USB (Universal Serial Bus) is a serial bus standard, used to connect devices
with the host computer. It provides hot-pluggable Plug and Play serial interface
that ensures a standard, low-cost socket for adding external peripheral devices
ranging from interactive HID devices such as joysticks and pointing devices
to isochronous devices such as telephony, audio, and imaging devices. USB allows
integration of cascading hubs into desktop devices such as monitors and keyboards.
Any device that plugs into or is connected to a USB port is considered a USB
device and must comply with the requirements defined as USB requirements. USB
system hardware and devices, including hubs, must comply with the Universal
Serial Bus Specification, Revision 1.1.

When a system has more than one host controller, each host controller must
provide full bandwidth and isochronous support. Host controllers must be located
on the PCI bus to meet the requirement. The host controller providing USB 1.1
functionality must comply with the specifications for either OpenHCI (Open
Host Controller Interface Specification for USB), or UHCI (Universal Host Controller
Interface). Host controllers providing USB 2.0 functionality must comply with
the Enhanced Host Controller Interface Specification for Universal Serial Bus
2.0.

Device and driver designs must provide maximum flexibility for interface options
so that the operating system or other vendor-supplied resource management tool
can coordinate user preferences, allowing multiple devices and applications
simultaneously. Devices and drivers must provide multiple alternate settings
for each interface where any alternate setting consumes isochronous bandwidth.
The USB host controller must support wakeup capabilities from S1, S2, and S3
states. If the system contains multiple USB host controllers, all host controllers
integrated on the system board are required to support wakeup from S1, S2,
and S3. USB devices and USB client software and drivers must support multiple
system suspend and resume cycles into and out of S3.

A USB device or driver that fits into one of the USB device class definitions
must comply with the related USB device class specification. If a device vendor
builds a device in a device class that does not have generic operating system
driver support, or if the vendor must exploit additional, unique hardware features
in their device, they can create additional WDM mini-drivers. No requirement
is needed for hubs integrated into USB keyboards or into mobile systems. To
minimize USB power consumption requirements, bus-powered hubs must provide
ports that can be individually power switched.

A user must not be required to install software before hot-plugging a USB device.
Instead, the user must be able to hot-plug the USB device and then load any
software in response to operating system detection of the newly attached device.
An internal device that connects to a mobile PC using USB must not continually
maintain the system when all component parts are on. Such a device will override
system power-management settings that control power-saving modes to protect
battery life. When any USB device is connected but not active, the driver must
allow system power management to suspend the mobile PC.