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Physical Connectivity
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Universal Serial Bus (USB) is an industry standard that defines the cables, connectors and communications protocols used for data and power supply between two devices. It was designed to standardise the connection of peripherals (such as for computers and peripherals, or personal devices).

Date of Release (YYYY-MM)
 Maximum Speed
 USB 1.1  1998-08
 12 Mbps
 USB 2.0  2000-04
 480 Mbps
 USB 3.0  2008-11
 5 Gbps
 also called USB 3.1 Gen 1
 USB 3.1  2013-07
 10 Gbps
 also called USB 3.1 Gen 2

USB 2.0

USB 2.0 was released in April 2000, adding a higher maximum signaling rate of 480 Mbit/s (High Speed or High Bandwidth), 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 Speed signaling rate is limited to 280 Mbit/s or 35 MB/s.

USB 3.0

The USB 3.0 specification was released on 12 November 2008, with its management transferring from USB 3.0 Promoter Group to the USB Implementers Forum (USB-IF), and announced on 17 November 2008 at the SuperSpeed USB Developers Conference.

USB 3.0 is the third major version of the Universal Serial Bus (USB) standard for interfacing computers and electronic devices. Among other improvements, USB 3.0 adds the new transfer rate referred to as SuperSpeed USB (SS) that can transfer data at up to 5 Gbit/s (625 MB/s), which is about ten times as fast as the USB 2.0 standard. Manufacturers are recommended to distinguish USB 3.0 connectors from their USB 2.0 counterparts by blue color-coding of the Standard-A receptacles and plugs, and by the initials SS. 

A USB 3.0 Standard-A receptacle accepts either a USB 3.0 Standard-A plug or a USB 2.0 Standard-A plug. Conversely, it is possible to plug a USB 3.0 Standard-A plug into a USB 2.0 Standard-A receptacle. Similar principle of backward compatibility applies to connecting a USB 2.0 Standard-A plug into a USB 3.0 Standard-A receptacle. The Standard-A is used for connecting to a computer port, at the host side.

USB 3.1

A January 2013 press release from the USB group revealed plans to update USB 3.0 to 10 Gbit/s.The group ended up creating a new USB specification, USB 3.1, which was released on 31 July 2013, replacing the USB 3.0 standard. The USB 3.1 specification takes over the existing USB 3.0's SuperSpeed USB transfer rate, also referred to as USB 3.1 Gen 1, and introduces a faster transfer rate called SuperSpeed USB 10 Gbps, also referred to as USB 3.1 Gen 2, putting it on par with a single first-generation Thunderbolt channel. The new mode's logo features a caption stylized as SUPERSPEED+. 

The USB 3.1 standard increases the data signaling rate to 10 Gbit/s, double that of SuperSpeed USB, and reduces line encoding overhead to just 3% by changing the encoding scheme to 128b/132b. 

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

USB Type-C

USB Type-C Specification 1.0 was finalized in August 2014 and defines a new small reversible-plug connector for USB devices. The Type-C plug connects to both hosts and devices, replacing various Type-A and Type-B connectors and cables with a standard meant to be future-proof, similar to Apple Lightning and Thunderbolt. 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. 

Type-A and Type-B adaptors and cables are required for older devices to plug into Type-C hosts. Adapters and cables with a Type-C receptacle are not allowed.

Standard Connectors

The type-A plug has an elongated rectangular cross-section, inserts into a type-A receptacle on a downstream port on a USB host or hub, and carries both power and data. Captive cables on USB devices, such as keyboards or mice, will be terminated with a type-A plug. 

A type-B plug has a near square cross-section with the top exterior corners beveled. As part of a removable cable, it inserts into an upstream port on a device, such as a printer. On some devices, the type-B receptacle has no data connections, being used solely for accepting power from the upstream device. This two-connector-type scheme (A/B) prevents a user from accidentally creating an electrical loop.

Mini and micro connectors

Various connectors have been used for smaller devices such as digital cameras, smartphones, and tablet computers. These include the now-deprecated (i.e. de-certified but standardized) Mini-A and Mini-AB connectors; Mini-B connectors are still supported, but are not On-The-Go-compliant. The Mini-B USB connector was standard for transferring data to and from the early smartphones and PDAs. The Mini-A connector and the Mini-AB receptacle connector were deprecated on 23 May 2007. 

The micro connector is also designed to reduce the mechanical wear on the device; instead the easier-to-replace cable is designed to bear the mechanical wear of connection and disconnection. The Universal Serial Bus Micro-USB Cables and Connectors Specification details the mechanical characteristics of Micro-A plugs, Micro-AB receptacles (which accept both Micro-A and Micro-B plugs), and Micro-B plugs and receptacles, along with a standard-A receptacle to Micro-A plug adapter.
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