← IGCSE Computer Science Paper 1

Interference as the cause of transmission errors

Unwanted disturbance to a signal, such as electrical noise or crosstalk, that damages data as it travels and can cause bits to be lost, gained or changed.

Part of: IGCSE Computer Science 0478 — 2.2 Methods of Error Detection — Module 5, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 55, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 60, WA Computer Science ATAR Year 11 — Network Communications — Module 76

Understanding Interference as the cause of transmission errors in IGCSE Computer Science Paper 1

What it does: The arrow shows the signal from cable A spilling into the neighbouring cable B. That is crosstalk, a type of interference. A longer link or a wireless link has more chances for noise to disturb the signal, so the risk of errors rises. Interference is the cause of errors. Methods such as parity or checksum only detect the damage afterwards. A weak or distorted signal is hard to read correctly, so bits can be lost, gained or changed. Interference affects signals while data travels along a cable or wireless link. It is a data transmission problem. Error detection does not stop interference. It helps the receiver notice when data has been damaged.

Common mistakes: The warning should appear when the data sent and received are different, because interference has changed a bit. The test should be sent != received. Noise flips a bit, so the result must still be 0 or 1. The XOR operator ^ flips the bit when noise is 1. Adding gives 2, which is not a valid bit.

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Learning
Duo
Which best describes interference in data transmission?
A method that checks data after it arrivesUnwanted disturbance to a signal that can damage data while it travels
Matches
interference = data sent twice
↓
Interference is not data being sent twice. It is unwanted disturbance, such as electrical noise or crosstalk, that damages the signal and the data it carries.
Applying
Meanings
Term
interference
Meaning
Unwanted disturbance to a signal, such as electrical noise or crosstalk, that damages data as it travels and can cause bits to be lost, gained or changed.
Practice
sent ← "01000001"
Testing
Gaps
IF sent received THEN
Check Answer
Spots
2received = "10100010"
3if sent == received:

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Quick Questions

1. Which best describes interference in data transmission?
A. A method that checks data after it arrives
B. Unwanted disturbance to a signal that can damage data while it travels ✓
Interference is the cause of errors. Methods such as parity or checksum only detect the damage afterwards.
2. Which is an example of interference?
A. Electrical noise from a motor near a cable ✓
B. A parity bit added to a byte
Electrical noise from nearby equipment disturbs the signal. A parity bit is an error detection tool, not a cause of errors.
3. What is crosstalk?
A. Signal from one cable leaking into a nearby cable ✓
B. Two computers confirming they received data
Crosstalk happens when a signal in one cable disturbs the signal in a cable next to it. Confirming receipt is acknowledgement, not interference.
4. Interference can cause a bit to change from 0 to 1 during transmission.
A. False
B. True ✓
Interference can distort the signal so the receiver reads the wrong bit. That is a data change error.
5. Which link is usually MORE exposed to interference?
A. A long wireless link ✓
B. A short cable between two nearby devices
Longer and wireless links pick up more noise and the signal gets weaker, so errors are more likely.

Match the Pairs

interference → Unwanted disturbance to a signal, such as electrical noise, that damages data while it travels
Sent: 10110010 Received: 10100010 → Interference has changed one bit during transmission, so the data has been changed
Crosstalk is when the signal in one cable leaks into a cable next to it and disturbs the data there → cableA signal -> leaks into cableB
Longer or wireless links are more exposed to interference, so errors are more likely → link_length up => error_risk up
interference = data sent twice → Interference is not data being sent twice. It is unwanted disturbance, such as electrical noise or crosstalk, that damages the signal and the data it carries.
risk(wired 1m) > risk(wireless 100m) → The comparison is the wrong way round. Longer and wireless links are more exposed to interference, so the wireless 100m link has the higher risk.

Key Term

interference
Unwanted disturbance to a signal, such as electrical noise or crosstalk, that damages data as it travels and can cause bits to be lost, gained or changed.

Fill in the Blanks

1. sent ← "10110010" received ← "10010010" IF sent ≠ received THEN
Hint: Type the comparison operator that means 'is not equal to', used to spot a bit changed by interference.
2. interference ← TRUE IF interference THEN OUTPUT "Bits may change"
Hint: Type the variable name that holds the unwanted disturbance condition.
3. signal ← "weak" IF signal = "weak" THEN OUTPUT "Data may be damaged"
Hint: Type the pseudocode keyword that displays a message.
4. IF cableLength > 90 THEN risk ← "high" ENDIF
Hint: Type the keyword that closes an IF statement.
5. IF link = "wireless" OR length > 90 THEN risk ← "high" ENDIF
Hint: Type the logical operator meaning either condition can be true, since wireless or longer links are more exposed.

Spot the Error

Question 1
A sent = "10110010"
B received = "10100010"
C if sent == received:
D print("Data changed")
Line C — The warning should appear when the data sent and received are different, because interference has changed a bit. The test should be sent != received.
Question 2
A sent = 1
B noise = 1
C received = sent + noise
D print(received)
Line C — Noise flips a bit, so the result must still be 0 or 1. The XOR operator ^ flips the bit when noise is 1. Adding gives 2, which is not a valid bit.
Question 3
A distance = 100
B limit = 50
C if distance < limit:
D print("More exposed")
Line C — Longer links are more exposed to interference. The message should print when the distance is greater than the limit, so the test should use >.
Question 4
A neighbour_signal = True
B if not neighbour_signal:
C print("Crosstalk risk")
Line B — Crosstalk happens when signals leak from a neighbouring cable. The risk exists when a neighbour signal is present, so the test should be if neighbour_signal: without not.
Question 5
A wire = "unshielded
B if wire == "unshielded":
C print("Picks up noise")
Line A — The string is missing its closing quote, which is a syntax error. It should be wire = "unshielded".

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