History
Apple Computer's Apple Desktop Bus or ADB, introduced in the mid-1980s, allowed all sorts of low-speed devices like mice and keyboards to be daisy-chained into a single port on the computer, greatly reducing the number of ports needed, as well as the resulting cable clutter. ADB was universal on the Macintosh line by the late 1980s, and offered a clear advantage over the profusion of standards being used on PCs.
ACCESS Bus, is a peripheral-interconnect computer bus developed by Philips (now NXP) and DEC in the early 1990s, based on Philips' I²C system. It is similar in purpose to USB, in that it allows low-speed devices to be added or removed from a computer on the fly. While it was made available earlier than USB, it never became popular as USB gained in popularity.
USB technology has been under development since 1993. The first official definition USB 1.0, was introduced in 1996.
It provides a Low-Speed transfer rate of 1.5 Mbits/s for sub-channel keyboards and mice, and a Full-Speed channel at 12 Mbits/s. USB 2.0, which came in 2001, made a leap to Hi-Speed transfer rates of up to 480 Mbits/s.
USB 2.0 (High-Speed USB 2.0)
In 2002, a newer specification USB 2.0, also called Hi-Speed USB 2.0, was introduced. It increased the data transfer rate for PC to USB device to 480 Mb/s, which is 40 times faster than the USB 1.1 specification. With the increased bandwidth, high throughput peripherals such as digital cameras, CD burners, and video equipment could now be connected with USB. It also allowed for multiple high-speed devices to run simultaneously.
USB 3.0 (Super-Speed USB)
USB 3.0 is the third major version of the USB standard for computer connectivity. Among other improvements, USB 3.0 adds a new transfer mode called "SuperSpeed", capable of transferring data at up to 5 Gbits/s (625 MBytes/s), which is more than ten times as fast as the 480 Mbit/s. Besides different connectors used on USB 3.0 cables, they are also distinguishable from their 2.0 counterparts by either the blue color of the ports or the SS initials on the plugs.
Technical aspects
Communication architecture differences
USB 2.0 employs a communication architecture where the data transaction must be initiated by the host. The host will frequently poll the device and ask for data, and the device may only transmit data once it has been requested by the host. The high polling frequency not only increases power consumption, it increases transmission latency because the data can only be transmitted when the device is polled by the host.
USB 3.0 improves upon this communication model and reduces transmission latency by minimizing polling and also allowing devices to transmit data as soon as it is ready.
Physical differences
The physical difference between USB 2.0 and USB 3.0 is the number of wire connections. This new topology greatly improves bus utilization, resulting in improved system throughput. USB 2.0 uses four wires (2 for single balanced data and two for the power), which supports half-duplex communication. In this architecture, a single bi-directional data pipe is used where data only flows in one direction at any given time.
In comparison, USB 3.0 adds five wires for a total of nine wires, and utilizes a unicast dual-simplex data interface that allows for two uni-directional data pipes, with each pipe handling communication for a single direction.
Power delivery
USB 3.0 also provides more efficient power management and increased power delivery over USB 2.0. The amount of current draw for USB 3.0 devices operating in Super-Speed mode is now 900 mA, resulting in an increase in total power delivery from 2.5 W to 4.5 W (at 5 V).
Cable length: USB 3.0 vs USB 2.0
The standard maximum cable length is 5 meters for USB 2.0 devices. The USB 3.0 standard does not specify a standard length; the maximum distance currently supported in USB 3.0 is 3 meters.
Conclusion
USB 3.0 or Super-Speed USB overcomes key limitations of other specifications all these limitations with six (over IEEE 1394b) to twenty (over USB 2.0) times higher bandwidth, better error management, higher power supply, longer cable lengths and lower latency and jitter times.
USB3.0 vs USB2.0
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