What is USB?

 

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USB, short for Universal Serial Bus, is a popular serial bus interface mainly used for connecting external devices to personal computers. Launched in 1996, USB was a huge leap forward in serial interfacing. USB was intended to replace all legacy computer interfaces, namely parallel (printers) and traditional serial (PDAs, other peripherals).

It features improved plug-and-play  via “hot-swap” (the ability to connect a peripheral without having to reboot); the ability to provide power to low-consumption devices, negating the need for an external power source; and, for the most part, driver-less  installation of devices. Two current USB standards exist: the older 1.1 and the newer 2.0. USB 1.1 cables are limited to maximum data transmit speeds of 12 Mbps, while 2.0 cables have a maximum speed of 480 Mbps.
  USB controllers, hubs, and attached devices are layered in a tiered star topology, with one root controller, several hubs, and devices connected to those hubs. A single root controller can handle up to 127 total USB connections. Actual device communication is achieved with pipes, or connections between the root controller and a specific function of a device (e.g., a USB webcam's built-in microphone would be one specific device function). A single USB controller can have up to 32 active pipes, 16 for upstream connections (device to controller) and 16 for downstream connections (controller to device)  
 

The actual computer hardware used to enable USB connections is called a host controller. USB 1.x host controllers utilize two different, and competing, standards: Open Host Controller Interface (OHCI), developed by Compaq and Microsoft, among others; and Universal Host Controller Interface (UHCI), developed by Intel. UHCI utilizes a more software-driven interface, making it more processor-intensive, but cheaper to implement, than OHCI. For USB 2.x, a standard controller interface, called Enhanced Host Controller Interface (EHCI), was developed. EHCI is the only interface capable of USB 2.0's 480 Mbps maximum transmit speed; OHCI and UHCI can only support “full speed” (12 Mbps) and “low speed” (1.5 Mbps) connections.
USB cables themselves are comprised of a twisted pair of data cables. This “twisted set” reduces noise and crosstalk during data transmission, ensuring a reliable and uninterrupted data stream.

  

 
     

Future Development:

Development of the USB 3.0 standard, promising speeds of 4.8 Gbps, is ongoing.

    Fun Facts & Usage Applications
    • As of 2008, USB devices numbered nearly two billion world-wide. USB was originally intended to replace the multitude of ports on computers; computing is still its main application. The list of USB devices for computers is nearly endless: Flash drives, printers, keyboards, mice, wireless network adapters, PDAs, digital cameras, computer-to-computer connections, MP3 players, and joysticks are a few of the more common applications. Additionally, USB has spread to many mobile phones, game consoles, and other portable devices.
    • A sub-class of USB devices, known as “USB mass storage” devices,
    • Apple Computer's iMac, first released in 1998, was the first PC to exclusively utilize USB as opposed to legacy serial/SCSI ports.
    • PoweredUSB is a specialized USB specification allowing more current to supply power to higher-consumption devices. It is mostly used in the retail industry to provide standalone power to barcode scanners, printers, and pin pads connected to POS (point-of-sale) systems.
    • PictBridge is an industry standard of the Camera and Imaging Products Association that utilizes USB connectors and standards to provide printer-to-camera connections—and the subsequent ability to print pictures-- without needing a computer. Typically, PictBridge cameras have a miniUSB-A connector that connects to a Regular-A receptacle on a PictBridge printer.

     
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