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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, similar to a power strip. USB hubs are often built into equipment such as computers , keyboards , monitors , or printers.
Physically separate USB hubs come in a wide variety of form factors: 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.
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.
A hub can be used as an active USB repeater to extend cable length for up to 5 metre 16 feet lengths at a time.
Active cables specialized connector-embedded one-port hubs perform the same function, but since they are strictly bus-powered, externally powered non-bus-powered USB hubs would likely be required for some of the segments. A bus-powered hub passive hub is a hub that draws all its power from the host computer's USB interface. It does not need a separate power connection.
However, many devices require more power than this method can provide and will not work in this type of hub. It may be desirable to use a bus-powered hub with self-powered external hard-disk units, as they may not go into sleep mode immediately upon computer shut-down or sleep mode when using a self-powered hub since they will continue to see a power source on the USB ports when using a self-powered hub. If a device requires more units of current than the port it is plugged into can supply, the operating system usually reports this to the user.
In contrast, a self-powered hub is one that takes its power from an external power supply unit and can therefore provide full power up to mA to every port. Many hubs can operate as either bus powered or self powered hubs. However, there are many non-compliant hubs on the market which announce themselves to the host as self-powered despite really being bus-powered.
Some self-powered hubs do not supply enough power to drive a mA load on every port. For example, many seven port hubs have a 1A power supply, when in fact seven ports could draw a maximum of 7 x 0.
Designers assume the user will most likely connect many low power devices and only one or two requiring a full mA. On the other hand, the packaging for some self-powered hubs states explicitly how many of the ports can drive a mA full load at once. For example, the packaging on a seven-port hub might claim to support a maximum of four full-load devices.
Dynamic-powered hubs are hubs which can work as bus-powered as well as self-powered hubs. They can automatically switch between modes depending on whether a separate power supply is available or not. While switching from bus-powered to self-powered operation does not necessarily require immediate renegotiations with the host, switching from self-powered to bus-powered operation may cause USB connections to be reset if connected devices previously requested more power than still available in bus-powered mode.
To allow high-speed USB 2. High-speed devices should fall back to full-speed USB 1. While high-speed hubs can communicate at all device speeds, low and full-speed traffic is combined and segregated from high-speed traffic through a transaction translator. Each transaction translator segregates lower speed traffic into its own pool, essentially creating a virtual full-speed bus. Having multiple translators is a significant benefit when one connects multiple high-bandwidth full-speed devices.
It is an important consideration that in common language and often product marketing USB 2. However, because the USB 2. Thus, not all USB 2. Each hub has exactly one upstream port and a number of downstream ports. The upstream port connects the hub directly or through other hubs to the host.
Other hubs or devices can be attached to the downstream ports. During normal transmission, hubs are essentially transparent: This way, what is sent by the host is received by all hubs and devices, and what is sent by a device is received by the host but not by the other devices an exception is resume signalling.
Downstream routing has been changed in USB 3. A route string sent in the packet header allows a USB 3. Hubs are not transparent when dealing with changes in the status of downstream ports, such as insertion or removal of devices. In particular, if a downstream port of a hub changes status, this change is dealt with in an interaction between the host and this hub; the hubs between them act as transparent in this case.
To this aim, each hub has a single interrupt endpoint "1 IN" endpoint address 1, hub-to-host direction used to signal changes in the status of the downstream ports. When the host polls this interrupt endpoint, it learns that the new device is present. It then instructs the hub via the default control pipe to reset the port where the new device was plugged in.
This reset makes the new device assume address 0, and the host can then interact with it directly; this interaction will result in the host assigning a new non-zero address to the device. For example, if a USB 1. However, the default design is that all lower-standard devices share the same transaction translator and thus create a bottleneck, a configuration known as the single transaction translator. Consequently, multi transaction translators Multi-TT were created, which provide more transaction translators such that bottlenecks are avoided.
Most USB hubs use one or more integrated controllers ICs , of which several designs are available from various manufacturers. Most support a four-port hub system, but hubs using port hub controllers are also available in the industry. The root hub is the first tier, and the last devices are on the seventh tier, which allows 5 tiers worth of hubs between them. The maximum number of user devices is reduced by the number of hubs. Also available are so-called "sharing hubs", which effectively are the reverse of a USB hub, allowing several PCs to access usually a single peripheral.
They can either be manual, effectively a simple switch-box, or automatic, incorporating a mechanism that recognises which PC wishes to use the peripheral and switches accordingly. They cannot grant both PCs access at once. Some models, however, have the ability to control multiple peripherals separately e. Only the simpler switches tend to be automatic, and this feature generally places them at a higher price point too.
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