← IGCSE Computer Science Paper 1

Drawbacks of USB

Limits of USB: short maximum cable length (unreliable over distance), slower speed than some interfaces (old ports slow new devices), and a limited number of ports.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 39

Understanding Drawbacks of USB in IGCSE Computer Science Paper 1

What it does: A computer has only a few USB ports, so more devices than ports means some cannot be connected unless a hub is used. Some other interfaces transfer data faster than USB, so USB can be a slower choice for very large amounts of data. USB cables can only be a few metres long before the signal becomes unreliable. USB is not limited to one direction. Backwards compatibility means the device works, but it is limited by the slower port. Automatic detection is a benefit of USB, not a drawback. A limited cable length is a key drawback of USB, because over long distances the data can be corrupted or lost.

Common mistakes: USB cables become unreliable when they are longer than the maximum length (about 5 m). The test should be cable_m > 5, not cable_m < 5. A fast device on an old port is slowed to the port's speed. The program should print speed (the slower value), not device.

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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

Which is a drawback of USB?

It can only send data in one directionIt has a limited maximum cable length

Matches

A 50 m USB cable gives the same reliable signal as a 2 m cable

↓

Wrong: USB cables have a limited length, and a much longer cable makes the data unreliable over the distance.

Applying

Meanings

Term

USB drawbacks

Meaning

Limits of USB: short maximum cable length (unreliable over distance), slower speed than some interfaces (old ports slow new devices), and a limited number of ports.

Practice

IF cableLength > 5 THEN

Testing

Gaps

IF cableLength maxLength THEN

Check Answer

Spots

1if cable_m < 5:

2 print('Signal may fail')

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Quick Questions

1. Which is a drawback of USB?

A. It can only send data in one direction

B. It has a limited maximum cable length ✓

USB cables can only be a few metres long before the signal becomes unreliable. USB is not limited to one direction.

2. What happens if a USB cable is made much longer than the maximum length?

A. Data transmission becomes unreliable ✓

B. Data transfers faster

Over long distances the signal weakens and errors become more likely, so USB cables have a limited length.

3. A new fast USB 3.0 drive is plugged into an old USB 1.1 port. What is the likely result?

A. The drive transfers data at the slower speed of the old port ✓

B. The old port speeds up to match the drive

Backwards compatibility means the device works, but it is limited by the slower port.

4. Which statement about USB speed is correct?

A. USB is always the fastest interface available

B. USB can be slower than some other interfaces ✓

Some other interfaces can transfer data faster than certain USB versions, so speed is a drawback.

5. A laptop has only two USB ports but the user wants to connect four devices. Which drawback is this?

A. A limited number of ports ✓

B. A limited cable length

Each computer has only a few USB ports, so extra devices need a hub or some to be unplugged.

Match the Pairs

USB cable length limit → A USB cable can only be a few metres long before the signal weakens and data becomes unreliable

USB 2.0 port + USB 3.0 device → The new device is slowed down and only transfers data at the speed of the older port

USB has a limited number of ports, so you may run out of sockets for your devices → 4 ports, 6 peripherals: 2 devices cannot be plugged in

USB can have a lower transfer speed than some other interfaces, which is a drawback for very large files → Copying a huge video file takes longer over USB than over a faster interface

A 50 m USB cable gives the same reliable signal as a 2 m cable → Wrong: USB cables have a limited length, and a much longer cable makes the data unreliable over the distance.

Plugging a fast USB device into an old slow port makes the port speed up → Wrong: the device is slowed down to the speed of the old port, because of backwards compatibility.

Key Term

USB drawbacks

Limits of USB: short maximum cable length (unreliable over distance), slower speed than some interfaces (old ports slow new devices), and a limited number of ports.

Fill in the Blanks

1. IF cableLength > maxLength THEN OUTPUT "Signal may be unreliable" ENDIF

Hint: Type the comparison operator meaning 'longer than', because USB cables have a limited length.

2. transferSpeed ← MIN(portSpeed, deviceSpeed)

Hint: Type the function that picks the smaller of two values, as a new device on an old port is slowed down.

3. freePorts ← totalPorts - usedPorts

Hint: Type the arithmetic operator that works out how many ports are left after some are in use.

4. freePorts ← totalPorts - usedPorts IF freePorts = 0 THEN OUTPUT "Use a USB hub"

Hint: Type the variable that holds the number of ports still available, since a computer has a limited number of ports.

5. IF portVersion = 2 THEN OUTPUT "Slower transfer speed" ENDIF

Hint: Type the pseudocode command that displays a message to the user.

Spot the Error

Question 1

A if cable_m < 5:

B print('Signal may fail')

C else:

D print('Signal is fine')

Line A — USB cables become unreliable when they are longer than the maximum length (about 5 m). The test should be cable_m > 5, not cable_m < 5.

Question 2

A port = 12

B device = 480

C speed = min(port, device)

D print('Speed:', device)

Line D — A fast device on an old port is slowed to the port's speed. The program should print speed (the slower value), not device.

Question 3

A devices = 6

B free = 4 + devices

C if free < 0:

D print('Not enough ports')

Line B — USB ports are limited. Free ports are found by subtracting the devices from the ports (4 - devices), not by adding.

Question 4

A port = 'USB 1.1'

B if port == 'USB 3.0':

C speed == 5000

D else:

E speed = 12

Line C — Line 3 must assign a value to speed using a single =. Using == compares values and does not store the speed.

Question 5

A ports = 2

B devices = ['mouse', 'pen']

C devices.append('phone')

D if len(devices) > ports

E print('Use a hub')

Line D — The if statement is missing its colon at the end. It should be if len(devices) > ports:

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