Drawbacks of USB

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Quick Questions

Shade in the A or B circle for each question.

1. Which is a drawback of USB?

A It can only send data in one direction

B It has a limited maximum cable length

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

A Data transmission becomes unreliable

B Data transfers faster

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

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

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

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1USB cable length limit		AA USB cable can only be a few metres long before the signal weakens and data becomes unreliable
2USB 2.0 port + USB 3.0 device		BWrong: USB cables have a limited length, and a much longer cable makes the data unreliable over the distance.
3USB has a limited number of ports, so you may run out of sockets for your devices		C4 ports, 6 peripherals: 2 devices cannot be plugged in
4USB can have a lower transfer speed than some other interfaces, which is a drawback for very large files		DThe new device is slowed down and only transfers data at the speed of the older port
5A 50 m USB cable gives the same reliable signal as a 2 m cable		EWrong: the device is slowed down to the speed of the old port, because of backwards compatibility.
6Plugging a fast USB device into an old slow port makes the port speed up		FCopying a huge video file takes longer over USB than over a faster interface

Definitions

USB drawbacks

Fill in the Blanks

Complete each line of code by filling in the missing parts.

IF cableLength _ maxLength THEN OUTPUT "Signal may be unreliable" ENDIF

transferSpeed ← ___(portSpeed, deviceSpeed)

freePorts ← totalPorts _ usedPorts

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

IF portVersion = 2 THEN ______ "Slower transfer speed" ENDIF

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A if cable_m < 5:

B print('Signal may fail')

C else:

D print('Signal is fine')

Question 2

A port = 12

B device = 480

C speed = min(port, device)

D print('Speed:', device)

Question 3

A devices = 6

B free = 4 + devices

C if free < 0:

D print('Not enough ports')

Question 4

A port = 'USB 1.1'

B if port == 'USB 3.0':

C speed == 5000

D else:

E speed = 12

Question 5

A ports = 2

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

C devices.append('phone')

D if len(devices) > ports

E print('Use a hub')

Answer Key

Drawbacks of USB

Section A: Quick Questions

B — It has a limited maximum cable length

A — Data transmission becomes unreliable

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

B — USB can be slower than some other interfaces

A — A limited number of ports

Section B: 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.

Section C: Definitions

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.

Section D: Fill in the Blanks

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

transferSpeed ← MIN(portSpeed, deviceSpeed)

freePorts ← totalPorts - usedPorts

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

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

Section E: Spot the Error

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.

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.

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

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

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