GSM vs. CDMA Explained

Posted: January 28, 2015

Overview

Before we get into the details of each technology, let's get the acronyms out of the way. CDMA stands for Code Division Multiple Access and GSM is the acronym for Global System for Mobile Communications. Now you can forget them, because everyone just uses the acronyms anyway. If you were to look up the Wikipedia entries for GSM and CDMA, you would find that they are so technical that they are not very helpful for most people. The point of this article is to simplify and summarize the differences in the two technologies so that no matter what your level of expertise, you can understand what kind of phones you can buy and the benefits and disadvantages of each.

CDMA and GSM are like different digital languages

Both CDMA and GSM can be broadly described as mobile networking protocols that use different technologies to send information back and forth via radio waves. They are primarily used in mobile phones, but many tablets, laptops, or anything else connected to a mobile network will also use one of these technologies. CDMA and GSM are like different digital languages. GSM devices can only talk to and understand GSM equipment (primarily cell phone towers) and CDMA devices can only communicate with CDMA networks.

The component in the mobile phone that enables it to communicate through these technologies is generally referred to as the “radio,” as its purpose is to convert information being sent out into radio waves and to interpret radio waves coming back into something that can be understood by your device.

Which Mobile Providers use GSM vs. CDMA?

In the US, Sprint and Verizon use CDMA while AT&T and T-mobile use GSM. In the rest of the world, GSM is the standard with only infrequent pockets of CDMA.

GSM vs. CDMA: What You Need to Know
GSM CDMA
SIM Card: X
Unlocked Phones: X
3G Network Speed:
4G LTE Networks: Carriers use GSM standard for LTE
US Networks: AT&T and T-mobile Verizon and Sprint

Compatibility

CDMA and GSM are not compatible with each other. As a rule you cannot take a GSM phone from AT&T or T-mobile and use it on Sprint or Verizon’s CDMA network. There is a minor exception to this rule: many of the newer phones currently on the market, like Google's Nexus 5 or 6 and Apple's iPhones, have a radio that supports both GSM and CDMA built in. However, even if your phone is compatible with both technologies, it can only be connected either to a GSM or CDMA network at one time.

What Makes them Different

One of the most important things to understand about the differences between GSM and CDMA devices is to understand how they are activated and authenticated with the mobile service provider. GSM providers use the Subscriber Identity Module (SIM) card. In other words, the SIM card is the thing that is activated on the network, so you can put the card into any compatible GSM phone and it will work on the mobile network with your phone number and data plan.

On GSM providers, the SIM card is the thing that is activated on the network, so you can put the card into any compatible GSM phone and it will work

On CDMA networks, the device itself must be activated. A unique identifying number is associated with the specific phone (called the MEID) is used to activate and authenticate the phone. Sprint and Verizon keep databases of those numbers, associated with the devices they sell, and will not activate any phones that are not in their database.

SIM cards

So, with a GSM service your SIM card is activated and you can place it in any compatible phone and it will work. You can even switch between GSM providers just by swapping out the SIM card.

On a CDMA service you must activate the specific phone you intend to use and can only switch to another one by getting the network provider to deactivate your current phone and activate a new one. This requires you to providing the specific number associated with the device (the MEID) to the mobile provider, either online or through customer service. Sprint or Verizon will then swap your phone number to your new CDMA phone.

More simply, you cannot switch between Sprint and Verizon, not because it is technically impossible, but because Sprint and Verizon will not allow it. Until recently all CDMA devices were “locked” to their original network and could not be unlocked and moved to another provider.

After a great deal of pressure from the public and eventually the White House and FCC, carriers made an agreement with the FCC that they would have unlocking procedures in place by February 2015. Essentially the agreement states that the carriers can no longer keep phones locked just to discourage people from switching providers. Sprint and Verizon will have to change some of their policies to comply with this agreement. At this point, both have made sure that most devices manufactured specifically for their networks can not be easily unlocked and used on other networks. It will be interesting to see how that plays out for the CDMA providers in 2015.

Which is Better?

I would argue that GSM is better due to the fact that it is inherently more open - it allows users more choice and flexibility in both device and mobile network selection. However, what is best for you will probably come down to two things: network coverage in your area and the cost of service. Those two things will vary greatly depending on where you live and how you use your mobile phone. Whichever network fits your needs is the best for you.

What is best for you will probably come down to two things: network coverage in your area and the cost of service

4G LTE Networks

At this point, you may be asking, “Hey, I am on a CDMA network but I still have a SIM card in my phone! I thought you said SIM cards were only for GSM networks, right?” Yes, both are correct! All mobile networks have adopted the GSM standard for their high speed (4G LTE) networks. That means that LTE equipped phones that are on a CDMA provider are really hybrid devices, using CDMA for communication up through 3G, and GSM for LTE data transfers (hence the SIM card).

Summary and the Future

So far we have talked about how GSM and CDMA are like different digital languages, how the radios in phones interpret the signals that are sent and received, and how the two technologies have very different methods of activation and authentication.

Some of this discussion about GSM vs. CDMA carriers will likely become irrelevant in the future. Carriers will eventually move all forms of communication including voice and texts over to LTE. Even today carriers are taking spectrum that was previously used for voice calls and allocating it to LTE data. When the day comes when the CDMA carriers send everything over LTE, they will become GSM providers as LTE is a universal GSM standard.

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