Giz Explains: What's the Difference between GSM and CDMA?

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America is divided! One faction is powerful and entrenched. The other is respected by the rest of the world, but can't seem to seize power here. I'm talking, obviously, about our cellphones.

Buying a phone is a tough choice. No wait, scratch that: It's a tough set of choices, a dozen decisions wrapped into one, made just once every two years. When you decide it's time to buy, you've got to select between operating systems, hardware, and feature lists. Just as importantly, you've got to choose a network, with its distinct coverage areas, rate plans and customer service. But built into your choice of network, is yet another dilemma: network technology. GSM or CDMA? These inconspicuous acronyms, which an awful lot of people deem fit to ignore, define the most basic functions of your phone

What, Which, and Who

GSM and CDMA both serve as shorthand for different mobile phone technologies. GSM stands for Global System for Mobile Communications; it's the world's most prolific mobile standard (a standard being a set of rules and suggestions about how a mobile network should work). CDMA stands for Code Division Multiple Access—in the context of cellphones and mobile networks, people tend to use it interchangeably to refer to two different mobile standards: CDMAOne or CDMA 2000.


What's the core difference? It all has to do with the way your data is converted into the radio waves that your cellphone broadcasts and receives. To keep from lulling you to sleep with the deep dive, I'll just scratch the surface and say that GSM divides the frequency bands into multiple channels so that more than one user can place a call through a tower at the same time; CDMA networks layer digitized calls over one another, and unpack them on the back end with sequence codes.


Image courtesy of National Instruments

CDMA was a late response to GSM, and in 1995 this more complex and modern channel access promised better security, fewer dropped calls, and more efficient infrastructure. But that was 1995, when car phones were still regularly spotted on city streets.


America is unique in that it's home to more CDMA users than GSM users, with the two largest CDMA carriers accounting for over 43% of the market. The two largest GSM carriers barely break 37%; worldwide, CDMA accounts for around 13% of phones, with GSM and its successor, UMTS making up of the remainder.

Which Acronym Are You?

If you just want to figure out which of these two sets of letters you're working with, well, that's easy:

• American CDMA carriers:
Verizon, Sprint and whoever uses their networks (Virgin, Boost, Alltel)


• American GSM carriers:
AT&T and T-Mobile, and whoever uses their networks (Suncom, Pure)

Of course, none of this tells us anything at all about what it means to use networks on either standard. Standards being basically a set of guidelines that participating companies abide by, most of the differences between CDMA and GSM are small details that you'll never have to concern yourself with: frequency bands, audio codecs, the physical specifications of the network infrastructure, the way a user is linked to a phone, and so on.


But these rules are very important to the AT&Ts, Verizons, Apples, and Samsungs of the world: They outline pretty much every technical aspect of a cellular network, and, to a lesser extent, the phones that are used on it. In the same way that web standards ensure that webpages render properly in our browsers, the GSM and CDMA standards give carriers a set of instructions to (for the most part) follow, and cellphone makers a guide for making devices that'll work on the world's wireless networks.

The Differences

Most of us will never have to think about whether or not our phones are CDMA or GSM-based. These acronyms are meant to be transparent, just like so many other tech standards are. (Most HDTV owners don't really care much if their images are delivered via Component or HDMI cable, nor do most music listeners mind if their music was encoded as a AAC file or an MP3—as long as the quality does not suffer.) But that's not to say that they aren't different.


Mildly Useful Network Trivia!

• US Frequency bands
GSM: 850MHz, 1900MHz
CDMA: 850MHz, 1700MHz, 1900MHz
• Audio Sampling/Bitrate
GSM: 8kHz @ 12.2kbps
CDMA: 8kHz @ 8.55kbps
• User ID systems

First, let's get this out of the way: I've been using GSM and CDMA as blanket names for a set of standards that have changed over time. Most new phones on AT&T and T-Mobile actually adhere to both GSM and the newer UMTS (Universal Mobile Telecommunications System) standards. UMTS isn't an official part of the GSM standard, but it is what GSM carriers use for 3G data transmission. Likewise, CDMA2000, based more directly on its predecessor includes a range of improvements over the original CDMAOne, key among them 3G data speeds. Though both GSM and CDMAOne standards are on their way out, I fully expect their names to live on as shorthands for what comes next. After all, they were the basis of the entire cellular industry as we knew it for decades.


Back in 1995, CDMA was an insurgent standard trying to supplant the dominant GSM, and the differences between the two technologies were more obvious. Old-school, 2G GSM phones worked better inside of buildings (neat trick: If you're having trouble getting a signal indoors, switch off your 3G), but caused interference in unshielded speakers (side-effect of aforementioned 'neat' trick). At the same time, CDMA phones had a slightly more refined method for handing off calls from tower to tower, so they dropped fewer calls. This is still true. It's also still true that 2G GSM networks can offer better coverage in mountainous terrain, since they utilize taller cell towers, though range of said towers is otherwise a bit shorter. Additionally, GSM (and UMTS) phones can send and receive data packets while making a call, which most CDMA networks still don't support.


Such were the arguments for and against CDMA when it barged into the scene in 1995, at time when GSM was the only game in town and most people didn't even own cellphones. So it follows that these original performance differences, which were striking at the time, now don't matter matter quite so much anymore. If a Droid gets better reception at your house than an iPhone, it's not because one is a CDMA2000 phone and the other is a GSM/UMTS phone. It's most likely because Verizon has a tower closer to your pad, and the backhaul to support your calls.

The real differences—the ones that you should care about—are more obvious.


Both GSM and CDMA standards outline a way that phones are identified by carriers. In GSM phones, it's a removable chip called a SIM card. In theory, you can pop a SIM card out of a GSM phone and stick it in any other GSM phone. (Although a lot of phones are "locked" to a specific carrier, which is majorly annoying.) The CDMA standard describes something similar, called the RUIM (removable user ID module), but that hasn't really caught on. Instead, CDMA phones ship locked to one network, and can only be switched to another with the cooperation of both the old and new carriers.

This isn't so important in a place like America, where phones are sold with contracts and discarded with after two years. But it's a huge deal in the developing world, where phones are sold unlocked, independently of carriers, and need to work with any and all local networks. And even in the first world, sometimes it's nice to be able to just switch numbers every once in a while. (A local pay-as-you-go SIM saved me a boatload of money on a recent trip overseas.)


And that leads us to the main reason you'll need to consider when choosing between CDMA vs. GSM: travel. Basically, CDMA phones suck at this. A CDMA-only phone from Verizon or Sprint is only able to roam on other CDMA networks, which simply don't exist in much of the world. Both carriers offer phones with built-in GSM support just for travelling, but this feature is missing from their most popular handsets.


Subtle as they may be, the outward differences between CDMA and GSM can tell you a lot about your phone, from where you can use it to how well it holds a call on the highway. I'm not saying that you should place more weight on a carrier's choice of wireless tech standards than its phone choice, customer service or coverage in your area. I'm just saying that you shouldn't ignore it.

Original art by guest artist Chris McVeigh (AKA powerpig). You can catch all his work at, and follow him on Twitter. (@Actionfigured)