Android A to Z: What is GSM?

What is GSM? When it comes to Android devices, especially for us here in the U.S., there are two major differentiators that really have nothing to do with Android and everything to do with the carrier you plan on using the device on. When you're talking about a mobile device, you're either talking about a GSM device, or a CDMA device. GSM stands for Global System for Mobile Communications and is the network standard for much of the world. 

Of the four major carriers here in the United States, T-Mobile and AT&T use GSM technology. In Europe, you'll be hard-pressed to find something other than GSM. In Asia, you'll still find some CDMA carriers. 

A major advantage, at least from an end-user perspective, of GSM is the ability to easily swap devices, thanks to the SIM card. That's the little card (like what you see in the picture above) that contains are information that allows you to connect to a network, and it also can contain contact information. Pop the card into a new phone, and your phone number and contacts come with you.

There are a couple caveats to that, of course. One is that the phone you're using has to have radios to work on specific frequencies. While T-Mobile and AT&T are both GSM carriers and share the same EDGE radio frequency, they use different 3G frequencies, and most devices released in the United States can only connect to one or the other. (That's not always the case though -- some phones, like the GSM Galaxy Nexus, have the ability to do both) Another hurdle is that carriers usually "lock" the device to only use their own SIM cards. That is, if you put an AT&T SIM card into a T-Mobile phone, it'll ask for an unlock code. You can purchase the codes online, or the carrier may give it to you for free, if your account is in good standing. Outside the U.S., this is less of an issue because phones often are purchased "unlocked," albeit at higher prices than you'll see here.

But, wait. It gets more confusing. The new 4G LTE is a GSM standard. Therefore, Verizon and (soon) Sprint are using GSM technologies on their otherwise-CDMA phones. And both of those carriers have had "world phones" in the past -- traditional CDMA devices with GSM radios tucked in for use outside the United States.

Is GSM preferred over CDMA? For some, it's just a personal thing. For others, it's a perceived technical thing (such as building penetration). For others, it's a business thing, like being able to more easily use your personal device overseas with a prepaid SIM.   

Previously on Android A to Z: What is fastboot?; Find more in the Android Dictionary

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Phil Nickinson
  • In the end, it should boil down to what carrier offers the best coverage for your needs. However, I have some reasons why GSM carriers should be considered first and CDMA carriers left for when no good GSM choice is available. 1. With CDMA you can't import a nice phone from overseas with supported radio bands for use with your carrier. You are stuck with whatever the CDMA carrier allows you to activate. 2. CDMA won't allow for simultaneous voice calls and data sessions. For people who talk via Bluetooth or wired headsets, it's super cool to be able to check some data on the web and respond to your caller with the appropriate info. LTE phones in Verizon may allow simultaneous voice and data but that's because LTE is used for data while CDMA is used for voice. Don't expect that flexibility in a 3G-only Sprint, Verizon or MetroPCS area. 3. CDMA doesn't allow for three-way calling, unless you initiate both calls. With GSM, you can join two incoming calls if you wish and turn the thing into a three-way call. 4. On the flipside of the previous item, CDMA won't allow you to initiate two calls and swap between them to consult with the parties in private. If you initiated the calls, your only choices are hanging up at least one call or make it a three-way call. In GSM you get to choose whether you want to consult the parties in private or join them for a three-way call.
  • I think GSM is the preferred technology for the reasons you have mentioned, with call quality being the only real advantage CDMA has over GSM. Good thing LTE will be the new standard. :) Now if only we could get a global radio frequency that all carriers and Government can agree on.
  • Doesn't CDMA generally offer a better signal? I thought I read somewhere (& can't find a link now) that a CDMA signal travels farther using less towers than a GSM signal. Maybe that is why a lot more people complain about dropped calls on AT&T & T-mobile than on Verizon & Sprint. CeluGeek is right though...go with the carrier that meets your needs best.
  • CDMA does warm hand-offs between towers. With CDMA, your phone can actually be connected to two towers at once. This leads to far fewer dropped calls. Especially while driving. Another point that no one ever mentions, is GSM has a functional distance limit of 22 miles. It is because the way GSM breaks up each second into discrete portions, and after 22miles, you won't be able to connect even with perfect line of sight. This doesn't matter by and large, but it gives CDMA overlapping fields of greater distance. It also allows CDMA to squeak off text messages from remote places. Most of the time, the 22 mile limit will not matter since you will not be connecting to a tower 22+miles away anyway due to signal strength. However, it does in some remote places. I have experienced the following first hand: If you are in a rural area like bush-Alaska, ATT stops working just a few miles from the highway. We were way the hell up a river in an area named Skwentna, and Sprint still worked! My buddy was able to update his facebook, make calls, and send text messages. My ATT Fuze had stopped getting a signal 2 hours ago. Question:
    Can anyone answer why VZW and Sprint 3G has a speed limit of about 2.2 Mb/s? While AT&T can increase the GSM 3G speed? If VZW was able to double their 3G to 4.4Mb/s that would be the greatest thing ever since their network is so strong.
  • CDMA cells resize dynamically depending on subscriber load. This is advantageous in areas where you must cover large areas with relatively few subscribers (I.e., the US).
  • Does this mean all Verizon 4g LTE phones are "global" phones (that is they take sim cards and can be unlocked)?
  • Close, but no cigar. if say, my verizon galaxy nexus' LTE chip can be unlocked, then yes, your DATA will be global, I.E. you can connect to a european LTE network, but you would have to do everything over a data connection (skype and whatnot). this needs more research, i would love to have an unlocked LTE phone.
  • Just like GSM 3G frequencies are different between T-Mo & AT&T, LTE frequencies are gonna end up being different between virtually every region (and between every US carrier)... Data roaming is actually about to get harder with LTE. Europe has yet to auction off it's LTE/4G usable airspace but it's likely not gonna matter what they do (for US residents) since they might match one carrier's frequency at best, none.