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 2021, 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 April 2022 and will shut down 2G GSM in December 2022.
- T-Mobile will shut down the 3G CDMA network used by some Sprint and Boost customers on January 1, 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 old25 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 isn’t terribly mature yet. In fact, we recently spotted carriers telling their own subscribers to turn it off if they weren’t thrilled with it. 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.
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) 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 T-Mobile are focused on 4G and 5G, not 3G. They’ll retire the older networks rather than switch.
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.
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. The 3GPP (the GSM governing body) has 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. 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.
What CDMA vs. GSM Means to You
You should be buying a 4G LTE or 5G NR phone now. CDMA and GSM should not be relevant to purchasing decisions in the year 2021.
Historically, there have been some real differences between the technologies. It was 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 had to accept any GSM-compliant phone. So the GSM carriers didn’t have total control of the phone you were using.
That wasn’t 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 US phones now have SIM cards, but that isn’t because of CDMA. The SIM cards are there for 4G LTE networks, because the LTE standard also uses SIM cards. If your phone or device doesn’t have a physical SIM, it almost certainly has an eSIM.
Verizon has been making it harder to move a Verizon SIM from device to device without requesting the move through Verizon first. But this doesn’t have to do with CDMA any more. It involves flags in Verizon’s 4G LTE provisioning systems.
3G CDMA networks (known as EV-DO or Evolution Data Optimized) also, generally, couldn’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. All 4G and 5G networks can handle voice and data at the same time.
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.