USB is an industry standard, ‘user-friendly’ method of transferring data between a host device (such as a computer) and a peripheral device (for example, a mouse). To most computer users, the system simply allows the use of various devices by attaching them via a USB port.
How does data transfer between devices?
USB stands for Universal Serial Bus. A ‘bus’ within a PC is a collection of wires that transfer data between components inside the computer, or between the computer and its peripheral devices, much as an electronic busbar distributes power throughout certain large, power-hungry environments such as factories and data centres.
Before the launch of USB, each peripheral device was attached to a computer with its own individually shaped port. As the number of peripheral devices increased over the years, a new standardised means of transferring data between the main host and a range of devices was sought. This ultimately resulted in the development of USB.
How is data sent across USB?
When a peripheral device is attached via USB, the host computer will detect what kind of device it is and automatically load a driver that allows the device to function.
Data is transferred between the two devices in small amounts known as ‘packets’. A set number of bytes (a unit of digital information) is transmitted with each packet.
Other information is also sent, including:
the source of the data
the destination of the data
the length of the data
details of any errors that have been detected
There are four types of data transfer that can occur:
Interrupt transfer. Peripheral devices such as keyboards and mice use this type of data message to send smaller amounts of data. Such transfers are often used for less frequent but important requests. The devices generate the requests, though they must wait for the host to inquire about the specific data the remote device needs.
Such requests are guaranteed to be reattempted if the first transfer fails. These transfers will also let you know about any changes to the status of the device.
Bulk transfer. Used by printers and digital scanners for large amounts of data, this type of transfer is low-priority and not time-critical. The transfer will slow down if the host computer has a number of USB devices connected.
Isochronous transfer. Audio, video and other real-time data uses isochronous transfer. Errors can occur during the transfer, though the transfer will not be interrupted in order to resend the packets. However, such transfers usually involve situations where the accuracy of the data is not critical, such as audio elements that may not be picked up by the listener. Missing these elements is preferable to retrying data, which could result in glitching audio.
Control transfer. This type of data transfer is used to configure and control a USB device. The host sends a request to the device and the data transfer follows. Control transfers are also used to check status. Only one control request is handled at any one time.
How does a USB cable physically transfer data?
USB cables are capable of transferring both power and data. To achieve this, every USB cable features two sets of wires. One set carries the current while the other transfers the data signals.
Within the standard USB 2.0 connector you can see four metal strips. The outer two strips are the positive and ground of the power supply. The two central strips are dedicated to carrying data.
With the newer USB 3.0 connector, the data transfer speed is increased by the addition of extra data-carrying strips; four extra signalling wires help USB 3.0 achieve its super speed.
How quickly do USB 2.0 and USB 3.0 transfer data?
USB 2.0 transfers data at a top speed of 480 megabits per second (Mbps), while USB 3.0 can transfer data at up to 5 gigabits per second (Gbps).
The following table shows the maximum transfer rates for each USB version:
You can identify USB 3.0 connectors by their blue colour and initials SS, which stand for ‘SuperSpeed’.
How do you transfer data from a PC to a USB flash drive/memory stick?
The following steps will show you how to download data on to a USB flash drive/memory stick:
Insert the USB flash drive/memory stick into a USB port on your computer.
This should usually bring up a window telling you the flash drive/memory stick is now a drive on your PC. If this occurs, click ‘Open folder to view files’ on the options menu. If the window does not appear, click on the Start button and go to Computer. Under ‘Devices with Removable Storage’ you should be able to see the flash drive as a removable disk. Click on this.
Go to the files on your computer you wish to move and right-click to copy them.
Go to the window for the removable disk and paste your files into the flash drive.
Can I transfer data between computers with USB?
A normal USB cable will not allow you to transfer data between computers. However, there are special USB cables called USB-USB bridged cables which contain technology that allows two PCs to communicate.
Warning: Connecting two PCs together using a normal A/A USB cable can damage both computers.
Identifying your USB connector
USB safety and security FAQs