Two basic technologies in mobile phones, CDMA and GSM, represent a gap you can't cross. They're the reason you can't use older 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 (2G and 3G) used in cell phones. In this story, I'll try to explain who uses which technology and what the real differences are.

We have been updating this story since 2012. In 2020, it's absolutely time to get off of CDMA and GSM. Verizon will turn off its CDMA network and T-Mobile will turn off its 2G GSM network by the end of 2020. In 2021, AT&T and T-Mobile will keep their 3G networks in very low-bandwidth modes mostly designed to support devices like electric meters and vending machines. Now that T-Mobile owns Sprint, it's likely to do the same with Sprint's old CDMA network. That means 2G/3G reception and call quality will likely be poor, if you get a signal at all. It's a 4G LTE world now, and if you aren't in it yet, it's time to switch over.

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 between 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's been slow going. All four carriers now support voice over 4G.

Now carriers are starting to install 5G, which after a few false starts will be a single global standard called 5G-NR.

The Samsung Galaxy S20 and its siblings are the first all-carrier 5G phones for the US

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 will 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 languageTo support the same frequency bands—being able to tune to the right channelTo be allowed on the network, in terms of permissions

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

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, for example 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 can't make calls.

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; and Google Pixel phones all work across all four 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.

Verizon still has considerable 3G CDMA coverage, but only until the end of 2020

Which Carriers Are CDMA? Which Are GSM?

In the US, Sprint, Verizon, and US Cellular use CDMA. AT&T and T-Mobile use GSM.

Most of the rest of the world uses 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 have done it, to get access to the wider variety of off-the-shelf GSM phones. But Verizon and Sprint are now focused on 4G and 5G, not 3G. They'll retire the older networks rather than switch.

The Technology Behind CDMA vs. 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.

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 has evolved faster than CDMA. WCDMA is considered the 3G version of GSM technology. To further speed things up, the 3GPP (the GSM governing body) released extensions called HSPA, which have sped GSM networks up to as fast as 42Mbps, at least in theory.

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

The Alcatel Go Flip 3 makes calls over 4G LTE

What CDMA vs. GSM Means to You

For call quality, the technology you use is much less important than the way your carrier has built its network. There are good and bad CDMA and GSM networks, but there are key differences between the technologies. Here's what you, as a consumer, need to know.

It's much easier to swap phones on GSM networks, because GSM carriers put customer information on a removable SIM card. Take the card out, put it in a different phone, and the new phone now has your number. What's more, to be considered GSM, a carrier must accept any GSM-compliant phone. So the GSM carriers don't have total control of the phone you're using.

That's not the case with CDMA. In the US, CDMA carriers use network-based white lists to verify their subscribers. That means you can only switch phones with your carrier's permission, and a carrier doesn't have to accept any particular phone onto its network. It could, but typically, US carriers choose not to.

All Sprint and Verizon phones now have SIM cards, but that isn't because of CDMA. The SIM cards are there for Sprint's and Verizon's 4G LTE networks, because the LTE standard also uses SIM cards.

3G CDMA networks (known as EV-DO or Evolution Data Optimized) also, generally, can't make voice calls and transmit data at the same time. Once more, that's an available option (known as SV-DO for Simultaneous Voice and Data Optimization), but one that US carriers haven't adopted for their networks and phones.

On the other hand, all 3G GSM networks have simultaneous voice and data, because it's a required part of the spec. Verizon 4G phones can do simultaneous voice and data because they route it all over LTE, avoiding CDMA entirely.

To find the right phone and carrier for you, our Readers' Choice and Fastest Mobile Networks awards are great places to start.

Further Reading