USB is a connection standard for computer peripherals. The term USB is an acronym for Universal Serial Bus. Specially designed, one-way connectors ensure that computer keyboards, flash drives, printers, digital cameras, media players, memory sticks, and hundreds of other devices can all connect to a desktop computer via a standard USB port. Not only does this save space on the motherboard, it also saves engineering effort and therefore lowers production costs. Mini-USB and Micro-USB specifications reduce the footprint of USB sockets and enable their use on ultra-portable devices such as bluetooth headsets and small digital audio players.
In contrast to more conventional cables such as telephone wire and network cabling, USB cables have different connectors at each end. This is because USB connections are generally not used for two-way communication between equal parties. Instead, USB is designed for transferring data from one device to another device, with the assumption that one device is a powerful computer and the other has limited or no processing power. A long, flat "A" type connector is connected to the USB host, or the main computer. A square-shaped "B" type connector is usually connected to the peripheral. This ensures that two peripherals or two computers will not be accidentally connected to each other. If the connection physically fits, than device compatibility is almost ensured. This is also means that gender-changing adaptors, common in network and telephone cable systems, are not needed. Extension cables, with a male "A" type connector at one end and a female "A" type connector at the other end are, strictly speaking, not USB compliant. They are, however, fully USB compatible and in most cases devices connected via extension cables will function correctly. USB hubs, both powered and non-powered, are in fact USB compliant.
Two notable changes to the USB specification were the introduction of USB-OTG, and the UBS 2.0 update. OTG is an acronym for On The Go, and the OTG addition the the USB spec has made USB more network-like in it's operation. Before OTG, only the host machine could initiate the transfer of data. That means that devices such as mice and keyboards had to wait for the computer to ask them if they have any information to give it. This led to a very small, but noticeable, latency in some applications. The OTG addition lets the mouse initiate data flow with the computer, thus sending information as soon as the user moves the mouse. OTG also defines three new USB connector types. The USB 2.0 update addresses some of the speed issues of USB, bringing it more in line with technologies such as Firewire. While the original 1.0 spec allowed for only 1.5 MB/second data transfer, USB 2.0 defines a protocol that allows up to 60 MB/second of data flow. The increased bandwidth is not all available to any one device, however. Maximum single device speed in USB 2.0 is half the theoretical maximum, at 30 MB/second. USB 2.0 devices are fully backwards-compatible with the older 1.0 and 1.1 level equipment.
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