NOTE: I had my acronym wrong on HSDPA. Now it's right...
So some people may be wondering what GSM, CDMA, TDMA, AMPS, iDEN, 1xRTT. WiDEN, EV-DO, GPRS, EDGE, UMTS and HSDPA all are. Well, let me tell you and show the advantages and disadvantages of each of the three main networks out now (CDMA, GSM, iDEN), and how all the rest of the aforementioned technologies fit into these three technologies.
But first let me start off with the "moldy oldies," AMPS and TDMA. AMPS, Advanced Mobile Phone System, is what everybody usually calls Analog. Sometimes, through some happenstance of chance caller ID information or text messaging somehow get through on this network, but usually the only thing that gets through is voice. I low signal areas that voice is staticy. But this is the network of the "bag phones" which can operate dozens of miles from a cell tower and still receive a signal. Of course, you can do that with a digital phone and an amplifier too. But analog was (and is, for those carriers that let people use it, or use it mainly) insecure (you can use a scanner to pick up conversations), prone to static and really hard on battery life. For example, the Nokia 3560, which I used to have, could last 11 days in standby on TDMA digital (mre on it later) but only a mere 42 hours on analog. And because of such high battery power requirements, even the best phones would heat up ater prolonged talking.
Then came TDMA. Because of the digital signal, as opposed to an analog one, phones usually had clearer voice quality, even in lower-signal areas. They also had much longer battery life, due to the fact that signals were usually not transmitted with as much power. And now Caller ID could work well, as well as text messaging and even data acess, though it was only a quarter as fast as a dialup modem and was not widely used. Oh, and the phones were smaller. But this technology was supplanted, as is evident by the activation cutoff for new prepaid lines by February of this year, by either CDMA or GSM, with GSM relying on much of the same technology...
...but usually better. GSM uses a SIM card (Subscriber Identity Module) to store people's phone number, network information, and even some or all of their address book. So if you need to change phones on GSM, merely swith out the SIM card with another phone that's either on the same carrier or is "unlocked," or ready for any carrier's SIM. GSM seems to be even lower-power than TDMA, probably because it does away with analog compatibility altogether (TDMA and CDMA largely keep this capability), and thus you can make smaller batteries and thus smaller phones and thus the Motorola Razr and SLVR and related phones, which are a half inch or less thick. By the way, TDMA (GSM is based on TDMA; GSM stands for Global Standard Mobile) stands for Time Division Multiple Access, meaning that the conversation's data is seperated from everyne else's by being in its own teensy time slot. As said, GSM is just a new network type using this way of working with calls.
But also GSM has faster data speeds, through GPRS (Global Packet Radio System) and EDGE (Enhanced Data GSM Experience? don't remember the last word), which are special improvements made to the network so data packets can travel faster over the airaves, making uch stuff as picture messaging (and thus camera phones) and relatively painless web browsing possible. GPRS boosts speeds to about that of a 56k modem., but with longer latencies, the time it takes for a request to get from point A to point B. And EDGE boosts speeds to twice or three times that, but again with latency problems. GPRS and EDGE have "classes" within them to determine how fast a particular device will go on that technology. That's right: not all EDGE and GRS handsets are created speed-equal. Some are class 4, some class 6, some class 11. And you would naturally want class 11. Both these technologies have been rolled out nationwide on all major GSM networks.
Short break here: people talk about 2G, 2.5G and 3G, and even 4G networks. What G means is generation, with each generation providing even faster data speeds. 1G was nalog, such as AMPS. 2G was digital, such as early CDMA (CDMAOne), TDMA, GSM and iDEN with slow data rates. 2.5G is a bit faster, inhabited mainly by GPRS, bt also by Nextel's new WiDEN (Wide iDEN). Some people say there's 2.75G, a spot for the likes of CDMA 1xRTT and EDGE, though usually the former goes to 3G even though it's sort of slow for the category and the latter goes into the 2.5G category though it's a little fast for that classification.
Then there's 3G, with speeds at least as fast as a slow DSL connection, but with the potential to get really, really fast. CDMA 1x EV-DO (aka EV-DO) is the big entry here, as well as its Revision A version (currently most systems that use this technology are Revision 0). The other player is UMTS and its EDGE-like enhancement, HSDPA, which can get really fast but is in few places as of yet. Oh, and 3G is usually only available in metro areas right now, but it will eventually be everywhere.
And 4G? Think fast! Really, really fast! As in DSL speeds on one bar of signal, cable speeds with a good signal. Yeah, that fast from a cell tower. Or try 11Mbps...yeah, WiFi in the sky aka WiMax...these networks are all in testing stages except for very, very small carriers' networks...
