USB

Overview

In general, there are three basic kinds or sizes related to the USB connectors and types of established connection: the older "standard" size, in its USB 1.1/2.0 and USB 3.0 variants (for example, on USB flash drives), the "mini" size (primarily for the B connector end, such as on many cameras), and the "micro" size, in its USB 1.1/2.0 and USB 3.0 variants (for example, on most modern cellphones).

Mode    Data rate             Introduced in

Low Speed          1.5 Mbit/s           USB 1.0

Full Speed           12 Mbit/s             USB 1.0

High Speed         480 Mbit/s          USB 2.0

SuperSpeed       5 Gbit/s                USB 3.0

SuperSpeed+    10 Gbit/s              USB 3.1

Unlike other data cables (Ethernet, HDMI etc.), each end of a USB cable uses a different kind of connector; an A-type or a B-type. This kind of design was chosen to prevent electrical overloads and damaged equipment, as only the A-type socket provides power. There are cables with A-type connectors on both ends, but they should be used carefully.[4] Therefore, in general, each of the different "sizes" requires four different connectors; USB cables have the A-type and B-type plugs, and the corresponding receptacles are on the computer or electronic device. In common practice, the A-type connector is usually the full size, and the B-type side can vary as needed.

The mini and micro sizes also allow for a reversible AB-type receptacle, which can accept either an A-type or a B-type plug. This scheme, known as "USB On-The-Go", allows one receptacle to perform its double duty in space-constrained applications.

Counter-intuitively, the "micro" size is the most durable from the point of designed insertion lifetime. The standard and mini connectors were designed for less than daily connections, with a design lifetime of 1,500 insertion/removal cycles.[5] (Improved mini-B connectors have reached 5,000-cycle lifetimes.) Micro connectors were designed with frequent charging of portable devices in mind; not only is design lifetime of the connector improved to 10,000 cycles,[5] but it was also redesigned to place the flexible contacts, which wear out sooner, on the easily replaced cable, while the more durable rigid contacts are located in the micro-USB receptacles. Likewise, the springy part of the retention mechanism (parts that provide required gripping force) were also moved into plugs on the cable side.[6]

USB connections also come in five data transfer modes: Low Speed, Full Speed, High Speed, SuperSpeed, and SuperSpeed+. High Speed is only supported by specifically designed USB 2.0 High Speed interfaces (that is, USB 2.0 controllers without the High Speed designation do not support it), as well as by USB 3.0 and newer interfaces. SuperSpeed is supported only by USB 3.0 and newer interfaces, and requires a connector and cable with extra pins and wires, usually distinguishable by the blue inserts in connectors.

History

A group of seven companies began the development of USB in 1994: Compaq, DEC,IBM, Intel, Microsoft, NEC, and Nortel.[8] 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;[9][10] the first integrated circuits supporting USB were produced by Intel in 1995.[11]

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".[11] 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.[12]

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 40-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), decrease power consumption, increase power output, and be backward compatible with USB 2.0.[13] USB 3.0 includes a new, higher speed bus called SuperSpeed in parallel with the USB 2.0 bus.[14] For this reason, the new version is also called SuperSpeed.[15] The first USB 3.0 equipped devices were presented in January 2010.[15][16]

As of 2008, approximately six billion USB ports and interfaces were in the global marketplace, and about two billion were being sold each year.[17]

In December 2014, USB-IF submitted USB 3.1, USB Power Delivery 2.0 and USB type-C specifications to the IEC (TC 100 – Audio, video and multimedia systems and equipment) for inclusion in the international standard IEC 62680 "Universal Serial Bus interfaces for data and power", which is currently based on USB 2.0.[18]

Version history

Prereleases

The USB standard evolved through several versions before its official release in 1996:

                USB 0.8 – released in December 1994

                USB 0.9 – released in April 1995

                USB 0.99 – released in August 1995

            USB 1.0 Release Candidate – released in November 1995

USB 1.x

 1996, USB 1.0 specified data rates of 1.5 Mbit/s (Low Bandwidth or Low Speed) and 12 Mbit/s (Full Bandwidth or Full Speed). It did not allow for extension cables or pass-through monitors, due to timing and power limitations. Few USB devices made it to the market until USB 1.1 was released in August 1998, fixing problems identified in 1.0, mostly related to using hubs. USB 1.1 was the earliest revision that was widely adopted.

