USB

in full Universal Serial Bus

Type of serial bus that allows peripheral devices (disks, modems, printers, digitizers, data gloves, etc.) to be easily connected to a computer. A “plug-and-play” interface, it allows a device to be added without an adapter card and without rebooting the computer (the latter is known as hot-plugging). The USB standard, developed by several major computer and telecommunications companies, supports data-transfer speeds up to 12 megabits per second, multiple data streams, and up to 127 peripherals.

(Universal Serial Bus) A widely used hardware interface for attaching peripheral devices. USB ports began to appear on PCs in 1997, and Windows 98 was the first Windows to support it natively. Within a few years, USB became popular for connecting nearly every external peripheral device. Replacing the serial and parallel ports on a PC, at least four USB ports are standard on every computer.

Hot Swappable
USB devices can be connected to without turning the computer off, enabling removable devices to be plugged and unplugged as needed. This feature, combined with easy-to-reach ports on the front of the computer case, gave rise to the ubiquitous USB drive, which is used for storage and numerous applications. See USB drive and U3.

USB 1.0 and USB 2.0
USB has a maximum bandwidth of 12 Mbps for USB 1.0 and 1.1 and 480 Mbps for Hi-Speed USB 2.0. Up to 127 peripheral devices can be attached to the bus, and USB 1.1 devices can plug into USB 2.0 ports. Fast devices can use the full bandwidth, while low-speed ones can use a 1.5 Mbps subchannel.

SuperSpeed USB 3.0
USB 3.0 (SuperSpeed USB) is expected to materialize in the 2009-2010 time frame. Augmented by fiber cable, USB 3.0 raises the data rate to 4.8 Gbps, a 10x increase. In addition, USB 3.0 does not continuously poll the devices that are attached as does USB 2.0, thereby reducing overhead on the CPU.

USB Hubs
A USB hub provides additional ports for the user. Like any other USB device, the hub plugs into a USB port on the computer and provides four or more USB ports for peripherals. The peripherals themselves may also contain hubs. For example, a monitor may have a built-in USB hub in order to provide a convenient desktop location for plugging in other devices. For more details, see USB hub.

Type A and B, Mini-A and B
The USB ports on computers and hubs have a rectangular Type A socket, and peripheral devices have a cable with a Type A plug. Peripherals that do not have an attached cable have a square Type B socket on the device and a separate cable with a Type A and Type B plug.

The Mini-A and Mini-B connectors are used on small portable devices such as PDAs, music players, digital cameras and cellphones. Hubs may use Mini-B instead of Type B for the computer connection. See PoweredUSB, USB device class, USB OTG, USB drive, USB printer, USB switch and USB toy.

USB Connections
Hubs can be stand-alone units or built into the monitor or keyboard. The rectangular Type A sockets are used on the PC and hubs, while Type B sockets are on the input side of hubs and on peripherals.

USB Plugs and Sockets

Sockets on a Hub
This Belkin hub, which plugs into a wall outlet for power, is connected to the computer by a Mini-B/Type A cable. It provides four Type A ports for peripherals, netting out to only three extra, because one Type A port on the computer is used to connect the hub.

USB-based Land Warrior
Not your ordinary computer, the Land Warrior is a body-worn system that uses USB as its "personal network." Developed by Pacific Consultants, its USB controller was created by TransDimension. (Image courtesy of the U.S. Department of Defense.)

1.	(architecture)	USB - Universal Serial Bus.
2.	(communications)	USB - Upper Side-Band modulation.