Universal Serial Bus (USB in short), is the serial converge-wires standard of connecting external devices. It is widely used on computer and also STB and PSP. The compelementory standard On-The-Go (OTG) makes it possible to exchange information between portable units.

Introduction
USB is the standard of external bus. It is used to normalizing the connection and communication between computer and external devices. USB supports the plug and play function and hot plug function of devices. USB can be used to connect as many as 127 external devices, such as mouse, modem, and keyboard, etc. USB is jiont applicated by a few companies such as Intel, CPQ, IBM, Microsoft, etc. Since the release of 1996, it has replaced the serial port and parallel port, and nowadays, it has become the most popular port of personal computer and a large amount of smart devices. From the first version USB V0.7 released in November 11, 1994, it has developed theough many years, and now the newest version is USB V3.0.

Standard
USB 1.0
USB 1.0 is released in 1996. The speed of it is only 1.5Mb/s; in 1998, it is upgraded to USB 1.1, and the speed is greatly advanced to 12Mb/s. There are still some old devices with such port of this standard. USB 1.1 is a common standard of USB. Its high speed system of transmission rate is 12Mnps. Its low speed system of transmission rate is 1.5 Mbps. (b stands for Bit). 1MB/s equals 8MBbPS. 12Mbps equals 1.5MB/s. Most of MP3 are with this kind of port.

USB2.0
USB2.0 is developed from USB1.1. Its speed has reached 480Mbps. That is 60MB/s. it satisfys the rate requirement of most external devices. The Enhanced Host Controller Interface (EHCI) of USB2.0 defines a compatible architecture with USB1.1. It can use the driver of USB2.0 drives USB1.1. That means every device supports USB1.1 can be directly used on USB2.0 port and there is no need to worry about the compatibility. And some accessories such as USB line, plug, and so on, can be used directly.

USB 3.0
　　 The USB3.0 Promoter Group made up by industry giants such as Intel, Microsoft, HP, TI, NEC, ST-NXP, etc, anouces that the new generation USB3.0 formulated by the Group has finished and public released. The new standard provides a 10 times speed more than USB2.0 and higher efficiency of energy conservation. It can be widely used by PC external devices and consumer electronics.

Advantages
The popularization of USB devices is because of the following advantages:
1. It can be hot plugged. This allows users to use USB devices directly while computer is working instead of repeat the action of "turn off the computer, then plug in the parallel port or serial port, and then turn on the computer."
2. It is convenient to carry. USB devices are mostly small, light, and thin. For users, hardwares of 20G, USB weighs half of IDE. If users want to carry on with large amount of data, USB will be the best choice.
3. Standard consolidated. IDE port hardware, sereial port mouse and ketboard, parallel port printer and scanner are the most common devices. But since USB came up, all of these external devices can be connected with PC with the same standard. At that time, USB hardware, USB mouse, USB printer, etc, came up.
4. It can be connected with multiple devices. There are always many USB ports on PC. It is possible to connect a few devices at the same time. If connect the PC with a USB HUB with four ports, there are four more ports to connect, and so on. There won't be any problems to connect all of your family's devices with one computer. (P.S: the greatest number of connected devices can be 127.)