The host computer is the root node of the USB tree and contains an implicit hub, called the root hub. A hub is a USB function that propagates USB data to one or more ports, thereby increasing the total number of functions that share the bus. A hub has one connection, called an upstream port, to higher levels of the USB tree. A hub can have any number of ports for connecting peripheral devices and other hubs. You can connect up to 127 total devices, including hubs, to the host computer. Peripheral devices are always leaf nodes within a USB bus. However, as a matter of practical implementation, many USB peripheral devices have hubs integrated into them, so a user may not need to purchase separate USB hubs.
The following illustration shows a USB bus with several common peripherals connected.
The association of the mouse with the keyboard's internal hub and the speakers with the monitor's internal hub is arbitrary. For example, you can connect the mouse to the monitor's internal hub, the modem to the keyboard's internal hub, and the speakers to the stand-alone hub in Tier 1 without affecting the system's functionality and without having to reconfigure any software on the host computer. USB devices and their corresponding USB device drivers behave identically, regardless of the specific bus topology.
Last updated on Tuesday, May 18, 2004
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