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CDMA versus GSM: What’s the difference?

If you’re in the market for a new phone or cell carrier, or you’re interested in cell phone networks, chances are you’ve run into the acronyms CDMA and GSM. You may be wondering what GSM and CDMA are, exactly, and how they affect your phone. Well, we’ve got the answers you seek right here.

Updated on 01-13-2017 by Kyle Wiggers: Updated OpenSignal survey data, added U.S. Cellular frequencies to reference chart, and updated references to the iPhone 7 and Google’s Pixel. 

GSM (Global System for Mobiles) and CDMA (Code Division Multiple Access) are two different radio technologies that perform the same basic function — wirelessly transmit voice, data, and texts from and to your phone — but thanks to a cellular landscape that’s far from cut and dry, each confer benefits and disadvantages that aren’t exactly clear from the outset. Practically speaking, for instance, signing up with a CDMA carrier could mean giving up the ability to talk and text at the same time, while opting for GSM service might land you with a mountain of SIM cards to juggle.

This handy guide aims to help you navigate a few of the most common GSM and CDMA pitfalls. By the end, you should be well enough informed to avoid any unpleasant surprises next time you’re forced to choose between one or the other.

Related: LTE Advanced explained | What is the difference between 4G and LTE?

GSM Explained

The iPhone 6 is a smartphone that typically functions on a GSM network.

The iPhone 6 is a smartphone that typically functions on a GSM network.

In the U.S., T-Mobile and AT&T use GSM.

We’ll get to the technology behind it in a bit, but GSM from a subscriber standpoint is fairly straightforward. Choosing a handset might be the easiest part: GSM carriers are prohibited by the governing body of cellular, the ITU (International Telecommunications Union), from disallowing any GSM phones on their networks. That means as long as a phone is unlocked (more on that in a bit) and supports the right bands (see below), it’ll work on your carrier of choice.

On GSM networks, your service is tied to a tiny, removable module called a SIM card, which stores your address book and phone number. You aren’t tied to a single handset — pop your SIM card into another phone and it’ll download your phone number and import your contacts. Furthermore, switching to a different GSM carrier is, in theory, as straightforward as switching SIM cards. The reality is a bit more complicated — a number of cellular providers arbitrarily “lock” phones to their network via software — but thankfully, U.S. federal law requires that carriers allow subscribers to migrate to other networks upon request (as long as they’ve paid all outstanding fees, of course).

That’s not the only benefit GSM confers. AT&T and T-Mobile’s implementation of one of GSM’s newer revisions, HSPA+, is typically quite zippy, with downloads averaging around 11Mbps (the speed of an average cable connection). And all GSM networks support simultaneous voice and data — you can talk and surf the web at the same time.

CDMA Explained

The Droid Turbo is a smartphone that runs on Verizon's CDMA network.

The Droid Turbo is a smartphone that runs on Verizon’s CDMA network.

CMDA networks, generally speaking, are more restrictive than their GSM equivalents. CDMA phones don’t use SIMs, instead relying on a server-based whitelist of subscriber numbers to verify your plan. That of course means that you don’t have to worry about losing service if you misplace your SIM card, but also grants CDMA carriers a greater degree of control. You can switch phones only with your carrier’s permission, for example, and even a phone compatible with a CDMA carrier’s network won’t work until it’s been approved by said carrier.

CDMA networks have a few technical advantages over GSM. They support a greater number of concurrent connections, which theoretically means less congestion (i.e. fewer dropped calls and Internet slowdowns) at cell towers. And the underlying technology is more secure. But CDMA is inferior in other ways, namely speed —  the most widely adopted variant in the U.S. — EV-DO, or Evolution Data Optimized — maxes out at 3.6Mbps, while modern GSM networks can theoretically hit 42Mbps. And CDMA networks lack some features, like simultaneous voice and data, that are a part of the GSM standard.

Bands

If there’s one big limitation that CDMA and GSM share, though, it’s compatibility. Carriers reserve different frequencies — “bands” — in order to transmit data, texts, and voice to and from phones without interference. But, while some carriers share bands, others do not — a GSM phone with specific band compatibility won’t work on a GSM network that doesn’t leverage the same frequencies.

