Universal Serial Bus(USB)

Universal Serial Bus (USB) is an industry standard developed in the mid-1990s that defines the cables, connectors and communications protocols used in a bus for connection, communication and power supply between computers and electronic devices.

USB was designed to standardize the connection of computer peripherals, such as keyboards, pointing devices, digital cameras, printers, portable media players, disk drives and network adapters to personal computers, both to communicate and to supply electric power. It has become commonplace on other devices, such as smartphones, PDAs and video game consoles. USB has effectively replaced a variety of earlier interfaces, such as serial and parallel ports, as well as separate power chargers for portable devices.
As of 2008, approximately 6 billion USB ports and interfaces were in the global marketplace, and about 2 billion were being sold each year.
A group of seven companies began the development of USB in 1994: Compaq, DEC, IBM, Intel, Microsoft, NEC and Nortel. The goal was to make it fundamentally easier to connect external devices to PCs by replacing the multitude of connectors at the back of PCs, addressing the usability issues of existing interfaces, and simplifying software configuration of all devices connected to USB, as well as permitting greater data rates for external devices. A team including Ajay Bhatt worked on the standard at Intel; the first integrated circuits supporting USB were produced by Intel in 1995.
The original USB 1.0 specification, which was introduced in January 1996, defined data transfer rates of 1.5 Mbit/s "Low Speed" and 12 Mbit/s "Full Speed" The first widely used version of USB was 1.1, which was released in September 1998. The 12 Mbit/s data rate was intended for higher-speed devices such as disk drives, and the lower 1.5 Mbit/s rate for low data rate devices such as joysticks.

A USB Standard Type A plug, the most common USB plug

The USB 2.0 specification was released in April 2000 and was ratified by the USB Implementers Forum (USB-IF) at the end of 2001. Hewlett-Packard, Intel, Lucent Technologies (now Alcatel-Lucent), NEC and Philips jointly led the initiative to develop a higher data transfer rate, with the resulting specification achieving 480 Mbit/s, a forty times increase over the original USB 1.1 specification.
The USB 3.0 specification was published on 12 November 2008. Its main goals were to increase the data transfer rate (up to 5 Gbit/s), to decrease power consumption, to increase power output, and to be backwards-compatible with USB 2.0. USB 3.0 includes a new, higher speed bus called SuperSpeed in parallel with the USB 2.0 bus. For this reason, the new version is also called SuperSpeed.The first USB 3.0 equipped devices were presented in January 2010.

USB 2.0 (High Speed)

The Hi-Speed USB Logo

• USB 2.0: Released in April 2000. Added higher maximum signaling rate of 480 Mbit/s (effective throughput up to 35 MB/s or 280 Mbit/s) (now called "Hi-Speed"). Further modifications to the USB specification have been done via Engineering Change Notices (ECN). The most important of these ECNs are included into the USB 2.0 specification package available from USB.org:
  • Mini-A and Mini-B Connector ECN: Released in October 2000.
    Specifications for Mini-A and B plug and receptacle. Also receptacle that accepts both plugs for On-The-Go. These should not be confused with Micro-B plug and receptacle.
  • Errata as of December 2000: Released in December 2000.
  • Pull-up/Pull-down Resistors ECN: Released in May 2002.
  • Errata as of May 2002: Released in May 2002.
  • Interface Associations ECN: Released in May 2003.
    New standard descriptor was added that allows multiple interfaces to be associated with a single device function.
  • Rounded Chamfer ECN: Released in October 2003.
    A recommended, compatible change to Mini-B plugs that results in longer lasting connectors.
  • Unicode ECN: Released in February 2005.
    This ECN specifies that strings are encoded using UTF-16LE. USB 2.0 did specify that Unicode is to be used but it did not specify the encoding.
  • Inter-Chip USB Supplement: Released in March 2006.
  • On-The-Go Supplement 1.3: Released in December 2006.
    USB On-The-Go makes it possible for two USB devices to communicate with each other without requiring a separate USB host. In practice, one of the USB devices acts as a host for the other device.
  • Battery Charging Specification 1.1: Released in March 2007 (Updated 15 Apr 2009).
    Adds support for dedicated chargers (power supplies with USB connectors), host chargers (USB hosts that can act as chargers) and the No Dead Battery provision which allows devices to temporarily draw 100 mA current after they have been attached. If a USB device is connected to dedicated charger, maximum current drawn by the device may be as high as 1.8 A. (Note that this document is not distributed with USB 2.0 specification package only USB 3.0 and USB On-The-Go.)
  • Micro-USB Cables and Connectors Specification 1.01: Released in April 2007.
  • Link Power Management Addendum ECN: Released in July 2007.
    This adds a new power state between enabled and suspended states. Device in this state is not required to reduce its power consumption. However, switching between enabled and sleep states is much faster than switching between enabled and suspended states, which allows devices to sleep while idle.
  • Battery Charging Specification 1.2: Released in December 2010.
    Several changes and increasing limits including allowing 1.5A on charging ports for unconfigured devices, allowing High Speed communication while having a current up to 1.5A and allowing a maximum current of 5A.

USB 3.0 (Super Speed)

Main article: USB 3.0

The Super-Speed USB Logo

USB 3.0 was released in November 2008. The standard claims a theoretical "maximum" transmission speed of up to 5 Gbit/s (625 MB/s). USB 3.0 reduces the time required for data transmission, reduces power consumption, and is backward compatible with USB 2.0. The USB 3.0 Promoter Group announced on 17 November 2008 that the specification of version 3.0 had been completed and had made the transition to the USB Implementers Forum (USB-IF), the managing body of USB specifications. This move effectively opened the specification to hardware developers for implementation in products. The new "SuperSpeed" bus provides a fourth transfer mode at 5.0 Gbit/s (raw data rate), in addition to the modes supported by earlier versions. The payload throughput is 4 Gbit/s (using 8b/10b encoding), and the specification considers it reasonable to achieve around 3.2 Gbit/s (0.4 GB/s or 400 MB/s), which should increase with future hardware advances. Communication is full-duplex during SuperSpeed; in the modes supported previously, by 1.x and 2.0, communication is half-duplex, with direction controlled by the host.

• Power Handling Capabilities: As with previous USB versions, USB 3.0 ports come in low-power and high-power variants, providing 150 mA and 900 mA respectively while simultaneously transmitting data at SuperSpeed rates. Additionally, there is a Battery Charging Specification (Version 1.2 - December 2010) which increases the power handling capability to 1.5A but does not allow concurrent data transmission. The Battery Charging Specification requires the physical ports themselves to be capable of handling 5A of current^{[citation needed]} but the specification limits the maximum current drawn to 1.5A.

A January 2013 press release from the USB group reveals plans to update USB 3 to 10Gb/s to put it on par with Thunderbolt by mid 2013