Typical hi-speed USB hard drives can be written to at rates around 25–30 MB/s, and read from at rates of 30–42 MB/s, according to routine testing done by CNet. This is 70% of the total bandwidth available.
According to a USB-IF chairman, "at least 10 to 15 percent of the stated peak 60 MB/s (480 Mbit/s) of Hi-Speed USB goes to overhead — the communication protocol between the card and the peripheral. Overhead is a component of all connectivity standards."
For isochronous devices like audio streams, the bandwidth is constant, and reserved exclusively for a given device. The bus bandwidth therefore only has an effect on the number of channels that can be sent at a time, not the "speed" or latency of the transmission.
USB supports the following signaling rates: The terms speed and bandwidth are used interchangeably. "high-" is alternatively written as "hi-".
A low-speed rate of 1.5 Mbit/s (~183kB/s) is defined by USB 1.0. It is very similar to full-bandwidth operation except each bit takes 8 times as long to transmit. It is intended primarily to save cost in low-bandwidth human interface devices (HID) such as keyboards, mice, and joysticks. The full-speed rate of 12 Mbit/s (~1.43 MB/s) is the basic USB data rate defined by USB 1.1. All USB hubs support full-bandwidth.
A high-speed (USB 2.0) rate of 480 Mbit/s (~57 MB/s) was introduced in 2001. All hi-speed devices are capable of falling back to full-bandwidth operation if necessary; i.e. they are backward compatible with USB 1.1. Connectors are identical for USB 2.0 and USB 1.x.
A SuperSpeed (USB 3.0) rate of 4800 Mbit/s (~572 MB/s).
The written USB 3.0 specification was released by Intel and partners in August 2008. The first USB 3 controller chips were sampled by NEC May 2009 and products using the 3.0 specification arrived beginning in January 2010. USB 3.0 connectors are generally backwards compatible, but include new wiring and full duplex operation.