Carrier Network Bands Frequencies (MHz)
Verizon CDMA 0, 1 850, 1900
AT&T GSM 2, 5 850, 1900
T-Mobile GSM 2, 4 1700/2100, 1900
Sprint CDMA 1, 10 800, 1900
US Cellular CDMA 0, 1 850, 1900

If your phone is only partially compatible with another carrier, you might have voice and texting capabilities, but no data. Alternatively, you might get Internet, but at slower speeds than fully compatible phones.

LTE, the great equalizer… or not

There’s a third type of network to consider.

Appropriately dubbed “Long-Term Evolution” (LTE for short), it boasts a litany of improvements over older CDMA and GSM technologies. Data upload and downloads are substantially faster. Capacity is higher. And call quality, thanks to a new data-based standard (Voice over LTE, or VoLTE), is vastly improved.

But LTE faces many of the same challenges as CDMA and GSM. LTE networks across carriers use different frequencies — Sprint, T-Mobile, Verizon, and AT&T all have LTE bands of their own. LTE also doesn’t rely on one variation uniformly — some networks, like Sprint, use a version of LTE (TD-LTE) with relatively low compatibility.

Carrier Bands (LTE) Frequencies (LTE)
Verizon 2, 4, 13 700 c, 1700 f, 1900
AT&T 2, 4, 17 700 bc, 1700 abcde, 1900
T-Mobile 2, 4, 12 700 a, 1700 def, 1900
Sprint 25, 26, 41 850, 1900 g, 2500
US Cellular 5, 12 850, 700 ab

To make matters worse, some networks continue to rely on legacy technologies while they transition to LTE. Verizon, for example, still uses its CDMA network to handle phone calls.

It isn’t all doom and gloom, though. Verizon pledged to shut down its CDMA voice network in favor of VoLTE by the end of 2016. T-Mobile has begun shutting off HSPA+ service as it transitions to higher-speed alternatives. And a handful of phones, including the iPhone 7 and iPhone 7 Plus and Google’s Pixel, support every LTE band and standard.

Technology

Older GSM networks used TDMA (Time Division Multiple Access), which works by assigning time slots to multiple conversation streams, alternating them in sequence, and switching between each conversation at very short intervals. But most carriers today use WCDMA (wideband CDMA), a far speedier technology.

The aforementioned SIM card is how GSM carriers know when you’re connected to the network. It’s also how the network assigns time slots to the phone conversation, and moreover tells the network what services you have access to.

The technology underlying CDMA, or Code Division Multiple Access, was developed by the Allied Forces during World War II as a method to prevent Nazi forces from jamming radio signals. It, unlike GSM, allows 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, meaning the voice data remains protected and filtered, so that only those participating in the phone call receive the data.

LTE is based upon CDMA technology, but mandates the use of MIMO (multiple in, multiple out) — essentially, multiple connections per device. That not only boosts stability, but dramatically boosts speed — network testing firm OpenSignal’s 2016 survey rates Verizon’s LTE service at an average of 12Mbps, about 10 times faster than the 0.64 CMDA speeds PC World measured in 2012.

Is One Better Than The Other?

If there’s a winner of the three, it’s LTE. It’s faster, more reliable, and the agreed-upon standard toward which a majority of carriers are moving. But if you’re on a budget, live in or frequent an area of the country without LTE, and/or intend to use a cellphone mainly to make calls, older-gen CDMA and GSM networks will suit you perfectly.

Of the two technologies, GSM is the clear winner, at least in the U.S. — it’s speedier and slightly more interoperable. Your decision will partly come down to coverage (check maps like Open Signal to see which networks are within reach), and factors that have nothing to do with networking tech. T-Mobile has its “Un-carrier” benefits. Sprint has cheap family plans. It may sound obvious, but compare features, phones, pricing, and quality before signing a contract or buying service. Unless you have a particular reason for choosing one or the other, go with the carrier that best fits your needs and taste.

When you do make your decision, and it’s time to buy a phone, here’s a good rule of thumb: for GSM carriers, pick up a phone compatible with HSPA+, the newest generation of GSM. If CDMA signals are all you can get, make sure you buy a phone that supports the fastest variant — EVDO Rev A.

Article originally published 8-8-2015.