A 3G modem is a device that allows a computer to connect to the Internet via a high-speed mobile broadband connection. This means that the Internet signal is broadcast over the airwaves rather than sent and received through a cable or telephone line. The technology is largely the same that is used for delivering Internet content to modern cellphones.
There are a variety of broadcasting standards that are covered by the 3G title, which is short for the "third generation" of mobile technologies. The most common standards in the United States are UMTS and CDMA. Because the various standards are not always compatible with one another, users must check carefully that a 3G modem is compatible with the cell phone network from which they intend to get 3G service.
In most cases, a 3G modem will come as a plug-in device, often referred to as a dongle. This is usually plugs into a USB slot on a computer and closely resembles a portable memory drive. The device houses the antennae that receive the signal, the SIM card that authenticates the user to communicate on the network, and the modem itself, which converts the wireless information into data the computer can understand.
Some forms of 3G modem also work as a router, meaning they can share the signal with multiple devices. Some such routers are relatively large devices and include Ethernet slots for making a wired connection with a computers. Others are smaller and communicate with the computers via Wi-Fi. This can be a useful solution for portable devices that have a Wi-Fi connection but do not have a USB socket,
A 3G modem user will often have to be much more careful about their Internet use than somebody a landline or cable-based service. This is because there are usually much lower monthly limits on the data that can be downloaded. These can be as low as 250MB and rarely exceed 5 GB a month. A user who passes this limit may be cut off for the rest of the month, but could also be allowed to continue using the service and charged extremely high penalty fees for the additional data use.
There are also some 3G technologies that are designed to work primarily for desktop computers. One example is WiMax, which can be used by some homeowners as their primary internet connection. This is only really practical in areas that have strong WiMax coverage but where fixed line or cable services are either unavailable or prohibitively expensive.