Sunday, December 15, 2013

CDMA vs. GSM: What's the Difference?

CDMA vs. GSM: What's the Difference?

If you're shopping for a mobile phone, you're in for a lot of acronyms. Here's what you need to know about two basic, yet important, terms.
By Sascha Segan December 5, 2013 08:47am ES PC Magazine

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.

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. 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.

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. Our Readers' Choice and Fastest Mobile Networks awards are a great place to start.

My head is spinning, but I'm determined to figure out how to change my communications situation. Just not today.  I now have a throbbing headache from trying to learn about technology changes that I've attempted to ignore for the last 18 months.  I AM tired of being the cash cow that allows my communications carriers to make changes that entice NEW subscribers.


  1. Wow, that was some cell phone technology speech!

    You speak of European paradise; well, I have discovered an even greater phone paradise, and it is chock-a-block full of yummy features: it goes by the name of "SS", short for Smoke Signals.

    It doesn't need to have batteries being recharged, you don't need to pay through the nose to ANY cell phone operator, it's compatible ANYWHERE in the world, and, bonus: it scares the mosquitoes away.

    Can't beat that, can ya?

    In other news: today is yet another yucky gloomy day. Clouds all over the sky, waiting for me to go out shopping to start pissing down on me from a great height. I really wonder what the fuck I am doing in this part of the country. I mean, it's the South of France, but it's NOTHING like Provence or the Côte d'Azur (which you guys call "the Riviera" - fuck knows why cuz nobody else in the world does, and certainly not the French).

    There, it rains 3 times a year. Here it's 3 time a fucking week. Plus, it's fuckin' freezing - Snows every winter. It's hell on earth, truly.

    1. Jebus, I forgot to credit the author, I didn't write that. I only wrote the part about my head spinning from trying to learn about the technology, lol.

      I'm sorry you are so cold, dear. :( We've got Santa Ana winds here, it's hot and dry. Right now, the sun just went down, it's 5:15 pm and it's 74 fahrenheit ( I dunno what that is in celsius and I'm too lazy to look it up). My azaleas are dying because I haven't watered them. Still, I sleep with the windows open even when it's 37 outside.

  2. Nunya,

    "it's 5:15 pm and it's 74 fahrenheit ( I dunno what that is in celsius and I'm too lazy to look it up)"

    Nice temperature. Must be around 23°C (25°C = 77°F). By the way, kudos for saying "Celsius". Most Americans are a bit retarded and think "C" stands for "centigrade".

    I have a question for you: could you try to find out about an Oz woman whose maiden name is Sarah Fogarty, but who is married to some Texan asshole (whose name I don't know)? Last I heard from her, she/they were living in a town in Texas named "Corpus Christi" (lol, yeah, I know... only in fuckin' Texas, right?).

    Sarah was my partner in crime in the musical duo I had in Oz for 7 years. She is quite simply the most prodigious singer I ever heard - in the league of Eva Cassidy and even better. She also plays acoustic guitar.

    Lastly, she and I had a long love affair, and I have very fond memories of our sex sessions, particularly our 69s - she had a strange hang-up about regular intercourse: because of a long lost failed relationship with a young asshole jazz pianist named Kevin; she kept "saving" her pussy for his pathetic cock. Duh!

    Poor Sarah: like me, she hate fuckin' country music, and she's now married to a Texan cunt, having to listen to that crap all day long, lol.

    1. Why Not,
      I Googled the maiden name. It is a common one. This came up on youtube. Can you watch it?

    2. THAT'S HER!!!! And I recognize Margie too very well. Wow, amazing, lol. I coulnd't get any sound from the video cuz I watched it from the townhall's public computer, but the images leave no doubt.

      Margie Salem is one of Elizabeth's sisters (Liz was my second spouse), and I started the idea of a duo with her. But Margie was too busy and not quite keen enough to go into it full time, and so she introduced me to Sarah.

      What a small world! I can't get over it. So, Sarah's back in Oz - presumably without the Texan dude, lol.

      I have to find out how I can get in touch with her.

  3. CDMA(Code Division Multiple Access) is a digital cellular technology that uses spread-spectrum techniques.CDMA consistently provides better capacity for voice and data communications than other commercial mobile technologies, allowing more subscribers to connect at any given time, and it is the common platform on which 3G technologies are built.It doesn't have sim card.
    The reason for more GSM usage is that the international Roaming of GSM is Very good and it has sim card,So you can easily swap your sim card when you need.If we got a locked phone means we can unlock it by getting the unlocking service from any one of the online service provider like