Universal Serial Bus (USB)

The Universal Serial Bus (USB) was designed to establish communication between devices and a host computer and is intended to replace many types of serial and parallel ports. The Universal Serial Bus (USB) is a standard for peripheral devices that began development in 1994. It was originally intended to make it essentially easier to connect external devices to computers by replacing the many connectors that computers had and simplifying software configuration of all the devices connected to USB. The first silicon for USB was introduced in 1995 by Intel. The USB 1.0 specification was introduced in January 1996, and four years later, the USB 2.0 specification was released.

A USB system has an asymmetric design that is made up of a host, several downstream USB ports, and many peripheral devices connected in a tiered-star topology. Additional USB centers may be included in the layers to create a branching structure with up to five levels. USB devices are connected in series through hubs. The root hub is built into the host controller, and sharing hubs allow multiple computers to access the same peripheral device. An actual USB device may consist of a number of logical sub-devices, known as device functions. The communication linked to USB is based on pipes. A pipe is a connection from the host computer or other device to a plausible unit called an endpoint. There are two types of pipes: stream and message pipes. A stream pipe is a one directional pipe connected to a one directional endpoint that transfers data using an isochronous, interrupt, or bulk transfer. A message pipe is a two directional pipe connected to a two directional endpoint that is solely used for the control of data flow. An endpoint is permanent as it is built into the USB device by the manufacturer. The type of pipe used in the UDB is dependant upon the type of data transfer that is being used. Isochronous transfers are known for their guaranteed data rate, which is usually quick. However, they also risk possible data loss. Interrupt transfers are used for devices that need guaranteed quick responses. Bulk transfers are bulky, irregular transfers that tend to use all of the remaining available bandwidth, but with no guarantees on bandwidth or latency. Control transfers are typically used for short, simple commands to the device.

Connecting USB connectors to the USB bus is generally easy due to the design of the plug. It is difficult to attach it incorrectly. However, it is not obvious at a glance whether the connector should be face up or face down. Thus, it may be necessary to try the insertion both ways. The maximum length of a standard USB cable is 16.4 feet. The USB standard specifies fairly slack tolerances for USB connectors with the intention of minimizing incompatibilities in connectors made by different manufacturers. It also specifies the limits to the size of a connecting device in relation to its coordinating plug. There are many different types of plugs, such as the Micro-B plug, Mini-B plug, Standard-A plug, and Standard-B plug. The Standard-A plug is a flattened rectangle that is inserted into a downstream-port receptacle on the USB host. It carries both power and data, and is usually seen on cables that are permanently attached to a device. A Standard-B plug is a square shape with beveled outer corners, and typically plugs into an upstream receptacle on a device that uses a removable cable. Type B plug carries data and delivers power.

USB can be compared with other device connection technologies. It was originally seen as an accompaniment to FireWire because it operated slowly with small peripherals while FireWire interconnected peripherals such as hard disks and audio interfaces. It can also be compared to Ethernet, digital musical instruments, and eSATA.
The most recent new feature of USB is the SuperSpeed bus that provides a quicker transfer mode. Full-duplex signaling is seen when the bus is operating, when two differential pairs separate from the non-SuperSpeed differential pair. The end result is two wires for power and ground, two wires for non-SuperSpeed data, and four wires for SuperSpeed data, as well as a shield that was not required in previous specifications. SuperSpeed institutes communication between the host and each device it is connected to.

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