From the point of the end user not much if all you're interested in is can I talk on it. CDMA has more domestic subscribers since Verizon and Sprint PCS are two of the larger providers of mobile service domestically in the US and Canada. GSM is newer and the first national GSM provider was VoiceStream which became T-Mobile. Both AT&T Wireless and cingular who had been using TDMA (IS-136) and AMPS (analogue) in their networks decided to upgrade their service by overlaying GSM on their systems and gradually retire the TDMA (IS-136) service.
GSM is far superior to CDMA as far as the ability to use the service on a world-wide basis since GSM has over 1.4 billion world-wide subscribers and CDMA only has over 100,000. CDMA is mainly in the US, Canada, New Zealand and Australia while GSM is in most every other major and minor country in the world except for Japan and South Korea. If international capability is not important to you either will do.
GSM also has an advantage in that it uses a SIM card (smart card) to store user information which essentially means that you can remove the smart card from one phone or other device (such as a PDA) install it in another device and you basically have your service in the new device. Since the SIM card contains your account basics you can pretty much change it to any compatible unlocked device.
CDMA also has the R-UIM which is a SIM-like device but it's not used at all with any North American provider at present.
AT&T Wireless, cingular & T-Mobile are the major GSM operators in the US. Verizon and Sprint PCS are the major CDMA operators.
Bottom line though for the end user is the mobile interface is not all that important. What is important is that the service works for you where you need it to work (home, business, sport, etc.) Some people think that GSM's voice codec sounds more natural and more like a land line than that used with CDMA. That's all subjective though.
There are two basic protocols: TDMA, and CDMA. TDMA chops up the clock into segments, and each user gets a time slice. CDMA is spread spectrum, everyone talks at once, and the system automagically figures out who said what. One Big Plus was CDMA SOFTLY degraded when stressed; TDMA hits a brick wall once out of time slots. The TDMA promoters claim that's hogwash. I don't know enough to call that fight.
GSM ran on TDMA. Other folks did as well. [ATT]
From there down, the fighting gets less and less clear. Take the SIM card issue: I can't envision any reason a CDMA phone can't be built with same, but I've never heard of one. Why?
That the real question is not technology, but can you reach a human at Customer Service...
I carried a pager from work for over a year before I discovered there was no coverage anywhere near my home. ;-)
There isn't any GSM anywhere in the county, as far as I know. The choice of carriers is AT&T (very recently) or USCellular. I used to give an address in a different city so I could get AT&T, and then roam onto USCellular. The USCellular deals were terrible. They've changed a little since AT&T put up a few towers.
There have been certain things recently added to the activations systems used by both Sprint PCS and Verizon Wireless which suggest the near-future implementation of SIM's or otherwise SIM-like devices. I have also seen many references in various online discussion forums as well as alt.cellular.sprintpcs and alt.cellular.verizon regarding new phones in development which utilize SIM's (or SIM-like gadgets) on the US CDMA platform.
CDMA subscriber base is much larger tham 100,000. In Thailand alone the subscriber is close to 1 M users and CDMA is still very widely used in countries like Korea.
That is weird. Maybe that's the choice of the NA providers. Most, if not all, CDMA phones in the Asia market uses R-UIM, which, from the customer point of view, offer the same convenience as SIM in GSM.
The bottom line probably is the quality of coverage. From my experience as a roaming GSM user in US, the GSM coverage there is not very good.
# Since "OFF-Time Slots" are efficiently handeled in CDMA, the voice would be clear in reception if same bandwidth is allocated for bot GSM And CDMA. These "OFF-Time Slots" can be described as time when no-one on either line is speaking anything. ... For a part of conversation please think something like this.. A: Please write my address. B: One Moment. ............silene, B is searching for a pen..... B: Yes please continue, A: abcdefthijk ..........(after some silence) ..... B Continue....
These silent slots can be used to increase sound clarity.
# GSM Mobile phones can be jaammed, they use FDMA, and TDMA simultaneously, This FDMA (Frequency Division Multiple Access) can be found to find recent elevation in power level in a frequency band and that band can be Flodded with random digital high power signal to increase BitErrorRate beyond limit to remove phone off network. CDMA has got Code Division Multple Access, using spread spectrum. So no specific frequency band can be received to decode the signal.
# Security. CDMA is secure. Since frequency Hops, and Code Division (The Code being the encryption Key ) The received signal can not be decrypted. This is not the case with GSM. A directional Antennae can find the probable bit pattern.
# SIM CDMA Mobile phones do not use SIM, because, the R-UIM used for authenticity has more space within to store. The encryption algorithms, etc.
# Recovering Bit Erros. CDMA Uses Viterbi code compression algorithm and hence has got an upper edge to recover Bit Error from Data. GSM only relies on CheckSum.
The time slots are a lot tighter than that. Years ago, listening to a channel of AT&T undersea cable would give unintelligible conversation. It would timeslice in between words, neatly inserting some white noise on both ends of the Pacific so that the callers didn't think they were disconnected.