CDMA vs. GSM: What's the Difference?

Two basic technologies in mobile phones, CDMA and GSM, represent a gap you can't cross. They're the reason you can't use old AT&T phones on Verizon's network and vice versa. But what does CDMA vs. GSM really mean for you?

CDMA (Code Division Multiple Access) and GSM (Global System for Mobiles) are shorthand for two older radio systems (also known as 2G and 3G) used in cell phones. We first published this article in 2012 and kept it updated it throughout the 2010s, when it was important to know the differences between these two technologies. But in 2022, it's absolutely time to get off of CDMA and GSM. Those networks are going away! If you have a phone that only uses 2G or 3G, you need to get a 4G or 5G phone, pronto.

• AT&T has already shut down its 2G GSM network and most recently said it will shut down 3G GSM/UMTS in February 2022.

• T-Mobile will shut down 3G GSM/UMTS in July 2022. It has not set a date to turn off 2G, but has greatly reduced its coverage and quality.

• T-Mobile will shut down the 3G CDMA network used by some Sprint and Boost customers on March 31, 2022.

• Verizon will shut down its 3G CDMA network on December 31, 2022.

In the twilight years of these networks, they're being turned down to levels primarily designed to support devices like electric meters and vending machines. That means 2G and 3G reception and call quality will likely be poor, even before the formal shutdowns. It's a 4G LTE world now, with 5G coming up fast. No matter how much you love your old phone, it's time to switch over.

Yes, there are some 2G-only and 2G/3G-only phones still for sale, especially unlocked GSM phones. Don't buy them. They'll work poorly, and pretty soon they won't work at all.

Don't weep for CDMA and GSM. They've had long lives. Sprint's CDMA network is 25 years old. The first GSM network launched in the US in 1995. There are more efficient ways to use our limited airwaves now.

1G, 2G, 3G, 4G, 5G

When cell phone providers talk about a "G," they mean a generation of wireless technology. Each generation is able to support more users and has better data transfer capabilities.

The first generation was analog cellular phones. When carriers switched to 2G digital systems in the 1990s, they chose among several competing options; some of them died out, but CDMA and GSM are the two 2G camps that survived. They remained split during the '00s through the third generation of cellular, which added better data speeds but stayed incompatible.

The CDMA/GSM split ended, in theory, as carriers all switched to LTE, a single, global 4G standard, starting in 2010. But the difference remained because phones still needed to access the older 2G and 3G networks, primarily for voice calls. AT&T, T-Mobile, and Verizon all started to phase in voice calling over 4G in 2014, but it took a while. All four carriers now support voice over 4G.

Now carriers are installing 5G, which (after a few false starts) will be a single global standard called 5G-NR. 5G is still in the early stages. You can't make voice calls over it quite yet, and it doesn't have true nationwide coverage. But 4G LTE is very mature now, and if you're still on 2G or 3G, you should have no concerns about upgrading to a 4G-compatible phone.

One Standard Doesn't Mean Compatibility

LTE, or Long Term Evolution, is the globally accepted 4G wireless standard. All of the US carriers use it. For more, see 3G vs. 4G: What's the Difference? 

And all of the carriers use the same 5G standard. (For more on that, see our explainer on 5G.) So you'd think, hey, that should make everyone compatible, right? Wrong.  

To be compatible, you need three things:

• To be using the same technology, like speaking the same language

• To support the same frequency bands—being able to tune to the right channel

• To be allowed on the network

In the 4G and 5G world, everyone will be using the same radio technology, but they may not have the same channels, and carriers may not permit other carriers' devices to be used on their networks.

The biggest problem is frequency band compatibility. Carriers operate on different radio channels, and one carrier's model of a phone may not include channels used by other carriers. This is frequently a problem across international borders, as with the six different international models of the Samsung Galaxy S20.

On Verizon and AT&T, 4G devices that haven't been certified by the carrier have trouble making voice calls or sending text messages over that network. They'll connect and get data, but they can't make calls.

AT&T has a list of which phones will work when it turns off 3G.

Many, but not all, popular phones now support all three major carriers' LTE networks. The Motorola Moto G4, E4, and later; the Samsung Galaxy S7 and later; the OnePlus 8 and later; and Google Pixel phones all work across all carriers. For iPhones, all iPhone 6 and later phones work on all carriers' LTE systems. 

