PC Hardware in a Nutshell, 3rd Edition

USB Speeds

USB defines the following three speeds, all of which can coexist on one bus:

Low Speed

Low Speed USB peripherals operate at a data rate of 1.5 Mb/s, and are supported by USB 1.1 and USB 2.0 interfaces. Low Speed USB is intended for such low-bandwidth devices as mice and keyboards, and is designed to be inexpensive to implement. Low Speed USB devices use a captive cable that can be no longer than 3 meters. Actual throughput on Low Speed USB is typically limited by overhead and other factors to about 1.2 Mb/s, or 150 KB/s.

Full Speed

Full Speed USB peripherals operate at a data rate of 12.0 Mb/s, and are supported by USB 1.1 and USB 2.0 interfaces. Full Speed is the fastest speed supported by USB 1.0 and USB 1.1, and is intended for such moderate-bandwidth devices as printers and scanners. Full Speed USB devices use active components, which are more expensive to implement than the passive components used by Low Speed USB. Full Speed USB devices use a detachable cable that can be no longer than 5 meters. Full Speed USB seldom exceeds actual throughput of 900 KB/s or so.

Hi-Speed

Hi-Speed USB peripherals operate at a data rate of 480 Mb/s, and are supported only by USB 2.0 interfaces. Hi-Speed USB is intended for such high-bandwidth devices as external drives. Hi-Speed USB devices use active components that are more expensive than Full Speed USB components. Also, Hi-Speed USB hubs require additional circuitry to arbitrate between mixed Hi-Speed and Full Speed devices ...