What is USB (Universal Serial Bus)?

The USB standard enables com­mu­ni­ca­tion between digital devices. Almost all pe­riph­er­al devices, such as a keyboard, mouse or printer, can be connected with a USB. There is however not much that’s “standard” about the tech­nol­o­gy. Over the years, it has gone through several upgrades and will probably continue to be modified in the future

You’re most likely aware of the term USB, as USB cables and ports are a part of everyday life. From the digital camera to the printer all the way to the external hard drive and the smart­phone, many digital devices can com­mu­ni­cate with each other using this type of con­nec­tion. But how did it all begin?

What is USB?

The de­f­i­n­i­tion of USB, short for Universal Serial Bus, is a universal interface based on a serial bus system. This interface allows external devices to be connected to a computer or other digital device, allowing for data transfer or power supply. When connected, data transfer can occur in both di­rec­tions. With the power supply function, a single USB cable enables certain pe­riph­er­al devices, such as external hard drives in 2.5-inch format, to be used without an external power chord.

USB was created in the late 1990s as a universal computer interface that even­tu­al­ly replaced other in­ter­faces such as serial and parallel ports. USB uses serial data transfer, or more precisely, bit-serial data transfer. Data is sent bit by bit in suc­ces­sion over one data channel instead of si­mul­ta­ne­ous­ly over several lines. This method enables higher data transfer speed. In addition, unlike parallel data transfer, different runtimes don’t have to be con­sid­ered for the re­spec­tive data lines. Nev­er­the­less, several lines can be used at the same time for serial data trans­mis­sion. Standards such as ethernet, HDMI and SATA also use serial data transfer methods.

Many USB devices can be connected to a computer or another end device while the system is running. This is called hot plugging. The USB interface of the computer usually rec­og­nizes the inserted device au­to­mat­i­cal­ly and starts the in­ter­ac­tion (plug and play or PnP). It’s also possible to unplug a USB device when the computer is running.

The current USB versions 3.1 and 3.2 are well suited for high data transfer rates. This has helped to make external hard drives a popular method for backing up data from computers. These hard drives are small and portable, and because they get their power via the USB cable, they don’t require an ad­di­tion­al power supply. Moreover, the ca­pac­i­ties of external hard drives have increased con­sid­er­ably over time and now offer multiple terabytes of storage.

What are the main features of USB?

• External interface with a universal standard for a wide range of end devices
• Capable of hot plugging (the ability to connect and dis­con­nect while in operation)
• Plug-and-play (being au­to­mat­i­cal­ly rec­og­nized and operated by the connected device)
• Capable of supplying power at the same time
• Ex­pand­able via a USB hub (one USB port can be turned into multiple ports)
• Stable, space-saving plug with reverse polarity pro­tec­tion
• Backwards com­pat­i­ble with older USB standards

What are the USB connector types?

Through­out its existence, the Universal Serial Bus standard has gone through many different designs and features. With each new model smaller than the previous one, the connector size has had to shrink as well. This has resulted in the emergence of various mini and micro variants. Boxes and boxes worth of obsolete computer cables can be found in many attics and garages, waiting for the right moment to be taken away. All USB connector types are protected against polarity reversal and uniquely designed to prevent them from being mixed up.

Below you can find a brief overview of the different types of USB con­nec­tors:

USB-A

This type is the most common one amongst the first-gen­er­a­tion con­nec­tors. Type A con­nec­tors were mostly used for keyboards or mice. The rec­tan­gu­lar, flat plug contains 4 lines and a shield. Mini and micro versions are available as well as a Su­per­Speed version with 9 wires.

USB-B

Type-B con­nec­tors are often found on devices used around the house or office, such as hard disks or printers. The sockets have square-like di­men­sions and four lines plus the shield. The B type also has a Su­per­Speed version as well as mini and micro versions.

USB-C

The minia­tur­ized USB-C connector was in­tro­duced in 2014 to slim PCs and smart­phones. This version also in­tro­duced a new re­versible design, allowing for the plug to be inserted re­gard­less of which way it is facing. The smaller C-type will also serve as the basis for the new USB-4 gen­er­a­tion.

How does USB work?

Data is trans­mit­ted in series via a bus. The in­di­vid­ual bits are sent and received one after the other on one line. The data signal is trans­mit­ted sym­met­ri­cal­ly: one line transmits the data signal, the other line an inverted signal. This method ensures a high data transfer re­li­a­bil­i­ty. External elec­tri­cal fields cancel each other out, which prevents in­ter­fer­ence during data transfer.

The older USB 1.0 and USB 2.0 versions transfer data in both di­rec­tions over the same signal line. Ad­di­tion­al lines were put into place for USB 3.0 and sub­se­quent versions. In order to allow the USB cable to also act as a power supply, ad­di­tion­al wires have been added. The connector supplies a constant voltage of at least five volts. The number of volts varies depending on the USB version and connector type. Other factors that determine the maximum current are the USB type and connected devices as week as other spec­i­fi­ca­tions.

What are the different USB versions?

While the letters A, B and C indicate the connector design, the different version numbers stand for a specific elec­tri­cal protocol. This defines what each format needs to achieve tech­ni­cal­ly. Dif­fer­ences between the versions are shown in the table below:

What are USB transfer rates?

The Universal Serial Bus fa­cil­i­tates pri­or­i­ti­za­tion among the different types of data transfer and provides a range of transfer rates within each version’s specified transfer rate capacity. The host con­troller takes care of the bus logic and co­or­di­nates the exchange of data between the connected USB devices.

The table below shows how the data transfer capacity differs between versions. In reality, these rates are, however, usually sig­nif­i­cant­ly lower:

Which devices are USB com­pat­i­ble?

One of the reasons for the de­vel­op­ment of the USB standard was to create a uniform transfer system for pe­riph­er­al devices. This is why today almost all external devices can be connected to PC or laptop via a USB cable. These devices include:

• External hard disks, USB sticks
• Printers and scanners
• Digital cameras and webcams
• Keyboards and computer mice
• Drives and burners for DVD or CD
• Smart­phones, tablets and smart­watch­es