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A USB hub is a device that expands a single Universal Serial Bus (USB) port into several so that there are more ports available to connect devices to a host system. USB hubs are often built into equipment such as computers, keyboards, monitors, or printers. When such a device has many USB ports, they all usually stem from one or two internal USB hubs rather than each port having independent USB circuitry. Physically separate USB hubs come in a wide variety of form factors: from external boxes (looking similar to an Ethernet or network hub) connectible with a long cable, to small designs that can be directly plugged into a USB port (see the "compact design" picture). In the middle case, there are "short cable" hubs which typically use an integral 6-inch cable to slightly distance a small hub away from physical port congestion and of course increase the number of available ports. Laptop computers may be equipped with many USB ports, but an external USB hub can consolidate several everyday devices (like a mouse and a printer) into a single hub to enable one-step attachment and removal of all the devices. == Physical layout == A USB network is built from USB hubs connected downstream to USB ports, which themselves may stem from USB hubs. USB hubs can extend a USB network to a maximum of 127 ports. The USB specification requires that bus-powered/passive hubs may not be connected in series to other bus-powered hubs. USB ports are often closely spaced. Consequently, plugging a device into one port may physically block an adjacent port, particularly when the plug is not part of a cable but is integral to a device such as a USB flash drive. A horizontal array of horizontal sockets may be easy to fabricate, but may cause only two out of four ports to be usable (depending on plug width). Port arrays in which the port orientation is perpendicular to the array orientation generally have fewer blockage problems. External "Octopus" or "Squid" hubs (with each socket at the end of a very short cable maybe 2 inches long), or "star" hubs (with each port facing in a different direction, as pictured) avoid this problem completely. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「USB hub」の詳細全文を読む スポンサード リンク
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