See Original Post on PcMag.com

BY SASCHA SEGAN

Two basic technologies in mobile phones, CDMA and GSM represent a gap you can't cross. They're the reason you can't use many 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 Carriers are CDMA? Which are GSM?
In the U.S., Sprint, Verizon and U.S. 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 story we'll focus on 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.

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.
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 evolved faster 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.

For more, see 3G vs. 4G: What's the Difference?
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. There are very few phones that support all of the carriers' LTE bands.

Verizon has said it aims to start selling LTE-only phones in 2015, but for now, those will require special Verizon software to make voice calls, so that move won't make it any easier to switch carriers with your phone. Even without CDMA, the CDMA philosophy of carrier control of your phone will remain intact.
A growing number of phones support all of these standards, but it can be hard to tell which ones.

The iPhone 6, the iPhone 6 Plus and the Google Nexus 6 are the most flexible. iPhone 6 and 6 Plus units from AT&T, T-Mobile, and Verizon can all be used on all three carriers, but they lack Sprint's special LTE bands.

Sprint iPhones have all the bands, but Sprint has strict unlocking policies. Nexus 6 phones will technically work on all four carriers, but Sprint only allows phones purchased from Google or Sprint on its network.

HTC One (M8) and Samsung Galaxy S5 phones from Verizon will work somewhat on AT&T's and T-Mobile's networks, albeit with limited coverage because while they have CDMA, GSM and LTE, they don't have all the frequency bands AT&T and T-Mobile use. Variants of those same models sold by AT&T and T-Mobile won't work on Verizon at all, because they lack the CDMA radio needed for Verizon. It's a mess.

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, just go with GSM. 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. Our Readers' Choice and Fastest Mobile Networks awards are a great place to start.

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