Un dispositivo USB puede ser dividido en tres capas:

			The bottom layer is a bus interface that transmits and receives packets.
			The middle layer handles routing data between the bus interface and various endpoints on the device. An endpoint is the ultimate consumer or provider of data. It may be thought of as a source or sink for data.
			• The top layer is the functionality provided by the serial bus device, for instance, a mouse or ISDN interface.

This chapter describes the common attributes and operations of the middle layer of a USB device. These attributes and operations are used by the function-specific portions of the device to communicate through the bus interface and ultimately with the host.