← IGCSE Computer Science Paper 1

How USB transmits data

USB sends data serially, one bit at a time, down a single cable. It works in both directions: half-duplex in older versions, full-duplex in later ones. The cable can also carry power.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 35

Understanding How USB transmits data in IGCSE Computer Science Paper 1

What it does: Full-duplex lets the host and the device send and receive simultaneously, which later USB versions support. A USB cable has wires for data and a wire for power, so a device such as a flash drive can be powered through the cable. USB stands for Universal Serial Bus. It uses serial transmission, sending one bit after another. Half-duplex allows two-way communication, but not at the same moment. Simplex would allow only one direction. Serial transmission sends each bit in turn along the cable, so the bits arrive in sequence.

Common mistakes: USB sends one bit at a time down a single cable, so it is serial transmission, not parallel. The line should be OUTPUT "so it is serial". Serial transmission sends only one bit at a time, so bitsPerTick should be 1. Sending 8 bits at once would be parallel.

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Learning
Duo
How does a USB cable send data?
In parallel, with many bits sent at once side by sideSerially, one bit at a time down a single wire
Matches
USB sends 8 bits at once on 8 wires (parallel)
↓
USB is serial, not parallel. It sends one bit at a time down the cable.
Applying
Meanings
Term
USB serial transmission
Meaning
USB sends data serially, one bit at a time, down a single cable. It works in both directions: half-duplex in older versions, full-duplex in later ones. The cable can also carry power.
Practice
Mode <- "full-duplex"
Testing
Gaps
usb.transmission ←
Check Answer
Spots
3OUTPUT "along a single cable"
4OUTPUT "so it is parallel"

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Quick Questions

1. How does a USB cable send data?
A. Serially, one bit at a time down a single wire ✓
B. In parallel, with many bits sent at once side by side
USB stands for Universal Serial Bus. It uses serial transmission, sending one bit after another.
2. Which statement about the direction of USB data is correct?
A. Data can travel in both directions along a USB connection ✓
B. Data can only travel from the device to the computer
USB is not simplex. It supports two-way communication, so a computer can send data to a device and receive data from it.
3. Older versions of USB are described as which type of transmission?
A. Simplex: one direction only, all of the time
B. Half-duplex: both directions, but only one at a time ✓
Half-duplex allows two-way communication, but not at the same moment. Simplex would allow only one direction.
4. Later versions of USB are described as which type of transmission?
A. Half-duplex: both directions, but never at the same time
B. Full-duplex: both directions at the same time ✓
Later USB versions can send and receive at the same time, which is full-duplex.
5. Which USB statement about power is true?
A. A USB cable can carry power as well as data ✓
B. A USB cable can only carry data, never power
USB can power small devices such as a keyboard or charge a phone, as well as transmit data.

Match the Pairs

USB: bit, bit, bit, bit -> → USB sends data serially, one bit at a time, down the cable
USB 1.x / 2.0: send OR receive → Older USB versions use half-duplex, so data travels in both directions but only one way at a time
Later USB versions use full-duplex, so data can travel in both directions at the same time → USB 3.0: send <-> receive at once
The USB cable carries power as well as data, so a simple device needs no separate power supply → USB cable = data lines + power line
USB sends 8 bits at once on 8 wires (parallel) → USB is serial, not parallel. It sends one bit at a time down the cable.
USB is simplex: data goes one way only → USB is not simplex. It carries data in both directions: half-duplex in older versions, full-duplex in later ones.

Key Term

USB serial transmission
USB sends data serially, one bit at a time, down a single cable. It works in both directions: half-duplex in older versions, full-duplex in later ones. The cable can also carry power.

Fill in the Blanks

1. usb.transmission ← serial
Hint: Type the transmission method that sends data one bit after another down a single wire.
2. usb.bitsAtATime ← 1
Hint: Type the number of bits that USB sends at one moment.
3. IF usbVersion = 1 THEN mode ← half_duplex ENDIF
Hint: Type the mode where data can travel both ways, but only one direction at a time.
4. IF usbVersion = 3 THEN mode ← full_duplex ENDIF
Hint: Type the mode where data can travel in both directions at the same time.
5. cable.carries ← [data, power]
Hint: Type what else a USB cable supplies to a device besides data.

Spot the Error

Question 1
A OUTPUT "USB sends data"
B OUTPUT "one bit at a time"
C OUTPUT "along a single cable"
D OUTPUT "so it is parallel"
Line D — USB sends one bit at a time down a single cable, so it is serial transmission, not parallel. The line should be OUTPUT "so it is serial".
Question 2
A mode ← "serial"
B bitsPerTick ← 8
C power ← TRUE
Line B — Serial transmission sends only one bit at a time, so bitsPerTick should be 1. Sending 8 bits at once would be parallel.
Question 3
A version ← "USB 1.1"
B duplex ← "full"
C OUTPUT "one way at a time"
Line B — Older versions of USB are half-duplex: data can travel in both directions, but only one direction at a time. duplex should be "half".
Question 4
A version ← "USB 3.0"
B duplex ← "full"
C OUTPUT "one way at a time"
Line C — Later versions of USB are full-duplex, so data can travel in both directions at the same time. The output should be "both ways at once".
Question 5
A bits ← "10110001"
B FOR i ← 1 TO 8
C SEND bits ALL AT ONCE
D NEXT i
Line C — USB is serial, so each bit is sent separately, one after another, along the cable. The loop should send bits[i] ALONG cable, not all the bits at once.

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