← IGCSE Computer Science Paper 1

Why data is checked after transmission

Checking received data, because it can be damaged in transit and the receiver cannot see the original. Spotting the error is the first step to fixing it.

Part of: IGCSE Computer Science 0478 — 2.2 Methods of Error Detection — Module 4, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 54, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 59, WA Computer Science ATAR Year 11 — Network Communications — Module 75, WA Computer Science ATAR Year 11 — Cyber Security — Module 99

Understanding Why data is checked after transmission in IGCSE Computer Science Paper 1

What it does: The receiver only has the received data, so it relies on a check result to decide whether an error has occurred. A corrupted file or message is a typical sign that data was damaged on its way from sender to receiver. Damage is often invisible. One changed bit can look fine, which is why a check is needed.

Common mistakes: The receiver gets data from the sender, not from itself. Line 2 should be RECEIVE msg FROM sender so the arrived data can then be checked. The damaged message should only be shown when an error WAS found. The condition must be error_found = TRUE, otherwise good data is reported as damaged.

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This module's key terms are also in a printable crossword (Modules 142–154) and word search (Modules 142–164).

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Learning
Duo
Why does a receiver need to check data after it has been transmitted?
The data may have been damaged on its way from the senderThe data always arrives exactly as it was sent
Matches
Data arrives, so it must be correct. No check needed.
↓
Data can be damaged by interference in transit, so arriving does not mean it is correct. It must be checked.
Applying
Meanings
Term
error detection
Meaning
Checking received data, because it can be damaged in transit and the receiver cannot see the original. Spotting the error is the first step to fixing it.
Practice
sentCheck ← 0
Testing
Gaps
IF checkReceived checkSent THEN
Check Answer
Spots
1OUTPUT 'Waiting for data'
2RECEIVE msg FROM receiver

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Quick Questions

1. Why does a receiver need to check data after it has been transmitted?
A. The data may have been damaged on its way from the sender ✓
B. The data always arrives exactly as it was sent
Data can be corrupted in transit (for example by interference), so the receiver must check that what arrived matches what was sent.
2. Which statement about the receiver is true?
A. It cannot see the original data, so it needs a method to check what arrived ✓
B. It can look at the original data and compare it with the received copy
The original stays with the sender. The receiver only has the received data, so checks such as parity or a checksum are needed.
3. A student opens a file they downloaded and it is full of strange symbols. What is the most likely cause?
A. The file was made smaller by the sender on purpose
B. The data was corrupted during transmission ✓
A corrupted file or message is a typical sign that data was damaged on its way from sender to receiver.
4. What is the purpose of error detection?
A. To stop interference from ever happening
B. To find out whether the received data has been damaged ✓
Error detection does not prevent errors. It checks received data to see whether an error has happened.
5. What is the first step to fixing a transmission error?
A. Detecting that an error has happened ✓
B. Ignoring the data and using it anyway
Detecting the error comes first. Only once an error is spotted can the data be sent again or corrected.

Match the Pairs

error detection → Checking received data to find out whether it was damaged on its way from the sender
corrupted file → A file whose data was changed or damaged in transit, so it no longer matches what was sent
The receiver cannot see the original data, so it needs a check to tell whether the data was damaged → if check_failed: error found
Detecting an error is the first step to fixing it, so a failed check leads to a request to send again → if error: request_resend()
Data arrives, so it must be correct. No check needed. → Data can be damaged by interference in transit, so arriving does not mean it is correct. It must be checked.
The receiver compares the data with the sender's original copy. → The receiver cannot see the original data, so it cannot compare directly. It must use an error detection check instead.

Key Term

error detection
Checking received data, because it can be damaged in transit and the receiver cannot see the original. Spotting the error is the first step to fixing it.

Fill in the Blanks

1. IF checkReceived <> checkSent THEN
Hint: Type the operator that means 'is not equal to', showing the data was damaged.
2. OUTPUT "Error detected"
Hint: Type the keyword that shows a message to the user.
3. RECEIVE message FROM sender errorFound ← FALSE
Hint: Type the keyword for the receiver getting the data that was sent.
4. IF checkReceived <> checkSent THEN errorFound ← TRUE ENDIF
Hint: Type the Boolean value stored when a mismatch shows the data was damaged.
5. IF errorFound THEN OUTPUT "Resend"
Hint: Type the keyword that begins a decision: once an error is detected, the receiver can act on it.

Spot the Error

Question 1
A OUTPUT 'Waiting for data'
B RECEIVE msg FROM receiver
C error ← CHECK(msg)
Line B — The receiver gets data from the sender, not from itself. Line 2 should be RECEIVE msg FROM sender so the arrived data can then be checked.
Question 2
A error_found ← CHECK(msg)
B IF error_found = FALSE THEN
C OUTPUT 'Data damaged'
D ENDIF
Line B — The damaged message should only be shown when an error WAS found. The condition must be error_found = TRUE, otherwise good data is reported as damaged.
Question 3
A error ← CHECK(original)
B IF error = TRUE THEN
C OUTPUT 'Ask for resend'
D ENDIF
Line A — The receiver cannot see the original data, only what arrived. The check must use the received data, so line 1 should be error ← CHECK(received).
Question 4
A SEND file TO sender
B damaged ← CHECK(file)
C IF damaged = TRUE THEN
D OUTPUT 'Send it again'
E ENDIF
Line A — The receiver must first receive the file before it can check it for damage. Line 1 should be RECEIVE file FROM sender, not SEND.
Question 5
A RECEIVE msg FROM sender
B error ← CHECK(msg)
C IF error = TRUE
D OUTPUT 'Damaged: resend'
E ENDIF
Line C — The IF statement is missing THEN. Line 3 should be IF error = TRUE THEN so the code runs when an error is detected.

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