GSM and CDMA are different standards for mobile communications. While GSM was developed in Europe, the US ? which usually does not like to follow Euro standards ? devised it?s own CDMA.
The two acronyms expand into Global System for Mobiles and Code Division Multiple Access, respectively, and they differ in the way the available bandwidth is shared between multiple users.
Code Division Multiple Access, or CDMA is the name of a generic technology for multiplexing data, which means that it is a method by which more than one user can use the bandwidth, so that there are multiple channels of communication.
This is done by assigning a code to each channel, which can be understood only by the receiver of that channel. For the other channel?s receivers, even if they try to listen to the signal, since they do not have the code, they will not be able to understand it.
Since CDMA uses this technology for bandwidth sharing, hence it?s name.
As for GSM, it uses two slightly different technologies, i.e. Time Division Multiple Access (TDMA) and/or Frequency Division Multiple Access (FDMA). While in TDMA, each channel is allocated a small time slot in which it can transmit/receive, in FDMA, each channel gets it?s own frequency range, much like analog TV or radio.
Now from the user?s perspective, what are the differences? firstly, CDMA voice quality is nowhere near as good as GSM. While this is not yet apparent in India, where the number of subscribers are relatively low, the fact remains that as the number of subscribers increase, the voice quality will go down. If you have used a CDMA phone in the US, you know what I am talking about.
The voice sounds harsh, metallic and unnatural. In GSM no such problem is there?the voice quality will not fall below a certain low limit. In fact, this is the main reason why GSM is now gaining popularity in the US, where previously CDMA was dominating.
Second obvious difference is that there is no SIM card in CDMA, while GSM phones will not work without a SIM card. Some CDMA providers do have a SIM card like device, which they call UIM, TIM etc. Thirdly, the power consumption is slightly higher in CDMA phones, because of the nature of transmission (always on kind of scenario).
Finally, GSM is more of a world wide technology, as it is in use in most countries in Europe and Asia, and now also in North America. CDMA, in contrast, is supported only in few countries ? US, Japan, Korea, India and probably a couple of others.
A GSM phone is the best bet for compatibility across the world, provided of course it has tri-band support (GSM uses three bands of frequencies).