Global System for Mobile Communications (GSM) and Code Division Multiple Access (CDMA) are two competing standards in cellular service. They both have derivatives for use with 3G phones known as Universal Mobile Telecommunications System (UMTS) and CDMA2000, respectively. The major difference between the two technologies is how they turn voice data into radio waves and how the carrier connects to the phone. Other differences include the coverage area, the data transfer speeds, and the type of hardware used.
Coverage
Some areas and countries only have one technology available, so users should be sure to review coverage maps before buying a phone. Generally speaking, CDMA is most commonly found in North America and some parts of Asia, while GSM is found in most other places. Some carriers do offer international or "world" phones that can work with both, but if the phone is going to be used predominantly in one area, it may make sense just to get one that's tied to one type.
Data Transfer Speed
Both technologies can be used with 3G standard phones, but 3G GSM speeds can be faster than 3G CDMA speeds, which can make a big difference for those who use their phones for social networking, email and streaming video. The fastest 3G standard used with CDMA2000 is EV-DO Rev B., which has downstream data rates of about 15.67 Megabits per second (Mbit/s). The fastest standard available with UMTS is HSPA+, with downstream speeds of up to 28 Mbit/s.
Hardware
GSM phones and CDMA phones also use different types of smart cards known as Universal Integrated Circuit Cards (UICC). These are small removable cards that can be used to store information like a contact list and activate, interchange, and upgrade phones without carrier intervention as long as the phone is unlocked. This means that the carrier makes it possible for phone to work even if the end user removes and changes the card. Though both types can be sold locked, this is more common with CDMA phones.
UICCs can be programmed to work with either GSM or CDMA and their derivatives, or with both. Those that only work with GSM phones are called Subscriber Identity Module (SIM) cards and those that only work with CDMA phones are called CDMA2000 Subscriber Identity Module (CSIM) cards. There are also a few types of UICCs that are programmed to work with GSM, UMTS, CDMA, and CDMA2000, including CSIM/USIM cards and Removable User Identity Module (R-UIM) cards.
Roaming
Generally speaking, both networks have fairly concentrated coverage in major cities and along major highways. GSM carriers, however, have roaming contracts with other GSM carriers, allowing wider coverage of more rural areas, generally speaking, often without roaming charges to the customer. CDMA networks may not cover rural areas as well, and though they may contract with GSM cells for roaming in more rural areas, the charge to the customer can be significantly higher.
International Roaming
Some GSM and CDMA carriers offer international roaming, which means users can still use their phones when traveling abroad. To work internationally, the phone has to be a quad-band phone, which means that it works with frequencies of 850, 900, 1800, and 1900 MHz. Additionally, the phone does have to be unlocked and the user does have to be trying to use a network that exists in the country, which is sometimes more difficult to do with CDMA since fewer countries have CDMA networks. If the phone is unlocked and there is a network present, though, then users can buy a UICC with minutes and a local number in the country in which they're traveling to avoid paying international rates.
4G
Things get a little bit more complicated when it comes to phones with 4G. Most US cellular carriers use CDMA. 4G phones generally use LTE (Long Term Evolution), a high speed wireless broadband technology. In most of the world, GSM is preferred and 4G phones with LTE outside of the US use this service. In the US, however, US cellular carriers offering 4G phones have made the switch to LTE on the CDMA service. It is possible to determine which service a smartphone uses with the model number of the phone. Phone manufacturers or the cellular carrier can clarify this information. As of 2014, users in the US are most likely using CDMA. With the exception of countries like Russia and Japan, which continue to use CDMA like the US, most other countries in the world use GSM for 4G.
