CDMA vs. GSM: What's the Difference?
If you're shopping for a mobile phone, you're in for a lot of acronyms.
Understanding the difference between CDMA vs. GSM is important, especially with
5G on the horizon.
Sascha Segan
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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
2019, we're getting close to the death date of these older technologies in the
US. (Verizon is turning its CDMA off this year.) But millions of people are
still using devices that require one or the other of these kinds of networks.
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 because most phones still need to
support 2G and 3G networks even to this day, you still see a difference in
compatibility. The difference will remain in any phone that needs to access an
older 2G or 3G network.
Now carriers are starting to install 5G, which after a few false starts will be
a single global standard called 5G-NR. But the CDMA/GSM split will still exist
in any area where 2G or 3G networks are still alive.
In the US, Verizon plans to turn off 2G and 3G at the end of 2019. T-Mobile
will turn off 2G and 3G at the end of 2020. AT&T has already turned off 2G.
AT&T and Sprint will keep 3G running until 2022.
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.
There are several variants and options carriers can choose, like toppings on
their technological ice cream. In this storywe're focusing on US networks.
The Technology Behind CDMA vs. GSM
CDMA and GSM are both multiple accesstechnologies. 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, 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 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.
CDMA and GSM Are Both Dying in the US
Don't buy a 2G GSM phone in the US in 2019. 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 came out in April and May 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, Sprint is using lower frequencies,
and T-Mobile is using a mix. 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 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 series and XS, XS Max, and XR phones work on all carriers. For the iPhone 7
and 8 generations, the Verizon, Sprint and unlocked models work on all
carriers; the AT&T and T-Mobile units only work on AT&T and T-Mobile.
Some other devices support three carriers—AT&T, T-Mobile, and Verizon—such as
the OnePlus 7 Pro. Those may not have a CDMA modem (which Sprint requires, but
Verizon doesn't any longer), or they may just not be certified for Sprint (as
Sprint has the power to reject uncertified phones.)
To find the right phone and carrier for you, our Readers' Choice and Fastest
Mobile Networks awards are great places to start.
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