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== GSM ==GSM stands for Global System for Mobile Communications; it's the world's most prolific mobile standard. This is what you will find if you are, say for example, traveling in Europe. Most new phones on AT&T and T-Mobile actually adhere to both GSM and the newer UMTS (Universal Mobile Telecommunications System) standards. UMTS isn't an official part of the GSM standard, but it is what GSM carriers use for 3G data transmission.Switching Providers: GSM phones use a removable Sim Card. If the phone is unlocked you can pop a SIM card out of a GSM phone and stick it in any other GSM phone, switching a phone from one provider to another.== CDMA ==CDMA stands for Code Division Multiple Access, including both CDMAOne or CDMA 2000. This is the most common standard in the United States of America throughout the 1990's and until present (2012).CDMA2000, based more directly on its predecessor includes a range of improvements over the original CDMAOne, key among them 3G data speeds.CDMA phones have a slightly more refined method for handing off calls from tower to tower, so they drop fewer calls as compared to GSM.Switching Providers: CDMA phones ship locked to one network, and can only be switched to another with the cooperation of both the old and new carriers.