What is CDMA and How Does it Work?

Adam Fendelman is a syndicated technology writer and senior web designer whose focus was on web analytics and web design among other things.

Michael Heine is a CompTIA-certified writer, editor, and Network Engineer with 25+ years' experience working in the television, defense, ISP, telecommunications, and education industries.

CDMA, which stands for Code Division Multiple Access, is a competing cell phone service technology to GSM on older networks that are gradually phasing out. In 2010, carriers worldwide switched to LTE, a 4G network that supports simultaneous voice and data use.

You've probably heard of CDMA and GSM when you were told that you can't use a certain phone on your mobile network because they're using different technologies that are not compatible with each other. For example, you may have an AT&T phone that can't be used on Verizon's network for this very reason or vice versa.

The CDMA standard was originally designed by Qualcomm in the U.S. and is primarily used in the U.S. and portions of Asia by other carriers.

Which Networks Are CDMA?

Of the five most popular mobile networks in the United States, Sprint, Verizon and Virgin Mobile use CDMA. T-Mobile and AT&T use GSM.

How CDMA Works

CDMA uses a “spread-spectrum” technique whereby electromagnetic energy is spread to allow for a signal with a wider bandwidth. This approach allows several people on different cell phones to be “multiplexed” over the same channel to share a bandwidth of frequencies. With CDMA technology, data and voice packets are separated using codes and then transmitted using a wide frequency range. Since more space is often allocated for data with CDMA, this standard became attractive for high-speed mobile Internet use.

CDMA vs. GSM

Most people don't need to worry about which cell phone network they choose in terms of which technology is better. However, the two standards do diverge in important technical ways.

CDMA Coverage

While CDMA and GSM compete head-on in terms of higher bandwidth speed, GSM offers more complete global coverage thanks to roaming and international-roaming contracts. GSM technology tends to cover rural areas in the U.S. more completely than CDMA.

Device Compatibility and SIM Cards

It's easy to swap phones on a GSM network because GSM phones use removable SIM cards to store information about the user on the GSM network, while CDMA phones do not. Instead, CDMA networks use information on the carrier's server to verify the same type of data that GSM phones have stored in their SIM cards.

This data-storage design means that the SIM cards on GSM networks are interchangeable. For example, if you're on the AT&T network, and therefore have an AT&T SIM card in your phone, you can remove it and put it into a different GSM phone, like a T-Mobile phone, to transfer all your subscription information over, including your phone number.

What this effectively does is lets you use a T-Mobile phone on the AT&T network.

An easy transition is typically not possible with most CDMA phones, even if they do have removable SIM cards. Instead, you typically need your carrier's permission to perform such a swap.

Since GSM and CDMA are incompatible with one another, you can't use a Sprint phone on a T-Mobile network, or a Verizon Wireless phone with AT&T. The same goes for any other mix of device and carrier that you can make out of the CDMA and GSM list from above.

CDMA phones that use SIM cards do so either because the LTE standard requires it or because the phone has a SIM slot to accept foreign GSM networks. Those carriers, however, still use CDMA technology to store subscriber information.

Simultaneous Voice and Data Usage

Most CDMA networks do not allow voice and data transmissions at the same time. This is why you may get bombarded with emails and other Internet notifications when you end a call from a CDMA network like Verizon. The data is basically on pause while you're on a phone call.

However, you'll notice that two-way data exchange works just fine on a CDMA network when you're on a phone call within range of a Wi-Fi network because Wi-Fi, by definition, isn't using the carrier's network.

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