← IGCSE Computer Science Paper 1

Data change

Bits arrive altered, such as a 0 flipping to a 1, so the same amount of data arrives but with wrong values. Comparing sent and received bits shows it.

Part of: IGCSE Computer Science 0478 — 2.2 Methods of Error Detection — Module 8, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 58

Understanding Data change in IGCSE Computer Science Paper 1

What it does: The fourth bit is 1 when sent but 0 when received. The other bits match, so that bit has changed. No bits were lost or gained, since the count is equal. The fifth bit changed from 1 to 0, which is data change. A flipped bit replaces a value with a different one. No extra bits are added, so it is not data gain. Compare bit by bit. Only the 6th bit differs (0 became 1). The other seven match. A (01000001) and C (01000011) differ by one flipped bit, so a single bit change can turn one character into another.

Common mistakes: The received value has 9 bits, so an extra bit has arrived. That is data gain, not data change. Data change keeps the same number of bits, so it should be 01000011 (8 bits) with one bit flipped. The test is the wrong way round. Data change is detected when the sent and received bits are NOT equal, so it should be IF sent ≠ received THEN.

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Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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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
Sent: 01000001. Received: 01000011. What type of error is this?
Data lossData change
Matches
Sent: 1010 Received: 101 -> data change
↓
Only 3 bits arrived instead of 4, so a bit was lost. That is data loss, not data change.
Applying
Meanings
Term
data change
Meaning
Bits arrive altered, such as a 0 flipping to a 1, so the same amount of data arrives but with wrong values. Comparing sent and received bits shows it.
Practice
sent ← "01000001"
Testing
Gaps
Error type:
Check Answer
Spots
1sent ← 01000001
2received ← 010000011

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Quick Questions

1. Sent: 01000001. Received: 01000011. What type of error is this?
A. Data change ✓
B. Data loss
The same 8 bits arrived, but one bit flipped from 0 to 1. Nothing was lost and nothing extra was added, so it is data change.
2. Which statement describes data change?
A. The same amount of data arrives, but some bits have wrong values ✓
B. Fewer bits arrive than were sent
In data change the length stays the same. Fewer bits arriving would be data loss.
3. A 0 flips to a 1 during transmission. This is an example of:
A. Data change ✓
B. Data gain
A flipped bit replaces a value with a different one. No extra bits are added, so it is not data gain.
4. Sent: 1100 0011. Received: 1100 0111. How many bits have changed?
A. 2
B. 1 ✓
Compare bit by bit. Only the 6th bit differs (0 became 1). The other seven match.
5. Sent character: A. Received character: C. What has most likely happened?
A. The character was lost on the way
B. Some bits changed, so a different character arrived ✓
A character arrived, but it is the wrong one. This shows that bits were changed in transit.

Match the Pairs

Sent: 01000001 Received: 01000011 → Data change: one bit has flipped from 0 to 1, so the same number of bits arrived but one value is wrong
Sent: 'A' Received: 'C' → A changed character: the letter that arrived is different from the letter that was sent
Comparing the sent and received bits position by position shows which bit was altered → Sent: 1 0 1 1 0 0 1 0 Received: 1 0 1 0 0 0 1 0
In data change the length stays the same: 8 bits are sent and 8 bits arrive, but one value is wrong → Sent: 11001100 (8 bits) Received: 11001000 (8 bits)
Sent: 1010 Received: 101 -> data change → Only 3 bits arrived instead of 4, so a bit was lost. That is data loss, not data change.
Sent: 0110 Received: 0110 -> data change → The sent and received bits are identical, so nothing has changed and the data arrived correctly.

Key Term

data change
Bits arrive altered, such as a 0 flipping to a 1, so the same amount of data arrives but with wrong values. Comparing sent and received bits shows it.

Fill in the Blanks

1. Sent: 01000001 Received: 01000011 Error type: data change
Hint: Type the two-word name for this error, where the same number of bits arrives but one has a wrong value
2. IF sent <> received THEN OUTPUT "Data change" ENDIF
Hint: Type the pseudocode operator meaning not equal to
3. IF LENGTH(sent) = LENGTH(received) THEN OUTPUT "Same amount of data" ENDIF
Hint: Type the operator that checks the two lengths are equal, because data change keeps the amount the same
4. Sent bit: 0 Received bit: 1 Bit flipped from 0 to 1
Hint: Type the value the bit has after it flips
5. FOR i ← 1 TO 8 IF sent[i] <> received[i] THEN OUTPUT "Bit changed"
Hint: Type the loop variable so the same bit position is compared in both bytes

Spot the Error

Question 1
A sent ← 01000001
B received ← 010000011
C IF sent ≠ received THEN
D OUTPUT "Data change"
E ENDIF
Line B — The received value has 9 bits, so an extra bit has arrived. That is data gain, not data change. Data change keeps the same number of bits, so it should be 01000011 (8 bits) with one bit flipped.
Question 2
A sent ← 10110010
B received ← 10100010
C IF sent = received THEN
D OUTPUT "Data change"
E ENDIF
Line C — The test is the wrong way round. Data change is detected when the sent and received bits are NOT equal, so it should be IF sent ≠ received THEN.
Question 3
A count ← 0
B FOR i ← 1 TO 8
C IF S[i] = R[i] THEN
D count ← count + 1
E ENDIF
F NEXT i
Line C — This counts the bits that match, not the bits that changed. To count flipped bits, it should test IF S[i] ≠ R[i] THEN.
Question 4
A sent ← A
B received ← "B"
C IF sent ≠ received THEN
D OUTPUT "Character changed"
Line A — The character A must be written in quotes as a string. Without quotes, A is treated as a variable name. It should be sent ← "A".
Question 5
A sent ← "11011100"
B received ← "11011100"
C IF sent[4] ≠ received[4] THEN
D OUTPUT "Bit 4 flipped"
Line A — The sent data must be 11001100 so that bit 4 is 0 when sent and 1 when received, which shows a flipped bit. With 11011100, nothing changed.

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