← IGCSE Computer Science Paper 1

Data skew in parallel transmission

Bits sent together down parallel wires arrive at slightly different times, so data can be received incorrectly. It gets worse as the cable gets longer.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 26, WA Computer Science ATAR Year 11 — Network Communications — Module 18

Understanding Data skew in parallel transmission in IGCSE Computer Science Paper 1

What it does: The 8 bits leave together but some wires are slightly slower, so the bits arrive at slightly different times. Skew grows with cable length, so parallel is only reliable over short distances, such as inside a computer. Over a longer cable the small timing differences between the wires add up, so the bits drift further apart.

Common mistakes: Skew gets worse as the cable gets longer, so a long cable (more than 5) should set skew_risk to HIGH, not LOW. Parallel transmission sends several bits at once down several wires. Sending one bit at a time describes serial. It should say 'Sent 8 bits at once'.

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This module's key terms are also in a printable crossword (Modules 121–130) and word search (Modules 121–141).

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Learning
Duo
What is skew in parallel transmission?
Bits are sent one after another down a single wireBits sent together arrive at slightly different times
Matches
Skew happens because a single wire sends bits one after another
↓
This describes serial transmission, not skew. Skew happens when bits sent together on several parallel wires arrive at different times.
Applying
Meanings
Term
skew
Meaning
Bits sent together down parallel wires arrive at slightly different times, so data can be received incorrectly. It gets worse as the cable gets longer.
Practice
cableLength ← 50
Testing
Gaps
= bits arrive at different times
Check Answer
Spots
1IF cable_length > 5 THEN
2 skew_risk ← LOW

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Quick Questions

1. What is skew in parallel transmission?
A. Bits sent together arrive at slightly different times ✓
B. Bits are sent one after another down a single wire
Skew happens when bits that were sent at the same moment down parallel wires do not arrive at the same moment.
2. Which type of transmission can suffer from skew?
A. Parallel transmission ✓
B. Serial transmission
Skew needs several wires sending bits at once. Serial sends one bit at a time down one wire, so bits cannot arrive out of step with each other.
3. What can skew cause?
A. Data is received incorrectly ✓
B. Data is sent faster
If bits arrive at different times, the receiver may read the wrong values, so the data can be corrupted.
4. What happens to skew as the cable gets longer?
A. It gets better
B. It gets worse ✓
Over a longer cable the small timing differences between the wires add up, so the bits drift further apart.
5. Why is parallel transmission only used over short distances?
A. Parallel wires cannot carry binary data
B. Skew becomes a bigger problem over long cables ✓
Parallel wires can carry binary data perfectly well. The problem is that over long distances skew makes the data unreliable.

Match the Pairs

skew → Bits sent together down parallel wires arrive at slightly different times, so the data can be received incorrectly
Cable length: 1 m -> 10 m → The longer the parallel cable, the bigger the timing differences between wires, so skew gets worse
Skew means bits sent at the same time over 8 parallel wires do not all arrive together → Sent together: 10110010 -> Arrive at different times
Because of skew, parallel transmission is only used over short distances → if cable_length <= short_distance then use parallel
Skew happens because a single wire sends bits one after another → This describes serial transmission, not skew. Skew happens when bits sent together on several parallel wires arrive at different times.
A longer parallel cable reduces skew, so it suits long distances → A longer cable makes skew worse, not better, so parallel is only used over short distances.

Key Term

skew
Bits sent together down parallel wires arrive at slightly different times, so data can be received incorrectly. It gets worse as the cable gets longer.

Fill in the Blanks

1. skew = bits arrive at different times
Hint: Type the name of the problem where bits sent together arrive at slightly different times
2. Longer cable = more skew
Hint: Type the word that shows how the amount of skew changes as the cable gets longer
3. Skew means data is received incorrectly
Hint: Type the word describing how data may be received when bits arrive at different times
4. Parallel is only used over short distances
Hint: Type the word for the cable distances where skew stays small enough to avoid errors
5. Skew happens on parallel wires
Hint: Type the transmission method that sends several bits at once down many wires

Spot the Error

Question 1
A IF cable_length > 5 THEN
B skew_risk ← LOW
C ELSE
D skew_risk ← LOW
E ENDIF
Line B — Skew gets worse as the cable gets longer, so a long cable (more than 5) should set skew_risk to HIGH, not LOW.
Question 2
A OUTPUT 'Sent 1 bit at a time'
B OUTPUT 'Wires used: 8'
C OUTPUT 'Skew can occur'
Line A — Parallel transmission sends several bits at once down several wires. Sending one bit at a time describes serial. It should say 'Sent 8 bits at once'.
Question 3
A cable ← 'short'
B IF cable = 'short' THEN
C OUTPUT 'Skew is large'
D ENDIF
Line C — Over a short cable the bits have little time to drift apart, so skew is small. It should output 'Skew is small'.
Question 4
A FOR wire ← 1 TO 8
B SEND bit ON wire
C NEXT wire
D OUTPUT 'Skew is harmless'
Line D — Skew is not harmless. Bits arriving at slightly different times can make the data be received incorrectly. It should say 'Skew can corrupt data'.
Question 5
A cable ← 1
B cable ← cable / 10
C OUTPUT 'Skew gets worse'
Line B — The program shows skew getting worse, which happens when the cable gets longer. The cable length should be multiplied (cable * 10), not divided.

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