USB hub

Monday, 13 November 2006

A universal serial bus (USB) hub is a standard peripheral interface for connecting a computer to different devices, such as digital cameras, monitors, modems, mouse, printers, scanners, game controllers, keyboards, and so on. The Universal Serial Bus (USB) is a peripheral bus specification that supports data exchange between a host computer and a wide range of simultaneously accessible peripherals. The bus allows peripherals to be attached, configured, used, and detached while the host and other peripherals are in operation. The capability eliminates the need to install interface card into dedicated computer slots and reconfigure the system each time a peripheral is attached or detached from a PC. Computer peripheral input/output (I/O) devices connect to a variety of data ports or external connectors of a host computer system, which includes a system processor and memory. Peripheral device interfaces that comply with the specification are referred to as USB interfaces and have been included in many recently developed personal computer systems. Such USB devices are generally referenced as either low-speed devices, capable of transferring data at a rate of 1.5 Megabits per second (Mb/s); or high-speed devices (also called full-speed devices) capable of transferring data at 12 Mb/s. Under the USB 2.0 specification, full-speed devices are capable of using 40 times multipliers for a transfer rate of 480 Mb/s, and such USB devices are typically known as true high-speed devices. A USB standard interface allows up to 127 peripheral devices to be attached to a host, usually a personal computer (PC), through a 4-wire bus. Such devices can be connected to the PC either directly, or via hubs. The hubs provide additional connections to the USB. The USB technology permits the user to connect and disconnect USB peripheral devices to or from the USB system without requiring system power-down, and generally with little or no configuration input required from the user. This capability provides considerable flexibility and possible cost reduction in comparison to many contemporary systems, particularly those which can only support one peripheral device per port. USB systems also can easily integrate various functions such as data, voice, and video, into the system through a single serial-data transfer protocol, without requiring add-on cards and the availability of their associated mainboard slots. 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.

The USB supports functional data and control exchange between the USB host and a USB device as a set of either unidirectional or bi-directional pipes. USB data transfers take place between host software and a particular endpoint on a USB device. A USB system comprises a USB host, a USB device and a USB inter-connection. The USB host is in charge of scheduling data transmission at the USB bus, and is generally embedded in a computer host. The USB device is classified into two categories: a hub for providing the USB bus with new USB ports, and a function device designating peripheral devices. The USB host interacts with USB devices through a USB host controller (HC). The USB system software on the host manages interactions between USB devices and host-based device software. The host is responsible for detecting the attachment and removal of USB devices, managing control flow between the host and USB devices, managing data flow between the host and USB devices, collecting status and activity statistics, and providing a limited amount of power to attached USB devices. USB uses a hub-based interconnect that provides a star configuration interconnect in order to interface a potentially large number of peripherals to the main processing unit of a computer system. Each USB bus system has a host, which is typically an interface adapter in a computer system. Each host has one or more hubs, each of which provides connectivity to one or more nodes or devices. A function is typically implemented as a separate peripheral device with a cable that plugs into a port on a hub. A physical package may implement multiple functions and an embedded hub with a single USB cable. This is known as a compound device. Each function contains configuration information that describes its capabilities and resource requirements. Before a function can be used, it must be configured by the host. This configuration includes allocating USB bandwidth and selecting function specific configuration options. The USB architecture automatically detects a device being connected and provides a unique address to each device which may then be used to communicate with the device over the common USB connection. The host determines if the newly attached USB device is a hub or a function and assigns a unique USB address to the USB device. The host establishes a control pipe for the USB device using the assigned USB address and endpoint number zero. If the attached USB device is a hub and USB devices are attached to its ports, then the above procedure is followed for each of the attached USB devices. If the attached USB device is a function, then attachment notifications will be dispatched by the USB software to interested host software.

A Universal Serial Bus (USB) hub is used in a network for connecting a set of computer peripheral apparatus to a host computer. All USB devices attach to the USB via a port on specialized USB devices known as hubs. A USB hub is an intelligent wiring connector coupled to a computing device and allows attachment for peripheral devices. Hubs serve to simplify USB connectivity from the user's perspective and provide robustness at low cost and low complexity. USB hubs are wiring concentrators that enable multiple attachment characteristics of USB. Attachment points on the hubs are referred to as ports. Each hub converts a single attachment point into multiple attachment points. The USB hub is connected to the computer via a single USB upstream connector. The upstream port of a hub connects the hub towards the host. The USB hub also includes a plurality of downstream ports for connecting the peripheral devices to the hub. Each of the other downstream ports of a hub allows connection to another hub or function. The USB hub uses a standardized connector at the downstream ports to provide universal connectivity between peripheral devices and the computer. USB hubs can detect attach and detach at each downstream port and enable the distribution of power to the downstream devices. Each downstream port can be individually enabled and configured as either full or low speed. The hub also isolates low speed ports from full speed signaling ports. The USB hub provides a convenient central data connection point for attaching multiple peripheral devices to a computer. The hub relays data from the computer to all enabled devices coupled to the data hub, and relays data from the enabled devices to the computer. This data relay is performed without any data storage or significant delay. A USB hub typically includes a hub controller, a hub repeater, and a transaction translator. The hub repeater provides a USB protocol-controlled switch between the upstream port and downstream ports as well as support for reset and suspend/resume signaling. The host controller facilitates communication to and from the host. The USB hub functions to carry out its internal function or to distribute power supplied to other hubs and functions connected at the lower part. Thus, the USB hub is supplied with bus power Vbus through a USB cable, or is supplied with self-power Vself from an additional independent power supply unit through a power cable.

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