← IGCSE Computer Science Paper 1

Advantages and disadvantages of serial transmission

Sends bits one at a time down one wire: cheaper, no skew and reliable over long distances, but slower than parallel for large amounts of data at the same clock speed.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 23

Understanding Advantages and disadvantages of serial transmission in IGCSE Computer Science Paper 1

What it does: Parallel sends several bits in every clock pulse, so serial takes longer to send a large file at the same clock speed. With a single wire the bits follow one another in a fixed order, so there is no chance of bits on different wires arriving at different times. Sending a whole byte at once is parallel transmission. Serial keeps data in step by sending one bit at a time. Serial is cheaper to wire and reliable over distance, so it suits long links. Using only one wire (or one data channel) means less cable and fewer connectors, which lowers the cost compared with parallel.

Common mistakes: Serial transmission sends bits down a single wire, so wires should be 1. Eight wires would be parallel transmission. Serial needs fewer wires than parallel, so it is cheaper to wire. The cost should be "low".

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Learning
Duo
Why is serial transmission cheaper to wire than parallel transmission?
It needs fewer wires, so less cable is usedIt needs more wires, so the cable is stronger
Matches
Serial is faster than parallel because it only uses one wire
↓
Fewer wires does not make serial faster. Serial sends one bit at a time, so it is slower than parallel for large amounts of data at the same clock speed
Applying
Meanings
Term
serial transmission
Meaning
Sends bits one at a time down one wire: cheaper, no skew and reliable over long distances, but slower than parallel for large amounts of data at the same clock speed.
Practice
OUTPUT "Serial: one bit at a time, one wire"
Testing
Gaps
serialWires ←
Check Answer
Spots
1wires ← 8
2skew ← FALSE

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Quick Questions

1. Why is serial transmission cheaper to wire than parallel transmission?
A. It needs fewer wires, so less cable is used ✓
B. It needs more wires, so the cable is stronger
Serial sends bits one at a time down a single wire, so it needs fewer wires and the cabling costs less.
2. Which statement about skew is correct for serial transmission?
A. There is no skew, so the bits stay in step ✓
B. Skew is a big problem because bits travel side by side
Skew happens when bits sent side by side arrive at different times. Serial sends one bit at a time down one wire, so there is no skew.
3. Which type of transmission is better suited to long distances?
A. Parallel transmission
B. Serial transmission ✓
Because serial has no skew, the data stays reliable over long distances. Parallel bits drift out of step over long cables.
4. A school needs to send a very large video file. What is a disadvantage of serial transmission here?
A. It is slower, because bits are sent one at a time ✓
B. It is faster, because every bit has its own wire
At the same clock speed, serial sends fewer bits per tick than parallel, so large amounts of data take longer.
5. Which is an advantage of serial transmission?
A. It sends a whole byte at the same time
B. Data arrives in the correct order and stays in step ✓
Sending a whole byte at once is parallel transmission. Serial keeps data in step by sending one bit at a time.

Match the Pairs

serial: 1 wire, bits sent one at a time → Serial transmission needs fewer wires, so cabling is cheaper
serial: long cable run, no skew → Serial transmission stays reliable over long distances because bits cannot get out of step
Serial is slower than parallel for large amounts of data at the same clock speed, because only 1 bit is sent per clock pulse → serial: 1 bit per pulse | parallel: 8 bits per pulse
Serial transmission has no skew, so the data stays in step and arrives in the correct order → wire 1: 1 0 1 1 0 0 1 0 (one wire, one order)
Serial is faster than parallel because it only uses one wire → Fewer wires does not make serial faster. Serial sends one bit at a time, so it is slower than parallel for large amounts of data at the same clock speed
Serial is unreliable over long distances because of data skew → Skew is a problem for parallel transmission, not serial. Serial has no skew, so it is reliable over long distances

Key Term

serial transmission
Sends bits one at a time down one wire: cheaper, no skew and reliable over long distances, but slower than parallel for large amounts of data at the same clock speed.

Fill in the Blanks

1. serialWires ← 1
Hint: Type the number of data wires that serial transmission uses.
2. skewHappens ← FALSE
Hint: Type the Boolean value that shows whether serial transmission suffers from data skew.
3. serialCost < parallelCost
Hint: Type the comparison operator showing serial costs less because it needs fewer wires.
4. IF bitsToSend > 1000000 THEN OUTPUT "Serial is slower" ENDIF
Hint: Type the pseudocode keyword that displays a message.
5. longDistanceOK ← TRUE
Hint: Type the Boolean value showing serial transmission is reliable over long distances.

Spot the Error

Question 1
A wires ← 8
B skew ← FALSE
C OUTPUT wires, skew
Line A — Serial transmission sends bits down a single wire, so wires should be 1. Eight wires would be parallel transmission.
Question 2
A mode ← "serial"
B cost ← "high"
C OUTPUT mode, cost
Line B — Serial needs fewer wires than parallel, so it is cheaper to wire. The cost should be "low".
Question 3
A distance ← 500
B IF distance < 100 THEN
C mode ← "serial"
D ENDIF
Line B — Serial suits long distances, so the test should be distance > 100. The < sign checks for short distances.
Question 4
A mode ← "serial"
B wires ← 1
C IF mode = "serial" THEN
D skew ← TRUE
E ENDIF
Line D — With one wire, bits cannot arrive out of step, so serial has no skew. skew should be set to FALSE.
Question 5
A bits_per_tick ← 8
B ticks ← 800 DIV bits_per_tick
C OUTPUT ticks
Line A — Serial sends only 1 bit per clock tick. Sending 8 bits per tick describes parallel transmission.

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