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The host controller connects a tree of USB devices to a host (computer, console, and so on). Multiple host controllers can be present. Each host controller anchors one device tree.
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A hub is a device with multiple ports, into which are plugged other hubs and functions. The root hub integrates with the host controller. All other hubs are external hubs.
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A function is a device that provides a capability to the host. It plugs into a hub (external or root). Mice, keyboards, cameras, scanners, and printers are examples.
USB is implemented in terms of software drivers (which I don't discuss here) and hardware. The hardware divides into host controllers, hubs, and functions:
This hardware arranges into a tiered-star topology where each star's center is a hub: both the root hub and external hubs sit at the center of their connected external hubs/functions. Figure 1 reveals this topology.
Configurations, Interfaces, and Endpoints
A function can have one or more configurations that control how the function behaves. Configurations can differ in how much power they consume, whether and how they remotely wake up a suspended computer, and more.
A trackball device that can be configured as a mouse or as a joystick is an example of a function with multiple configurations. Another example is an Integrated Services Digital Network (ISDN) communications device that offers one 128Kb channel or two 56Kb channels.
Each configuration contains one or more interfaces that specify how software accesses the hardware. Interfaces frequently have alternate settings corresponding to different bandwidth requirements.
An interface exposes endpoints that each serve as a data transfer source or a data transfer destination. Each endpoint supports data transfer in a single direction and has a unique numeric identifier. Figure 2 relates endpoints to interfaces and configurations.
