|Friday, 03 November 2006|
A Universal Serial Bus (USB) is an interface standard for connecting a personal computer and a peripheral device. The Universal Serial Bus (USB) is a peripheral bus specification developed by personal computer (PC) and telecommunication industry companies that brings the plug and play features of computer peripherals outside the PC box. This eliminates the need to install cards into dedicated computer slots and reconfigure the system. Personal computers equipped with USB allow computer peripherals to be automatically configured as soon as they are physically attached. This eliminates the need to reboot or run setup software. USB supports data exchange between a host computer and a wide range of simultaneously accessible devices which share USB bandwidth through a token scheduled protocol. The bus allows peripherals to be attached, configured, used, and detached while the host is in operation. USB also allows multiple devices to run simultaneously on a computer, with dedicated hubs and peripherals such as monitors and keyboards also acting as additional plug-in sites. The USB interface is functioned with the master/slave architecture. Therefore, the data transfer between a computer and a peripheral can be established only after the computer is recognized to be a host and the peripheral is recognized to be a device. Computer systems typically include a bus over which data and control signals are exchanged with peripheral devices. These buses are often categorized as either parallel buses or serial buses. Parallel buses include multiple data lines, whereas serial buses include a single data line. The USB specification supports four basic types of data transfers: control transfers, bulk transfers, interrupt transfers and isochronous transfers. Control data transfers are used by USB software to configure devices when they are first attached. Bulk data transfers typically consist of larger amounts of data such as used for printers or scanners. Interrupt data transfers are small, spontaneous data transfers from a device. Isochronous data transfers are continuous and real-time in creation, delivery and consumption. The USB data transfer model between a source or destination on the host and an endpoint on a device is referred to as a pipe. There are two types of pipes: stream and message. Stream data has no USB defined structure while message data does. Pipes have associations of data bandwidth, transfer service type, and endpoint characteristics such as directionality and buffer sizes. Pipes come into existence when a USB device is configured.
A USB bus connects USB devices with a USB host. There is only one USB host on any USB system. A host controller interfaces the USB bus to the host computer system. The host controller may be implemented in a combination of hardware, firmware or software. A USB host may be a desktop computer, a laptop computer, or some other USB enabled device. The host acts as master of the bus, acknowledging attachment and removal of peripherals, initiating enumeration processes and all subsequent USB transactions on the bus, collecting status and activity statistics, and controlling electrical interface between the host and USB peripherals. A USB hub may be used to control USB connections between a plurality of USB hosts and peripheral devices. USB devices are divided into device classes such as hub, locator, or text devices. USB devices are required to carry information for self-identification and generic configuration. All USB devices are accessed by a unique USB address. A USB (Universal Serial Bus) hub is used in a network for connecting a set of computer peripheral apparatus to a host computer. The USB transfers signal and power over a four wire cable. USB cables have an upstream and a downstream connector. The upstream and downstream connectors are not mechanically interchangeable to eliminate illegal loopback connections at the hubs. The USB device that is addressed selects itself by decoding the appropriate address fields. In a given transaction, data is transferred either from the host to a device or from a device to the host. The peripherals attached to a USB share the bandwidth of the USB through a host scheduled token based protocol. A USB connection generally includes both a high-speed data bus and a power bus, and typically may be coupled to as many as 127 peripheral devices. A bus-powered USB peripheral device obtains its power from a host computer or a USB hub through the power bus in a USB connection. Both USB peripheral devices and USB hubs may be either self-powered or may obtain power from a USB host (bus-powered). Mobile communication devices generally include a portable power supply, such as a rechargeable battery, and thus typically operate as self-powered USB peripheral devices.
The USB cable is a four wire cable, and the maximum cable length is about 5 meters. A USB connector is disposed at an end of the USB cable. The USB connector is inserted into a mount port disposed on a computer. When the USB connector is inserted into the mount port, a fastening claw in the USB connector is fastened to a fasten protrusion formed in the mount port, then the USB connector is fastened to the computer. The Universal Serial Bus (USB) connector is widely used for connecting a computer and a peripheral device. The USB connector can transmit signals at a high speed and has become a standard auxiliary device for computer systems. There are typically two connector types, and no cross-over cables and adapters are needed. The properties of the USB cable must be adapted to carry information in accordance with the outlined specifications for the particular cable, and to comply with the adapter plugs that accompany USB outlets. These specifications include transmission rates or bandwidths, voltage ratings, temperature ratings, insulation resistance, conductor resistance or impedance at specified temperatures. A typical round USB cable conforming to the USB 2.0 includes double shielded twisted pairs of conductors. The two twisted pairs of wires, which are used as the data conductors, along with power lines and encased within a round jacket formed of polypropylene thread. A circular configuration is used to facilitate easier placement of shielding and extrusion of the outer jacket. In some USB cables, shielding is provided and the conductors are generally configured of four wires, arranged in two insulated, twisted pairs of data transmission signal wires. Higher quality USB cables, which include twisted, paired conductors and shielding, are generally capable of data transmission rates of 12 Mbps. A polypropylene thread sealer is filled around the four wires, including the two pairs of twisted conductors and the two power wires, thereby forming a round, cross-sectional shape, which is an easy shape for extrusion and wrapping with a shield.