CDMA vs. GSM: What's the Difference?

CONTENTS

Two basic technologies in mobile phones, CDMA and GSM represent a gap you

can't cross. They're the reason you can't use 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 the two major radio systems used in cell phones. Both acronyms

tend to group together a bunch of technologies run by the same entities. In this

story, I'll try to explain who uses which technology and what the real differences

are.

Which Carries are CDMA? Which are GSM?

In the U.S., Sprint, Verizon and U.S. Cellular use CDMA. AT&T and T-Mobile use

GSM.

That means we're mostly a CDMA country. It also means we're not part of the

norm, because most of the world is 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 story we'll be talking about U.S. networks.

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 U.S., 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, U.S. carriers choose not to.

In other words, you can take an unlocked AT&T phone over to T-Mobile (although

its 3G may not work well because the frequency bands are different). You can't

take a Verizon phone over to Sprint, because Sprint's network rejects non-Sprint

phones.

Many Sprint and Verizon phones now have SIM cards, but that isn't because of

CDMA. The SIM cards are generally there for Sprint's and Verizon's 4G LTE

networks, because the LTE standard also uses SIM cards. The phones may also

have SIM slots to support foreign GSM networks as "world phones." But those

carriers still use CDMA to authenticate their phones on their own home networks.

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 U.S. 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 U.S. 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 big enough that they can get custom phones built for them, so they

don't see the need to waste money switching 3G technologies when they could be

building out their 4G networks.

BACK TO TOP

CDMA vs. GSM: What's the Difference?

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 explain later) isn't a time division technology.

CDMA required 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.

Since its inception, GSM has had many more add-ons and evolutions than CDMA.

As I mentioned above, 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, U.S. carriers chose not to install them and have instead turned

to 4G LTE to be more compatible with global standards.

The Future is LTE

The CDMA vs. GSM gap will close eventually as everyone moves to 4G LTE, but

that doesn't mean everyone's phones will be compatible. LTE, or "Long Term

Evolution," is the new globally accepted 4G wireless standard. All of the U.S.

carriers are turning it on.

The problem is, they're turning it on in different frequency bands, with different 3G

backup systems, and even, in the case of the new Sprint Spark network, using an

LTE variant (TD-LTE) that doesn't work with any other U.S. carrier's phones.

Furthermore, it's not like the 2G and 3G networks are going away any time soon.

Carriers have told us they're leaving their UMTS and EVDO networks live until at

least 2020. So we will not enter a European-style paradise of interchangeable

phones anytime soon.

So what does all of this mean for you? If you want to switch phones often, use

your phone in Europe, or use imported phones, go with GSM. Otherwise, pick

your carrier based on coverage and call quality in your area.