USB Physical layer and components description
USB 1.0 – Low speed – 1.5Mbps
USB 1.1 – Full Speed – 12 Mbps
USB 2.0 – High Speed – 480 Mbps
USB 3.0 – Supper Speed – 3.2 Gbps
Refer. Sec 4.1.1 – USB 2.0 spec
Due to timing constraints allowed for hub and cable propagation times, the maximum number of tiers allowed is seven (including the root tier). Note that in seven tiers[above fig. shows only 4] , five non-root hubs maximum can be supported in a communication path between the host and any device.
USB Host – There is only one host in any USB system. The USB interface to the host computer system is referred to as
the Host Controller. A root hub is integrated within the host system to provide one or more attachment points.
USB Devices – USB devices are one of the following
• Hubs – which provide additional attachment points to the USB
• Functions – which provide capabilities to the system. such as, pen drive, usb keyboard, mouse