What is a USB Hub?
Since USB (Universal Serial Bus) can work with simultaneous connection of up to 127 devices, hubs are used to attach additional USB peripherals. When a computer's USB ports fill up, simply attach a device called a hub, which provides additional ports (usually four or seven), and keep on plugging in more peripherals-and hubs-as needed.
Self-powered hubs
This type of hub has an AC adapter that provides external power to the hub. To use the hub, you need to plug the adapter cord into an electrical outlet or power strip. Since this hub supplies its own power, you can often connect several devices to it. Typical self-powered hubs have four USB ports.
Bus-powered hubs
This type of hub gets power from another source, such as a computer. You can plug other devices into a bus-powered hub. The Apple USB keyboard acts as a bus-powered hub. However, if a device requires high power, such as a printer, a bus-powered hub will not be able to supply enough power.
Note
: You should use self-powered hubs versus bus-powered hubs for best capability and performance.
Important:
Under the USB Specification you cannot connect two bus-powered hubs together because there won't be enough power for the second hub. To connect two hubs together, at least one hub must be self-powered. For this reason, you cannot connect an Apple USB keyboard to the USB port on another Apple USB keyboard. If you want to use two bus-powered hubs with your computer, connect each to a USB port on the computer.
For a partial list of USB Hubs, please visit the USB Peripherals web page at
http://www.apple.com/usb/