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The physical phenomena on which the device relies (such as spinning platters in a hard drive) will also impose limits; for instance, no spinning platter shipping in 2009 saturates SATA revision 2.0 (3 Gbit/s), so moving from this 3 Gbit/s interface to USB 3.0 at 4.8 Gbit/s for one spinning drive will result in no increase in realized transfer rate.
USB 2.0 provides for a maximum cable length of 5 metres (16 ft 5 in) for devices running at high speed (480 Mbit/s). The primary reason for this limit is the maximum allowed round-trip delay of about 1.5 μs. If USB host commands are unanswered by the USB device within the allowed time, the host considers the command lost.
For high-power SuperSpeed devices, the limit is six unit loads or 900 mA (4.5 W)—almost twice USB 2.0's 500 mA. [ 13 ] : section 9.2.5.1 Power Budgeting USB 3.0 ports may implement other USB specifications for increased power, including the USB Battery Charging Specification for up to 1.5 A or 7.5 W, or, in the case of USB 3.1, the USB Power ...
USB 2.0 was released in April 2000, adding a higher maximum signaling rate of 480 Mbit/s (maximum theoretical data throughput 53 MByte/s [25]) named High Speed or High Bandwidth, in addition to the USB 1.x Full Speed signaling rate of 12 Mbit/s (maximum theoretical data throughput 1.2 MByte/s).
Specified USB data rates of 1.5 Mbit/s (Low-speed), 12 Mbit/s (Full-speed), 480 Mbit/s (High-speed) and 5 Gbit/s (SuperSpeed). xHCI 1.0, errata files 1-4: Released on January 17, 2011. Incorporated initial review feedback from larger 1.0 public audience, Save-Restore clarifications, and Hardware LPM support.
While storage devices usually have their size expressed in powers of 10 (for instance a 1 TB Solid State Drive will contain at least 1,000,000,000,000 (10 12, 1000 4) bytes), filesystem limits are invariably powers of 2, so usually expressed with IEC prefixes.
Cables are also allowed to support up to 5 A (with 20 V limit up to 100 W of power). However, the 20 V limit for 5 A cables has been deprecated in favor of 50 V. The combination of higher voltage support and 5 A current support is called EPR and allows for up to 240 W (48 V, 5 A) of power according to the USB PD specification.
Furthermore, the older PCI clocking scheme limits the bus clock to the slowest peripheral on the bus (regardless of the devices involved in the bus transaction). In contrast, a PCI Express bus link supports full-duplex communication between any two endpoints, with no inherent limitation on concurrent access across multiple endpoints.