Global System for Mobiles is the primary technology used globally for 3G mobile networks, with about a 73 percent market share. GSM competes primarily with Code Division Multiple Access technology, which is the technology used by five of the seven largest carriers in the United States. While GSM provides compatibility, multitasking and speed advantages over CDMA on a 3G network, most carriers around the world are switching to the Long Term Evolution standard for their 4G networks.
Every GSM phone has an International Module Equipment Identification number to identify the handset. GSM also uses a Subscriber Identity Module card to store a customer's account information. If you buy a new GSM phone, you can simply remove the SIM card from your old phone, place it in your new phone and begin using the new phone right away. You are not required to register the new phone's IMEI number with a GSM provider.
The ease of switching handsets and the dominance of GSM mobile networks globally provide customers with a larger selection of phones to choose from. However, while you can generally use a GSM phone on any carrier's 3G GSM network in Europe, that's not also true in the United States. AT&T and T-Mobile, the two U.S. GSM carriers, use different frequencies for their 3G networks. A 3G T-Mobile phone might connect to the AT&T network, but it probably won't be able to transmit voice or data on the right frequencies to work well, and might revert to 2G speeds.
When you use a GSM network, you can talk on the phone and surf the Internet or sync your email at the same time. That's usually not an option if you're using a phone on a CDMA network. CDMA released an add-on option called Simultaneous Voice and Data Optimization that would enable callers to use voice and data at the same time, but the add-on would require changes to both the CDMA network and CDMA phones. Carriers in the United States have not adopted the add-on for their networks.
A GSM network is generally much faster than a CDMA network. Most GSM carriers adopted the High Speed Packet Access extension for 3G networks that enable data transfers as fast as 42Mbps. On a 3G CDMA network, the maximum data transfer rate is 3.6Mbps.
Most carriers have adopted the LTE standard for their 4G networks, which enables very fast simultaneous transfers of both voice and data. However, all U.S. carriers will keep their 3G networks in place until at least 2020. Customers who live in an area without 4G coverage will need to continue to use a 3G network. iPhone users will have to upgrade to the iPhone 5 or a later version to use the popular phone on a 4G network.
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