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.
Phones using GSM can do both voice and data at the same time, which cannot be done with CDMA.
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.
U.S. Providers and Technologies
- Verizon - CDMA, UMTS
- AT&T Mobility - GSM, UMTS, HSDPA
- Sprint - CDMA, UMTS
- Nextel - iDEN
- US Cellular - CDMA
- T-Mobile - GSM, UMTS, GAN
- Cricket Communications - CDMA
- Virgin Mobile - CSMA
Roaming
A CDMA-only phone from Verizon, Sprint, or US Cellular is only able to roam on other CDMA networks. This is fine in North America, but CDMA networks are rare in Europe and other countries. These three carriers offer phones with built-in GSM support for International traveling, but this feature is missing from their most popular handsets.
Using Another Provider's Phone With Verizon, Sprint, or US Cellular
Verizon's phones and network are CDMA, which doesn't use sim cards.
CDMA phones do not use sim cards, while GSM phones do.
CDMA and GSM are different technologies that are incompatible with each other.
The term "unlocking" referring to an unlocked phone is used when discussing GSM and iDEN phones.
There are other CDMA providers besides Verizon, which include Sprint and US Cellular.
Verizon will only activate another Verizon phone. A non-Verizon CDMA phone's ESN number will not be in Verizon's database of phones, and therefore, Verizon will not activate it.