Many mobile phones use tri-band technology, allowing them to communicate on multiple cellular frequency bands. This enables them to use more frequencies if the airwaves are too busy and allows them to operate in other parts of the world where different frequencies are used. Tri-band technology is generally used with the Global System for Mobile Communications (GSM) standard which, in the U.S., is used by AT&T and T-Mobile.
Four frequency bands carry GSM cellular signals throughout the world. GSM850, GSM900, GSM1800, and GSM1900 all refer to the approximate frequency of the band in megahertz (MHz), although actual communications occur on both ends of the stated frequency. For instance, the GSM850 band contains a number of channels for both uploading and downloading and actually spans 824.2 to 894.2 MHz.
Different parts of the world use different GSM frequency bands. While the GSM1900 band is prevalent throughout most of the United States, the GSM850 band is also used regularly, especially in outlying areas. Other countries typically use GSM900, which was the original GSM band, and have since expanded into the GSM1800 band as well. Many countries also use the same frequencies as the United States.
A phone with tri-band technology is able to communicate on three of the four GSM bands. Some tri-mode phones that are designed for the US operate on both the GSM850 and GSM1900 bands, as well as supporting GSM1800 for international travel. A world traveler should look for a tri-mode phone supporting GSM900, GSM1800, and GSM1900 which trades some US capability in exchange for better international roaming coverage. A quad-band phone will support every GSM band, eliminating the tradeoff between US and global performance that is necessary in choosing a tri-band phone.
In addition to tri-band phones, many manufacturers also make tri-mode phones. These phones can not only operate on the GSM system, but also on Code Division Multiple Access (CDMA) networks, such as those used by Verizon and Sprint in the US, and on the analog Advanced Mobile Phone System (AMPS). While some tri-mode phones support GSM, CDMA, and AMPS, others just support two network systems with dual-band support for one of them. These phones are technically dual-mode, dual-band sets. International travelers can be well served with one of these phones that supports both dual-band GSM communications as well as AMPS for countries with older cellular networks.