Most cellular phone carriers support one of the following incompatible technologies for transmitting and decoding phone calls.
Each technology has its proponents and detractors. The most important consideration for consumers is ensuring that your phone and service provider both support the same technology. Example: you can use a CDMA-compatible phone only on a CDMA-compatible network.
TDMA (Time Division Multiple Access), sorts messages into time slots in order to send them. Voice and data are converted to digital signals that are sent and received in bursts in specific time slots.
CDMA (Code Division Multiple Access) was developed by Qualcomm for military antijamming systems, which translates into highly private conversations. It spreads a low power signal across a wide frequency width, which provides for a significant increase in subscriber capacity.
GSM (Global System for Mobile Communications) was originally developed in Europe and works similarly to the TDMA system: it sorts messages into time slots. The unique feature of this technology is that it allows for a short message service, up to 124 characters, while simultaneously carrying voice traffic.
Disadvantage of GSM Europe and North America operate at different frequency bands, you can't use your North American GSM phone while traveling through Europe.
Advantage of GSM Most GSM phones use a removable Subscriber Identity Module (SIM Cards), more commonly known as a smart card, containing a user's account information, phone book, and security parameters. By inserting the smart card from your North American GSM phone into a European GSM phone, you can successfully roam throughout most of Europe if your provider has roaming agreements with European carriers. (The reverse is also true for European users in North America.)
Furthermore, manufacturers are currently developing multi-band GSM that, once made available to the public, will allow GSM phone owners from either side of the Atlantic to roam on both continents with a single phone (again, as long as your service provider has roaming agreements with carriers abroad).