USB uses four wire cable, which is used to transmit data to the serial port, and the other two for the downstream (Downstream) equipment to provide power, for high-speed and requires high bandwidth peripherals, USB to full speed 12Mbps transmission data; for low-speed peripherals, 1.5Mbps is the transmission rate of USB to transmit data. USB bus will be automatically converted according to the two kinds of transmission mode of the peripheral. USB is based on token bus. Similar to a token ring network or a FDDI based token bus. The USB master controller broadcasts a token, and the address of the device detects whether the token is in line with its own, and is responsive to the host by receiving or transmitting data. USB manages the USB bus power by supporting the suspension / recovery operation. USB system uses Lianxing level topology. This topology consists of three basic parts composition: host (host), hub (hub) and functional device.
　　Host, also known as root, root knot or root Hub, it does on the motherboard or as adapter mounted on the computer, the host contains a master controller and a root hub (Hub Root), controlling the flow of data and control information on the USB bus, each USB system can only have a root set line, which is connected to the main controller.
　　The hub is a particular component of the USB structure, which provides a device called port (Port) that is connected to the USB bus, and detects the device connected to the bus, and provides power management for these devices, responsible for fault detection and recovery of the bus. Hub can provide energy for the bus also for their own provide energy (from the outside to get a power), provided by their own energy devices can be inserted into the bus to provide energy hub, but the bus provides energy devices can not insert themselves provide energy hubs or support more than four downstream ports, such as bus provides energy equipment needs more than 100mA power, not the same bus provide power from the hub connection.
　　Function device is connected to the bus through port. USB at the same time to do Hub use.