Universal Serial Bus (USB) — Types and Methods of Data Transmission | IGCSE Computer Science

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Past paper frequency (2018 to 2024)

This topic accounts for approximately 4% of your exam marks.

stable

Rare

Stable4%

Serial vs parallel and simplex/half-duplex/full-duplex appear as definition or comparison questions.

USB (Universal Serial Bus) is the most common wired connection between computers and external devices. As the name says, USB is a serial method, and it operates asynchronously (with no separate clock signal; the data is self-clocking).

Devices that use USB include: keyboards, mice, printers, scanners, external hard drives, flash memory sticks, webcams, mobile phones, audio interfaces, and most other peripherals.

USB connector shapes

The letter in a USB name describes the physical shape of the plug:

USB-A: the rectangular plug. Found on flash drives, mice, keyboards, older external storage devices.
USB-B: square-ish plug. Found on the back of printers and older external storage.
USB-C: small, oval, symmetrical (can be plugged in either way up). The current standard for phones, laptops and many modern peripherals. Can carry both data and large amounts of power.

USB generations and speeds

The number in a USB name describes the generation, which sets the maximum speed:

Generation	Maximum speed
USB 1.1	12 Mbps
USB 2.0	480 Mbps
USB 3.0 / 3.1 / 3.2	5 Gbps to 20 Gbps
USB4 / USB4 2.0	Up to 80 Gbps

Newer USB generations are backwards-compatible with older ones. Plugging a USB 2.0 device into a USB 3.0 port works, but the device runs at USB 2.0 speeds.

Plug-and-play

When a USB device is connected, the operating system automatically:

Detects that a new device has been plugged in.
Looks up the device's identifier in its list of drivers.
Loads the matching driver if it is already installed, or downloads and installs one if not. (For most common device types, a generic built-in driver works without any extra installation.)
Makes the device available to applications.

This plug-and-play behaviour means the user does not have to manually install drivers or restart the computer in most cases.

Advantages of USB

Advantage	Why it matters
Standardised connector	One port shape works across thousands of products and manufacturers; the plug only fits in one orientation so it cannot be inserted wrongly (USB-C goes in either way up)
Plug-and-play	Devices are detected automatically; the correct driver is loaded without user effort
Carries power as well as data	Many devices (mice, keyboards, flash drives, phones charging) need no separate power supply
High speeds and backwards compatibility	Newer USB generations offer multi-gigabit data rates while still working with older devices
Wide industry support	Drivers, cables and documentation are easy to find for any operating system

Disadvantages of USB

Disadvantage	Why it matters
Limited cable length	Maximum cable length is around 5 m before the signal degrades, so USB is not for long distances
Older USB generations are slow by modern standards	USB 2.0 maxes out at 480 Mbps, far below USB 3.0 and beyond
Older USB standards may be phased out	USB 1.1 and 2.0 may eventually lose support in future operating systems