You cannot remove the sim card in CDMA.So even if your CDMA phone is theft ,he cannot change the sim card.But in GSM you can change the simcard and use it.

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How ever some smart phones have both GSM and CDMA for an example the Motorola Razr M 4G LTE phone as it has (CDMA 800 1900, LTE B13, WCDMA 850 900 1900 2100, GSM 850, 900, 1800, 1900 EVDO Rev. A, HSDPA 21.1 Mbps (Cat 14), HSUPA 5.76 Mbps)If you have an unlocked smart phone with both GSM/CDMA your wireless carrier and network options are unlimited as you can use any mobile network rather is based on GSM or CDMA.

1.) Using the Global System for Mobile Networks with a mobile device unlocked with a GSM baseband or quadband will let you use you mobile phone anywhere in the world. GSM mobile phones are also known as "World Phones".2.) You can use any unlocked GSM Mobile phone on any wireless carrier that operates on the T-Mobile or AT&T Networks.3.) Verizon has better coverage for 4G LTE since it is based on Code Division Multiple Access.4.) Both GSM and CDMA share a similar speed like 3G while all other speeds are different.5.) Some smart phones come with both GSM/CDMA quad base bands as you can use it only any network if both are unlocked. The Motorola Droid Razr and the iphone 4S for an example has both GSM and CDMA quad base band integrated.

Another thing about unlocked GSM mobiles, each carrier may use different frequencies to carry voice and data across, which means that a mobile phone made for one company might not use the same frequency ranges and cause data speeds to be lower than possible. GSM is much easier to be used worldwide; just get the unlock code or an unlocked phone, then just pop in the SIM card of your choice. From most reviews and studies, GSM frequencies that carriers use are faster in data speeds, both download and upload, than what CDMA offers. In general, basic users would not see as much difference, only the coverage areas for the carrier. Heavy data users would see a bigger difference.

1.) AT&T and T-Mobile are Global System Mobile Communication Networks. Sprint and Verizon are based Code Division Multiple Access.2.) GSM and CDMA have different band radio frequencies and transfer voice and data differently.3.) CDMA have integrated Identification modules thats built in to the mobile device while GSM have removable Identification modules known as the SIM card.

GSM uses SIM cards (or microsim or nanosim)CDMA uses the phone hardwareThis means with GSM you can (theoritically) easily switch phones and networks vs CDMA where you have to go through a specific process and are much more limitedGSM also means you could buy a phone not available for your carrier like a nokia that's only available in europe, unlock it and use it over here

In GSM Phone you can use any sim cards but while in CDMA , it is fixed with operator

Also check this, very informative : http://en.wikipedia.org/wiki/Comparison_of_mobile_phone_standards

Check this url for your answer : http://www.diffen.com/difference/CDMA_vs_GSM