USB HUB

A 'USB hub' is a device that allows many USB devices to be connected to a single USB port on the host computer or another hub.
USB hubs are often built into equipment, normally keyboards or, more rarely, monitors or printers. Separate USB hubs come in a wide variety of form factors from boxes that look similar to a network hub to small designs intended to be plugged directly into the USB port on a computer (that is, without a connecting cable).

Contents

Power

Speed

Physical layout

Length limitations

References

Power

A 'bus-powered 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.
In contrast a 'self-powered hub' is one that takes its power from an external power supply unit and can therefore provide full power to every port. Many hubs can operate as either bus powered or self powered hubs.
USB power is allocated in units of 100mA up to a maximum total of 500mA per port. Therefore a compliant bus powered hub can have no more than four downstream ports and cannot offer more than four 100mA units of power in total to downstream devices (since one unit is needed for the hub itself). If more units of power are required by a device than can be supplied by the port it is plugged into, the operating system usually reports this to the user.
However, there are many non compliant hubs on the market which announce themselves to the host as self powered despite really being bus powered. Equally there are plenty of noncompliant devices that use more than 100mA without announcing this fact (or indeed sometimes without identifying as usb devices at all). These hubs and devices do allow more flexibility in the use of power (in particular many devices use far less than 100mA and many usb ports can supply more than 500mA before going into overload shutoff) but they are likely to make power problems harder to diagnose.

Speed

To allow 'high-speed' devices to operate in their fastest mode all hubs between the devices and the computer must be high-speed. High-speed devices should fall back to full-speed when plugged in to a full-speed hub (or connected to an older full-speed computer port). While high-speed hubs support 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. Some designs use a single transaction translator, while other designs have multiple translators. Having multiple translators is only a significant benefit when connecting multiple high-bandwidth full-speed devices.^[1]
It is an important consideration that in common language (and often product marketing) USB 2.0 is used as synonymous with high-speed. However, because the USB 2.0 specification, which introduced high-speed, incorporates and supersedes the USB 1.1 specification, any compliant full-speed or low-speed device is still a USB 2.0 device. Thus, not all USB 2.0 hubs operate at high-speed.

Physical layout

A USB network with many devices will require one or more hubs connected to each other. USB hubs can extend a USB network a maximum of five times. The USB specification requires that bus-powered hubs may not be connected in series to other bus-powered hubs.
USB ports on computer housings are usually closely spaced, so that plugging devices into one port may block an adjacent port. This may be a reason to want to use an external USB hub. However, many hubs also have closely spaced ports, replicating the problem of inability to utilize all of the ports. Star shaped hubs with each port pointing in a different direction, such as pictured top left, avoid this problem. Aside from practical layouts novelty USB Hubs have also been produced, for example a USB Hub shaped like the TARDIS, a fictional time-travelling space ship from the BBC science fiction series ''Doctor Who'', or another shaped like a nuclear missile launch console complete with a big red button (which shuts down the PC).
Also existing 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.g. 2 PCs and 4 peripherals, assigning access separately). Only the simpler switches tend to be automatic, and this feature generally places them at a higher price point too.

Length limitations

There are limits on USB cable length. A hub can be used as an active USB repeater, to extend cable length. Powered hubs can be connected among several layers. Specialized connector-embedded one-port hubs are available for use solely as extension cables. Active extension cables are bus-powered by design creating limitations to the types of devices that can be connected to them.

References

1. USB Technology: Multi-TT Hub Goes Head-to-Head With Single-TT ''Tom's Hardware UK and Ireland'' URL last accessed on August 24 2006.