OK, now back to GSM, whose next leap upward is to UMTS, or rather to incorperate UMTS. At first, phones using this technology were quite bulky, but soon they assumed a normal profile. Except that they work at 300 to 400 kbps in UMTS areas, around the speed of a low DSL connection. Interestingly, UMTS, aka WCDMA\Wide CDMA, is a totally different system than GSM, not just a simple upgrade, and that's why it's taking so long to happen. But this is the whiz-bang tech Europe and everywhere else is using, mainly because they're GSM junkies and this is the upgrade path.
The next step in the progression is HSDPA (High Speed Downlink Packet Access), which is again sorted n classes. Class 10 promises 14.4Mbps data rates. Probably it will really work around 10, if that. Right now they're only using up to Class 8 or 9, about the speed of a cable connection i it's maxed out, slower if you don't hold your mouth right ;). Again, this technology is only a few places in the US.
And thus the saga of GSM is completed. Now for CDMA...
CDMA started as the variant now called CDMAOne. Instead of time it uses a code to divide up people's conversations. And later in its progression it got CDMA2000 1xRTT, a fully backward compatible system (with CDMAOne) that could pack twice as many voice users in the same amount of radio "space" and extremely raise data speeds as well. That's why Qualcomm, the company who makes a ton of CDMA stuf, calls it 3G. The maimum speed for this network is 135kbps, and on multisource downloads, as everyone knows, I have gotten perilously close to that speed. :)
Then 3G happened. EV-DO is more widespread than UMTS or HSPDA, though it's supposedly slower than the latter. But it's still smoking, with normal data rates hovering between 400 and 700 kbps. Small problem: upstream bandwidth is only about a tenth of that. But no matter; just get Revision A, while UMTS\HSPDA is still trying to make video calling work. Revision A, due out soon, ups upstream speeds to 200-400 kbps. Ahh, much better. Oh, and the technology follows a logical, easy succession. Thus the pretty darn nice EV-DO coverage across the US, though it is sadly by no means nationwide.
Lastly, iDEN. This network, similar to GSM in that it now uses SIM cards (and has since 2003 or so), is built (by Motorola) around wakie talkie. It uses a special region of the frequency spectrum that normal cell carriers don't use (usually two carriers fit on the 850 "cellular" band and lots of carriers fit on the 1900MHz "PCS band, more on that later) for its glorified two-way radio existence. Yep, all Nextel phones are are glorified two-way radios. Yeah, I know, iDEN isn't ONLY Nextel, but that's what it is mostly. Anyway, we all know that iDEN\Nextel s famous for it's ultra-fast "Bleep Bleep" walkie-talkie]Direct Connect service. And by the way it's digital and based on TDMA as well. Only, it's not ad advanced in many areas as CDMA or GSM. For example, its fastest data technology, WiDEN (Wide integrated Digital Enhanced Network) is only about the speed of a 56k modem. But it's not nationwide. What's nationwide is about 1\3 of that speed. And, due to the technology, iDEN phones are comparatively huge, and if they're not huge (merely medium-sizzed) battery life suffers even more. As in standby battery life goes down from 75 hours at standby to 60 or even 42. But thats the price you pay for fast walkie talkie coast to coast. Though another perk is that all new phones have reasonably good GPS, so you can find your way around. But CDMA carriers are getting GPS too.
OK. That's it. Let me rehash the pluses and minuses of CDMA, GSM and iDEN...
CDMA (Verizon, Sprint PCS)
+Fast data speeds (1x or EV-DO)
+Great voice quality
+Very good 3G coverage thusfar (talking about EV-DO)
+GPS aided services coming very, very soon
+Very developed network
-Phones usually aren't as small as GSM
-No SIM cards
-Roaming not good for features
GSM (T-Mobile, Cingular)
+Phones are either small or have great battery life or both
+Future for high speed data is bright
+Roaming, if available, is seamless and full-featured
-Low 3G penetration
-Not fast for nationwide data
-Not as great voice quality
-Looks like no GPS stuff coming
-Hard transition to UMTS\HSPDA for high speed data
+Very mature GPS solution
+Walkie service is still the fastest on the market
-Not very many walkie talkie features
-Low voice quality
-Low data speeds
-Poor Battery life
Personally, I'm a CDMA junkie because of high data speeds and large coverage footprint. But I keep GSM because the phones and services become more interesting every other day. And most of my extended family has Nextel, and I like using TeleNav, so I keep Boost Mobile around.
Hope this post has served to unconfuse everyone about what this, that r the other technology is all about. If not, please comment and I'll see what I can do to clear things up.