As illustrated in Figure C.1, USB uses a tiered topology, allowing you to attach up to 127 devices to the bus simultaneously. USB currently supports up to five tiers, and each device can be located up to five meters from its hub.
Figure C.1 Example of the USB topology
The three types of USB components are:
Host . Also known as the root , the root tier , or the root hub , the host is built into the motherboard or installed as an adapter card in the computer. The host controls all traffic on the bus and can also function as a hub.
Hub . Provides a point, or port, to attach a device to the bus. Hubs are also responsible for detecting devices which are attached or detached from them and for providing power management for devices attached to them. Hubs are either bus - powered , drawing power directly from the bus, or self - powered , drawing power from an external source. You can plug a self-powered device into a bus-powered hub. You cannot connect a bus-powered hub to another bus-powered hub or support more than four downstream ports. You cannot connect a bus-powered device that draws more than 100 milliamperes (mA) to a bus-powered hub.
Device . A USB-capable device, which is attached to the bus through a port. USB devices can also function as hubs. For example, a USB monitor can have ports for attaching a USB keyboard and a mouse. In this case, the monitor is also a hub.
When you plug a device into a particular port for the first time, Windows 2000 Professional must go through the detection and enumeration process with that device. In the enumeration process, Plug and Play devices are identified by the operating system.