While GSM is the nearly universally accepted form of cellular technology, several carriers still use CDMA for their cell phones and cellular networks. Understanding the difference between GSM and CDMA boils down to several easily identifiable differences.
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ESN vs. SIM Cards
Remove the battery cover from your device. If the sticker under the phone's battery displays an ESN (Electronic Serial Number), the phone is CDMA based. If a SIM card slot is featured, the phone is GSM based (SIM cards can also appear in side-loaded slots, not located under the phone's battery).
What GSM Does That CDMA Doesn't
GSM technology requires a SIM card. This means users can simply slip their SIM card into another GSM-enabled device and start using the phone (if that phone is unlocked or supported by their carrier). CDMA customers must call their carrier (Verizon, Sprint, etc.) and request that an ESN change be performed to switch between phones.
Calling and Data Differences
The terminology is basically different for GSM vs. CDMA. For example, GSM carriers typically call data networks "3G" while CDMA networks use the terminology EVDO (a third-generation mobile network). Lately, however, "3G" has become the standard terminology for marketing purposes. GSM also typically offers more calling frequencies than CDMA. For example, GSM frequencies include 850, 900, 1800 and 1900MHz, which are offered throughout most of the world, with a quad-band 3G data spectrum also available. CDMA phones typically only offer two frequencies (those associated with the carrier of the phone).
Worldwide Use Differences
GSM technology is universally accepted in almost every country, and users can even use international SIM cards when travelling. CDMA is typically not usable in most countries outside the United States and parts of Asia unless the phone features a secondary SIM card slot. The United States is one of the last few countries to offer any CDMA networks (Verizon, Sprint).
Unlocked Phones vs. Carrier-Locked Devices
GSM phones often come in an "unlocked" format, which allows them to be used on a variety of GSM networks throughout the world, oftentimes without a carrier contract required. CDMA phones typically work only on the network they are sold on and are not offered in unlocked formats.
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