The Global System for Mobile Communications (GSM) is the most popular mobile communications system in the world, boasting over three billion subscribers as of 2010. A second-generation technology, the GSM system allows users to use their phone as usual almost anywhere in the world. GSM phones operate on a Subscriber Identity Module (SIM) card, which tells the network which phone is which and allows the user to store data such as phone numbers, calendars, and notes, which can then be transferred instantly to other GSM handsets. There are many different types of GSM handsets, offering a number of different features; in addition, there are multi-band and multi-mode phones.
Phone companies often race each other to develop the next hot phone with all new bells and whistles, which results in a constantly growing market with ever-expanding technology. Basic GSM handsets come with features like voice encryption, text messaging, caller ID, and call waiting, but newer models can include Short Message Service (SMS), multi-call conferencing, and Global Positioning Satellite (GPS) support. Even with the lengthy list of bonus features, the basic categories of GSM handsets remain the same, and refer to the way the phone operates within the network.
The first thing to consider when buying a phone is the frequency at which it operates. In the western hemisphere, cell phones operate at 850 or 1900 megahertz, while the eastern hemisphere uses 900 or 1800. A cell phone will already be equipped to operate at the frequencies used in the country where it is purchased, but some phones offer the capability to run on another frequency if the original frequency is not supported. Referred to as "multi-band" phones, these devices have become very popular with travelers, as they are operational in most places around the globe. Multi-band phones come in dual-, tri-, and quad-band varieties; the more frequencies supported, the better the roaming capabilities.
In some cases, coverage is not limited by international borders, but by geographic area. While GSM is widely supported, some areas may only be covered by Code-Division Multiple Access (CDMA), or even analog transmission. In this case, GSM handsets will not work. Some phone companies now offer multi-mode phones, which can switch to a different transmission type when the original type is not available. Dual- and tri-mode phones are available, but the meaning behind these terms can be unclear.
A dual-mode GSM handset will support GSM and CDMA transmission, and a true tri-mode phone will support GSM, CDMA, and analog transmission. Some companies label phones as tri-mode if they support two different GSM frequencies as well as analog, which is not entirely accurate. This type of phone is really a dual-frequency, dual-mode phone. The word "band" refers to frequency, which varies by country. The word "mode" refers to the transmission type, which varies by geographic area.