The USB interface expands a computer's plug and play capabilities, as it allows a wide array of devices that have been specifically created for USB to be connected with the PC, either with or without device drivers installed. USB's standard design was accomplished by the USB Implementers Forum (USB-IF), a consortium of leading computer and electronic companies that serves as the industry standards body. As a result of the standardization, third-party hardware is identical on all platforms and differs only in the required OS-specific software.
The advantages from USB aside from expandability are lower production cost, auto-configuration, improved performance and reliable speed. The USB interface also provides power to peripherals without the need for a power adapter. Common USB devices include mice, keyboards, joysticks, webcams, game pads and other low-bandwidth devices.
USB controllers
Before USB 2.0, there are two competing Host Control Device implementations: the Open Host Controller Interface (OHCI) developed by Microsoft, Compaq and National Semiconductor, and the Universal Host Controller Interface (UHCI) made by Intel. The USB-IF mandated a unified implementation in USB Version 2.0, resulting in the Enhance Host Controller Interface (EHCI). Only EHCI supports hi-speed transfers of up to 480 Mbit/second, while the original competing implementations were retained as companion or virtual host controllers for backward compatibility. The virtual host controllers support full speed (12 Mbit/second) and low-speed (1.5 Mbit/second) transfers.
Device Classes
Among the many defined device classes, ordinary users will most likely encounter the following:
oAudio interface devices ? include speakers, microphone, sound card
oCommunications devices ? Ethernet and serial adapter, modem
