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USB can connect a series of devices using a tiered star topology. The key elements in USB topology are the host, hubs, and devices, as illustrated in in Figure - USB Bus Topology. Each node in the illustration represents a USB hub or a USB device. At the top level of the graph is the root hub, which is part of the host. There is only one host in the system. The specification allows up to seven tiers and a maximum of five non-root hubs in any path between the host and a device. Each tier must contain at least one hub except for the last tier where only devices are present. Each USB device in the system has a unique address assigned by the host through a process called enumeration (see section Enumeration for more details on enumeration).
The host learns about the device capabilities during enumeration, which allows the host operating system to load a specific driver for a particular USB device. The maximum number of peripherals that can be attached to a host is 127, including the root hub.
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USB Host
The USB host communicates with the devices using a USB host controller. The host is responsible for detecting and enumerating devices, managing bus access, performing error checking, providing and managing power, and exchanging data with the devices.
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