USB 2.0

USB 2.0 was released in April 2000, adding a higher maximum signaling rate of 480 Mbit/s called High Speed, in addition to the USB 1.x Full Speed signaling rate of 12 Mbit/s. Due to bus access constraints, the effective throughput of the High Speedsignaling rate is limited to 35 MB/s or 280 Mbit/s.[19][20]

Further modifications to the USB specification have been made via Engineering Change Notices (ECN). The most important of these ECNs are included into the USB 2.0 specification package available from USB.org:[21]

            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.

           Pull-up/Pull-down Resistors ECN: Released in May 2002

           Interface Associations ECN: Released in May 2003.

New standard descriptor was added that allows associating multiple interfaces with a single device function.

             Rounded Chamfer ECN: Released in October 2003.

A recommended, backward 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 specified Unicode, but 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 and updated on 15 April 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 "sleep", 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:[22] Released in December 2010.

Several changes and increasing limits including allowing 1.5 A on charging ports for unconfigured devices, allowing High Speed communication while having a current up to 1.5 A and allowing a maximum current of 5 A.

USB 3.0

USB 3.0 standard was released in November 2008, defining a new SuperSpeed mode. A USB 3.0 port, usually colored blue, is backward compatible with USB 2.0 devices and cables.

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.[23] This move effectively opened the specification to hardware developers for implementation in products.

The new SuperSpeed bus provides a fourth transfer mode with a data signaling rate of 5.0 Gbit/s, in addition to the modes supported by earlier versions. The payload throughput is 4 Gbit/s[citation needed] (due to the overhead induced by used 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 in SuperSpeed transfer mode; in the modes supported previously, by 1.x and 2.0, communication is half-duplex, with direction controlled by the host.[24]

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.[25] Additionally, there is a Battery Charging Specification (Version 1.2 – December 2010), which increases the power handling capability to 1.5 A but does not allow concurrent data transmission.[22] The Battery Charging Specification requires that the physical ports themselves be capable of handling 5 A of current[citation needed] but limits the maximum current drawn to 1.5 A.

USB 3.1

A January 2013 press release from the USB group revealed plans to update USB 3.0 to 10 Gbit/s.[26] The group ended up creating a new USB version, USB 3.1, which was released on 31 July 2013,[27] introducing a faster transfer mode called "SuperSpeed USB 10 Gbit/s", putting it on par with a single first-generationThunderbolt channel. The new mode's logo features a "Superspeed+" caption (stylized as SUPERSPEED+). The USB 3.1 standard increases the data signaling rateto 10 Gbit/s in the USB 3.1 Gen2 mode, double that of USB 3.0 (referred to as USB 3.1 Gen1) and reduces line encoding overhead to just 3% by changing theencoding scheme to 128b/132b.[28] The first USB 3.1 implementation demonstrated transfer speeds of 7.2 Gbit/s.[29]

The USB 3.1 standard is backward compatible with USB 3.0 and USB 2.0.

The USB Type-C Specification 1.0 defines a new small reversible-plug connector for USB 3.1 devices. The type-C plug will be used at both host and device side, replacing multiple type-B and type-A connectors and cables with a future-proof[30] standard similar to Apple Lightning and Thunderbolt.[31][32] The 24-pin double-sided connector provides four power/ground pairs, two differential pairs for USB 2.0 data bus (though only one pair is implemented in a type-C cable), four pairs for high-speed data bus, two "sideband use" pins, and two configuration pins for cable orientation detection, dedicated biphase mark code (BMC) configuration data channel, and VCONN +5 V power for active cables.[33][34] Type-A and type-B adaptors/cables will be required for legacy devices in order to plug into type-C hosts, however adaptors/cables with a type-C receptacle are not allowed.

Full-featured USB 3.1 type-C cables are active, electronically marked cables and contain a chip with an ID function based on the configuration data channel and vendor-defined messages (VDMs) from the USB Power Delivery 2.0 specification. USB 3.1 type-C devices also support power currents of 1.5 A and 3.0 A over the 5 V power bus in addition to baseline 900 mA; devices can either negotiate increased USB current through the configuration line, or they can optionally support the full Power Delivery specification using both BMC-coded configuration line and legacy BFSK-coded VBUS line.

Alternate Mode dedicates some of the physical wires in the type-C cable for direct device-to-host transmission of alternate data protocols. The four high-speed lanes, two sideband pins, and - for dock, detachable device and permanent cable applications only - two USB 2.0 pins and one configuration pin can be used for Alternate Mode transmission. The modes are configured using VDMs through the configuration channel. As of December 2014, Alt Mode implementations includeDisplayPort 1.3[35] and MHL 3.0;[34][36] other serial protocols like PCI Express and Base-T Ethernet are possible.