As shown in Figure 5-2, the simple connection of a host to a device requires interaction between a number
of layers and entities. The USB Bus Interface layer provides physical/signaling/packet connectivity
between the host and a device. The USB Device layer is the view the USB System Software has for
performing generic USB operations with a device. The Function layer provides additional capabilities to
the host via an appropriate matched client software layer. The USB Device and Function layers each have a
view of logical communication within their layer that actually uses the USB Bus Interface layer to
accomplish data transfer.
The physical view of USB communication as described in Chapters 6, 7, and 8 is related to the logical
communication view presented in Chapters 9 and 10. This chapter describes those key concepts that affect
USB implementers and should be read by all before proceeding to the remainder of the specification to find
those details most relevant to their product.
To describe and manage USB communication, the following concepts are important: