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 used in cell phones. In this story, I’ll try to explain who uses which technology and what the real differences are.
We first ran this story in 2012 and have updated it almost every year since. In 2018, we’re seeing some major changes coming that you need to know about—most notably, how Verizon is preparing to shut down its CDMA network at the end of 2019, and how 2G GSM is going extinct in the US.
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.
There are several variants and options carriers can choose, like toppings on their technological ice cream. In this
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
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, are 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.
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
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
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. (3G GSM is also actually a type of CDMA. I’ll explain that later.)
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.
CDMA and GSM Are Both Dying in the US
Don’t buy a 2G GSM phone in the US in 2018. US carriers are moving away from both CDMA and 2G GSM, which will make some basic, flip, older, or foreign phones stop working in the US—if they haven’t stopped working already.
These 1990s-era technologies are typically cheap to buy and use, but they’re very inefficient users of the airwaves. As Americans try to cram more and more data into the air, carriers are switching their channels from inefficient older technologies to the more efficient 4G and 5G systems, which can pack many more users and much more data into the same sized channels.
AT&T has already turned off its 2G GSM. T-Mobile has reduced its 2G coverage so it’s only useful for machine-to-machine devices like fleet trackers and vending machines, not phones. (Both carriers still have 3G networks.) Verizon plans to turn off 2G and 3G CDMA at the end of 2019. Sprint has not said when it will shut down CDMA, but that will probably come in 2020 or 2021.
That’s the stick. Here’s a carrot: 4G LTE supports much higher-quality voice calling technologies that will make your phone calls sound better.
If you have a 2G-only flip or voice phone, it will stop working soon, if it hasn’t already. Ideally, you should switch to a 4G phone as soon as possible for the best coverage and quality. For more, see Why We Don’t Recommend 2G GSM Phones in the US.
4G LTE and 5G Bring It All Together
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, known as 5G NR. (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 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.
For instance, on Verizon, 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.
The first 5G phones will come out in March or April of 2019. AT&T/Verizon and Sprint/T-Mobile are using two sets of very different radio channels for 5G. AT&T and Verizon are using high frequencies, while Sprint and T-Mobile are using much lower frequencies. So while they’re all using common technology, the phones may not work on each other’s networks because they don’t have the right antennas to tune to each others’ channels.
Right now, some phones support all four carriers by combining CDMA, GSM, and LTE. The iPhone 6 and later; the Motorola Moto G4, E4, and later; the Samsung Galaxy S7 and later; and Google Pixel phones all work across all four carriers.
Some other devices support three carriers—AT&T, T-Mobile, and Verizon—such as the OnePlus 6T, the Razer Phone 2, and the LG G6+. Those devices don’t have CDMA (which Sprint requires) and are certified for use on Verizon’s LTE network.
So what does all of this mean for you? If you want to switch phones often or use imported phones, just go with AT&T, T-Mobile, or virtual carriers on those networks. Otherwise, pick your carrier based on coverage and call quality in your area, and assume you’ll probably need a new phone if you switch carriers. To find the right phone and carrier for you, our Readers’ Choice and Fastest Mobile Networks awards are great places to start.