Dual band is a feature allowing a device to function in two different frequency bands. In the context of mobile phone networks, a dual band device enables broader roaming capabilities to users.
There are four primary frequency bands in a Global System for Mobile Communications (GSM) network, as follows: 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz. The United States uses the 850 MHz and 1900 MHz bands, and Europe uses the 900 MHz and 1800 MHz bands.
If a GSM phone supports one band, such as 850 MHz in the U.S., then a user can use a mobile device and communicate in a supported GSM network. However, if a user only has a 1900 MHz GSM signal, he cannot communicate or make calls from the 850 MHz-supported location.
The U.S. requires that mobile devices have dual band capability for GSM network roaming and support 850 MHz and 1900 MHz bands. A dual band phone with these specifications cannot be used in Europe.
To function in at least one U.S. network and one European network, a dual band device must support a 850 MHz or 1900 MHz band combined with a 900 MHz or 1800 MHz band.