections 2.1–2.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Who recalculates a transmitted checksum?
What you will learn
- Describe how a checksum is produced and checked
- Describe how an echo check compares sent and received data
- Compare their communication overhead and limitations
A checksum is recalculated at the receiver
The sender calculates a checksum from the data and sends it alongside the data. The receiver calculates a new checksum from what arrived and compares the two. A mismatch signals that the data may have changed.
Apply and check the rule
For an echo check, the receiver sends the received data back to the sender. The sender compares it with the original. Echoing uses extra transmission time and bandwidth; a checksum is compact, but different data can sometimes produce the same checksum.
Pause and explain
What is one drawback of an echo check?
Worked example
The receiver recalculates a checksum and gets a different value from the one sent. What should it infer?
Show the worked solution
- The values do not match.
- The received data may have changed or been corrupted.
- The check detects a possible error but does not itself restore the data.
Answer Treat the data as potentially corrupted and request a reliable resend if the protocol provides one.
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.
A sender's checksum is 57, but the receiver calculates 61. What does the mismatch indicate?
Give me a hint
Compare the two checksum values.
Compare my reasoning
- The values are different.
- The received data may contain an error.
- The receiver should not treat it as verified.
A possible transmission error is detected; the data should be treated as suspect.
Look for these in your work
- Mismatch identified
- Error inference stated
- Safe handling stated
Describe the sequence of an echo check.
Give me a hint
Follow the data from sender to receiver and back.
Compare my reasoning
- The sender transmits the data.
- The receiver sends the received data back.
- The sender compares the returned copy with the original.
Send, echo back, compare with the original; a difference signals an error.
Look for these in your work
- Original send
- Echo step
- Comparison
A sensor network has very limited bandwidth. Compare an echo check with a checksum for routine error detection.
Give me a hint
Weigh extra return data against a compact summary and its limitations.
Compare my reasoning
- An echo returns the full data, which can consume additional bandwidth and time.
- A checksum sends a smaller summary for comparison.
- A checksum is more efficient but does not guarantee that every possible change produces a different value.
A checksum usually has less communication overhead than echoing the full data, which suits limited bandwidth. It can miss some changes if different data produce the same checksum, so no check is perfect.
Look for these in your work
- Echo cost
- Checksum benefit
- Limitation
Common mistakes
- Saying a checksum is the original data
- Claiming a matching checksum proves that corruption is impossible
What can you explain now?
In an echo check, what does the sender compare?
Compare with the explanation
The returned data with the original data
The receiver echoes the data so the sender can compare it.
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Make it stick
How does an echo check differ from a checksum check?
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.