Understanding the difference between Code Division Multiple Access (CDMA) and Global System for Mobiles (GSM) — two legacy radio telecommunications standards — is critical for consumers buying a new smartphone or switching carriers. International travelers should take special note of the differences to avoid getting stuck with a phone that does not work somewhere they have to be. Both CDMA and GSM convert data from your smartphone into radio waves. Today, they represent increasingly deprecated 2G and 3G radio systems based on competing technologies.

What is GSM?

GSM is the standard system for most of the world. Some 80% to 90% of the world uses GSM technology for wireless calls, with notable exceptions being the United States and Russia. GSM networks use Time Division Multiple Access (TDMA), which assigns time slots to multiple conversation streams, alternating them in sequence and switching between each conversation in very short intervals. During these intervals, phones can transmit information. In order for the network to know which users are connected to the network, each phone uses a subscriber identification module (SIM) card.

SIM cards are one of the key features of GSM networks. They house your service subscription, network identification, and address book information. The cards also assign time slots to the phone conversation and tell the network what services you can access. They can even be used to pass information between phones if a carrier allows it.

What is CDMA?

CDMA is often found in the U.S. and Russia, alongside GSM. This technology was originally developed by Allied forces during World War II, primarily as a method to prevent the Nazis from jamming radio signals. Unlike GSM, CDMA grants users full access to the entire spectrum of bands, thus allowing more users to connect at any given time. It also encodes each user’s individual conversation via a pseudo-randomized digital sequence to protect voice data and keep it private.

Phones on CDMA networks do not have to use SIM cards because each phone is built specifically to work on that carrier’s network. For consumers, this once meant that phones were tied to a carrier and their bands, so if you wanted to switch providers, you’d have to buy a new phone. That’s not always the case now, thanks to enhanced network technology and since CDMA carriers routinely use SIM cards to service LTE networks.

Which one is better?

Both CDMA and GSM are the global standards for cell phone communication, and one is not inherently better than the other. Keep the following in mind when you have to choose between CDMA and GSM phones, though.

CDMA phones without SIM slots are forever hitched to their carriers and cannot be transferred to other networks. A Verizon phone cannot be transferred to Sprint’s network, for instance, or vice versa. Sometimes it’s not that straightforward, though. Although some Verizon devices do use CDMA, they also have an unlocked SIM slot for use on other networks. You can often find ways to get your carrier to unlock your phone when you’re done with their contract.

In contrast, GSM phones are fairly easy to unlock and transfer between networks. Third-party manufacturers often sell phones designed for GSM networks since they don’t require access to specific carrier bands. GSM phones can even work in countries with compatible GSM networks and offer wider international roaming. GSM also covers rural areas more completely than CDMA in the U.S.

CDMA networks allow for a greater number of users, meaning their capacity is greater than that of GSM networks. Moreover, CDMA is the infrastructure on which all 3G networks are based — for both GSM and CDMA carriers. However, LTE is fast becoming ubiquitous. LTE technology represents an evolved form of GSM and uses similar technology to GSM networks. The standard boasts enhanced voice quality and functions as the base of high-speed, 4G data networks. In this case, LTE does have the edge over the competition in terms of overall speed and quality. The next step to offer further improvements is 5G, which is rolling out now. As mobile network technology evolves, the CDMA versus GSM split becomes less relevant.

What do carriers use?

For U.S. cell phone customers, AT&T and T-Mobile are GSM carriers, while Verizon, Sprint, and US Cellular are CDMA, taking into account updates in carrier policies and plans to close off networks. The services, features, phones, and service quality a network offers aren’t solely dependent on their infrastructure, so it’s best to go with the carrier that suits your tastes, needs, and budget. If you’re buying a new phone, you’ll want to make sure that it’s going to work with your network, or choose a network and phone that suits your needs, geographical location, and travel lifestyle.

The sunset years

The era of CDMA versus GSM is gradually drawing to a close. Verizon long ago announced plans to shut down its CDMA network, and T-Mobile will switch off its 2G GSM network by the end of 2020. With the recent T-Mobile-Sprint merger, the combined company will also likely sunset Sprint’s old CDMA network. There is, however, some good news for customers that stick to old school ancillary devices—T-Mobile and AT&T will maintain their 3G networks until early 2022.

The world has now largely converged around 4G LTE cell phone technology, a transformation that was, until recently, halting and incomplete because of the need to connect with older networks for voice calling. Today, 4G is still the standard for most carriers, at least those in the US, but 5G is the next big step.

Even if it comes with higher speeds and super-low latency, there are still plenty of kinks to work out with 5G, especially when it comes to radio. Channels may differ, and disparate carrier device permissions may obstruct genuine universal compatibility because carriers can still operate on different radio channels — one carrier’s phone may not include channels used by other carriers. However, most smartphones, even lesser-known brands like ZTE or Nubia, now support LTE networks from major carriers.

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