Yes, this is more complicated than the old 2G world. One advantage of GSM was that if a phone and carrier both adhered to the standard, and the phone supported the right channels, the network had to accept the phone. That isn't the case any longer.

Which Carriers Are CDMA? Which Are GSM?

In the US, Verizon, US Cellular, and the old Sprint network (now owned by T-Mobile) used CDMA. AT&T and T-Mobile used GSM.

Most of the rest of the world used GSM. The global spread of GSM came about because in 1987, Europe mandated the technology by law, and because GSM comes from an industry consortium. What we call CDMA, by and large, is owned by chipmaker Qualcomm. This made it less expensive for third parties to build GSM equipment.

So why did so many US carriers go with CDMA? Timing. When Verizon's predecessors and Sprint switched from analog to digital in 1995 and 1996, CDMA was the newest, hottest, fastest technology. It offered more capacity, better call quality, and more potential than the GSM of the day. GSM caught up, but by then those carriers' paths were set.

It's possible to switch from CDMA to GSM. Bell and Telus in Canada did it to get access to the wider variety of off-the-shelf GSM phones. But Verizon and T-Mobile are focused on 4G and 5G, not 3G. They are retiring the older networks rather than switching.

What Is 5G?

The Technology Behind CDMA and GSM

CDMA and GSM are both multiple-access technologies. They're ways for people to cram multiple phone calls or internet connections into one radio channel.

GSM came first. It's a "time division" system. Calls take turns. Your voice is transformed into digital data, which is given a channel and a time slot, so three calls on one channel look like this: 123123123123. On the other end, the receiver listens only to the assigned time slot and pieces the call back together.

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The pulsing of the time division signal created the notorious "GSM buzz," a buzzing sound whenever you put a GSM phone near a speaker. That's mostly gone now, because 3G GSM (as I'll explain) isn't a time division technology.

CDMA requires a bit more processing power. It's a "code division" system. Every call's data is encoded with a unique key, then the calls are all transmitted at once; if you have calls 1, 2, and 3 in a channel, the channel would just say 66666666. The receivers each have the unique key to "divide" the combined signal into its individual calls.

Code division turned out to be a more powerful and flexible technology, so "3G GSM" is actually a CDMA technology, called WCDMA (wideband CDMA) or UMTS (Universal Mobile Telephone System). WCDMA requires wider channels than older CDMA systems, as the name implies, but it has more data capacity.

(GSM is actually only the formal name for the 2G system. But the name is also widely used to refer to any technology on the "GSM path" and approved by the same industry body, so I'm referring to WCDMA as 3G GSM so people don't confuse it with the separate 2G CDMA.)

Since its inception, GSM evolved faster than CDMA. WCDMA is considered the 3G version of GSM technology. The 3GPP (the GSM governing body) released extensions called HSPA, which sped GSM networks up to as fast as 42Mbps, at least in theory.

Our CDMA networks, meanwhile, got stuck at 3.6Mbps. Faster CDMA technologies exist, but US carriers chose not to install them and instead turned to 4G LTE to be more compatible with global standards.

Why Carriers Are Shutting Down 2G and 3G

Everyone's scrolling, Snapping, texting, FaceTiming, and more. Demands for mobile data use continue to rise. 2G and 3G, CDMA and GSM, are inefficient uses of the airwaves. 4G and 5G compress more information into each hertz of airwaves, and can combine channels much more flexibly for more efficient operation. So the carriers are retiring the older, less efficient technologies in exchange for newer networks which make better use of a scarce resource.

The carriers believe that most people have already switched to 4G through the natural process of upgrading phones. Phones making 4G voice calls have been in the market for years now. Apple's first iPhone with 4G voice was the iPhone 6, from 2014. While it's annoying to have to switch out an older phone because of this network transition, it's far from sudden.

There's no technological difference between "CDMA" and "GSM" carriers any more in the age of 4G LTE and 5G. Some cultural differences remain, though.

Specifically, Verizon makes it really hard to move your SIM card between devices without their permission, while it's easier to do so on AT&T and T-Mobile. That's an option in 4G. Verizon does it because historically, with CDMA, it could control which phones were on its network, and it wants to keep as much of that control as possible. AT&T and T-Mobile don't in part because in the GSM era, the GSM spec meant they had to accept anyone with a compatible phone.

Ready to Upgrade?

To find the right phone and carrier for you, our Readers' Choice and Fastest Mobile Networks awards are great places to start, along with our list of the best phones we've tested.