Universal Serial Bus (USB) is a communication specification designed to form two-way connections from a desktop computer or a laptop to computer peripheral devices. USB has become the standard way to connect your computer to many peripherals: keyboards, mice, printers, flash drives, external hard drives, digital cameras, and personal media players. USB has also become the standard for connections to PDAs, smart-phones, and video game consoles.
Most of today’s laptops and desktop computers have only one or two USB sockets. So, the question then becomes, “How do I connect all my USB devices at once?” — and the answer is a USB hub.
A USB hub is simply a small unit with additional USB ports. One cable goes out from a USB port on your computer to the single “upstream” port of your hub. You then string out more USB cables from the hub’s multiple “downstream” ports. A USB hub usually has four “downstream” ports, but some hubs may have eight, ten, twelve, or more ports. You plug in all your USB devices (you can also plug in more hubs), chaining them together up to a total of 127 devices.
Remember that USB cables are only five meters long. You can set up another hub as an active USB repeater to extend your USB “reach”, or you can extend your chain with an Active Cable (which is actually a connector-embedded one-port hub — but, because these devices are bus-powered, you will still likely need several self-powered USB hubs on your chain segments).
But, we’re getting ahead of ourselves here, talking about self-powered hubs and bus-power. A hub can be self-powered, with a power cord that plugs into a power socket. Or a hub can be unpowered, drawing its power via the USB bus from your computer, pulling up to 500 milliamps at 5 volts. Most high-powered USB devices (such as scanners and printers) have their own plug-in power supply, but low-powered USB devices (such as mice) draw their power from the USB bus. If your USB devices are self-powered (such as scanners and printers), then you do not need a powered hub. If, however, your USB devices are unpowered, and draw power from the USB bus, you will need a self-powered hub to provide that power, so that you do not want to overload your computer’s own power supply with an enormous USB power draw.
Use a self-powered USB hub for connecting such low-draw devices as memory card readers, digital cameras, memory sticks, mice, keyboards, MP3 players, and other small USB items. However, a self-powered USB hub should be used for high-draw unpowered USB components such as external drives or fax machines. USB scanners and printers are usually self-powered and can be plugged into either type of hub. You may need an AC-adapter for some self-powered hubs — check whether the AC-adapter is an extra, or included with the hub.
The original USB standard supports transfer speeds of up to 12 megabits per second; USB 2.0 supports up to 480 megabits per second. Most USB devices require, at most, 6 megabits per second, so you shouldn’t have to worry about taxing the total transfer rate for your entire USB chain.
Another feature to look for in a USB hub is whether it is USB 1.1 or 2.0 compliant. The first USB devices used USB 1.1.; newer devices support USB 2.0. A USB 2.0 hub supports USB 1.1 devices, but not at the faster USB 2.0 rate. A USB 1.1 hub, on the other hand, might or might not support a USB 2.0 device (and, if it does, the transfer speed will be at the USB 1.1 rate, only 12 megabits per second).
All USB devices can be plugged in or unplugged as needed (such an action being called “hot-swapping”). But, be careful trying to “hot-swap” a USB external hard drive — data could be lost if the device is unplugged at an inopportune moment.