GSM and CDMA are competing wireless technologies with GSM enjoying about an 82% market share globally. In the U.S., however, CDMA is the more dominant standard. Technically GSM (Global System for Mobile communications, originally from Groupe Spécial Mobile) is a specification of an entire wireless network infrastructure, while CDMA relates only to the air interface — the radio portion of the technology.
Code division multiple access (CDMA) describes a communication channel access principle that employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code). CDMA also refers to digital cellular telephony systems that use this multiple access scheme, as pioneered by QUALCOMM, and W-CDMA by the International Telecommunication Union (ITU), which is used in GSM’s UMTS.
edit Subscriber Identity Module (SIM Card)
SIM (subscriber identity module) card, the onboard memory device that identifies a user and stores all of his information on the handheld. You can swap GSM SIM cards between handsets when a new one is necessary, which enables you to carry all of your contact and calendar information over to a new handset with no hassle. CDMA operators answer this flexibility with their own service that stores user data, including phone book and scheduler information, on the operator’s database. This service makes it possible to not only swap over to a new handset with little trouble, but it also gives users the ability to recover contact date even if their phone is lost or stolen.
edit International Roaming with GSM and CDMA
Where international business travel is an issue, GSM leaps forward in the race for the title of “Most Accessible.” Because GSM is used in more than 74% of the markets across the globe, users of tri-band or quad-band handsets can travel to Europe, India, and most of Asia and still use their cell phones. CDMA offers no multiband capability, however, and therefore you can’t readily use it in multiple countries. However, certain phones like the iPhone 5 now have Quad-band GSM built in so they can be used overseas with special calling plans from carriers.
edit Data Transfer Methods in GSM vs. CDMA
Another difference between GSM and CDMA is in the data transfer methods. GSM’s high-speed wireless data technology, GPRS (General Packet Radio Service), usually offers a slower data bandwidth for wireless data connection than CDMA’s high-speed technology (1xRTT, short for single carrier radio transmission technology), which has the capability of providing ISDN (Integrated Services Digital Network)-like speeds of as much as 144Kbps (kilobits per second). However, 1xRTT requires a dedicated connection to the network for use, whereas GPRS sends in packets, which means that data calls made on a GSM handset don’t block out voice calls like they do on CDMA phones.
edit Interaction between GSM and CDMA
In cities and densely populated areas, there are often high concentrations of GSM and CDMA connection bases. In theory, GSM and CDMA are invisible to one another and should "play nice" with one another. In practice, however, this is not the case. High-powered CDMA signals have raised the "noise floor" for GSM receivers, meaning there is less space within the available band to send a clean signal. This sometimes results in dropped calls in areas where there is a high concentration of CDMA technology. Conversely, high-powered GSM signals have been shown to cause overloading and jamming of CDMA receivers due to CDMA’s reliance upon broadcasting across its entire available band.
The result of this little cross-broadcasting joust has led some cities to pass ordinances limiting the space between cell towers or the height they can reach, giving one technology a distinct advantage over the other. This is something to note when choosing a wireless provider. The distance between towers will severely affect connectivity for GSM-based phones because the phones need constant access to the tower’s narrow band broadcasting.
edit Popularity and Market Share
GSM is a lot more widespread in Europe and Asia. In the United States, Sprint and Verizon networks are CDMA whereas AT&T and T-Mobile are on GSM. Most of Europe uses GSM and so does China. In India, Hutch, Bharti and BSNL are on GSM whereas Reliance and Tata Tele are on CDMA networks.
edit Video explaining the difference