When T-Mobile dropped contract requirements in 2013, it shook up the wireless world. The other carriers that make up the “big four” – AT&T, Sprint, and Verizon – eventually followed suit. As a result, smartphone manufacturers began making their “unlocked” wares available for purchase, bringing an entirely new level of freedom to smartphone vets and would-be smartphone owners. While this big change may have simplified things financially, it has also made things a bit murkier for discerning customers who need to know: Which carrier will meet my needs the best, and which one should I choose?

We’ve got the answers.

Coverage Technology

Digging deep into the various carrier offerings, you’ll find terms you may be familiar with (LTE) and others you might only have heard of in passing, if at all (CDMA, GSM, HSPA+, and VoLTE).

GSM vs CDMA

GSM (Global System for Mobiles) and CDMA (Code Division Multiple Access) refer to the two primary technologies used by wireless carriers throughout the world to facilitate 3G and 2G networking. GSM and CDMA are incompatible with one another, which explains why in the past you couldn’t take a smartphone manufactured for AT&T and just jump over to Verizon – not without first making hardware changes.

While there are technical differences in how each handles network traffic, for consumers the difference boils down to the fact that GSM-based smartphones store customer ID data in SIM cards. This is why you’re able to pop your SIM card out of a GSM smartphone and pop it into another GSM smartphone (provided it’s unlocked) and get straight to business. Things are different with CDMA, which stores customer plan data within the smartphone. Carrier politics also play a role, as GSM is a more open standard, developed by an industry association, whereas today’s CDMA technology was developed by Qualcomm. CDMA carriers have traditionally been restrictive, “whitelisting” only certain devices, but that’s changing for the better.

Most CDMA-based phones these days include SIM cards, but this is only to facilitate the requirements of 4G LTE.

GSM is the predominant technology used throughout the world, and especially in Europe. Most international carriers use GSM technology, although there are some holdouts (including two of the major U.S. carriers) who remain CDMA-based. Here’s how the four main carriers are split along the GSM/CDMA divide within the U.S.

• AT&T: GSM
• T-Mobile: GSM
• Sprint: CDMA
• Verizon: CDMA

4G LTE

LTE (short for Long-Term Evolution and frequently marketed as 4G LTE) is now the current network technology, and is considered the next evolutionary step from both the GSM and CDMA standards. It’s widely considered the fourth generation of consumer networking technology, hence the “4G” designation. LTE is the protocol over which data is transferred, and at the present time is the fastest available data transfer platform for consumers. It’s also fast becoming the dominant technology. Over the past few years carriers have been phasing out older 3G networks in favor of LTE. Some analysts estimate that the move away from 3G could be complete as early as 2018.

VoLTE stands for Voice Over LTE, and is yet another step ahead in the technology that allows for telephone communication to be broadcast over LTE. Historically, voice conversation has been based on much older 2G technology. VoLTE also enables for more high quality voice calls (which is why it’s sometimes referred to as “HD Voice”), in addition to quicker call connections, Wi-Fi calling integration (more on that below) and what industry types call Rich Communications Services. This includes things like video calling and real-time translation built directly into a smartphone’s native dialing app, rather than a third-party app like Skype.

VoLTE will also help hasten the demise of the old GSM and CDMA dichotomy as all carriers will be using LTE for both data and voice, making it much easier to change carriers and keep the same smartphone. That said, LTE is segmented by bands, with each carrier supporting a handful with only a little overlap. We’ll explain that further in a future installment.

WiMAX & HSPA+

WiMAX and HSPA+ are two older “4G” technologies. Sprint launched WiMAX as the first fourth-generation network in 2010, about a year before Verizon launched the first LTE network in the US. It’s since been abandoned in favor of Sprint’s LTE network. T-Mobile and AT&T were marketing their HSPA+ networks as 4G prior to their LTE networks, though it was actually an extension of their 3G networks capable of speeds approaching LTE levels in actual use. HSPA+ is still available, though is being phased out in favor of LTE.

Wi-Fi Calling and Texting

Wi-Fi calling and texting has been possible for a long time, leveraging third party apps like Skype to serve as an alternative to communicating via cellular signal, but never through the traditional smartphone dialer and messaging apps.

This is in such high demand that now the carriers are starting to offer their own Wi-Fi calling and texting capabilities. The convenience is clear: with carrier-offered Wi-Fi calling and texting, you can use your own phone number instead of having to launch a third-party app. This enables calls and texts to be made in areas where there may be limited or zero cellular coverage. Additionally, cellular calls that lose signal can automatically toggle over to Wi-Fi to ensure a continued connection, as long as there’s an open signal available.

T-Mobile was the first to offer it, but AT&T and Sprint have followed in suit, albeit in a more limited capacity. Verizon is the only carrier that still hasn’t caught up to speed. Wi-Fi calling and texting isn’t available on all phones offered by the three participating carriers, but the list is growing. Be sure to check with the carrier of your choice to determine which phones support it.

Speed Ratings

Researching the estimated download and upload speeds for the various carriers can be a grueling exercise. Pulling facts and figures from any of the four main carriers is hard, as many of them appear to under-promise, while others (like T-Mobile) only declare best-case scenarios. According to data provided by the carriers themselves, here are the average 4G LTE download and upload speeds you can expect.

Average 4G LTE Download/Upload Speeds (in Mbps)

• AT&T: 12 Mbps (download) / 5 Mbps (upload)
• Sprint: 6-8 Mbps (download) / 2-4 Mbps (upload) Sprint claims peak download speeds can reach up to 25 Mbps, and peak upload speeds can reach 10 Mbps.
• T-Mobile: 21.7 Mbps (download) / 12 Mbps (upload)
• Verizon: 5-12 Mbps (download) / 2-5 Mbps (upload). Peak download speeds can “approach” 50 Mbps.

Again, some of these averages can (and should be) taken with a grain of salt, such as the T-Mobile claim which appears to dwarf the other carriers.

A recent independent report by OpenSignal determined the average 4G download speed in the U.S. is 9.9 Mbps. The report also ranked how the major carriers performed with respect to download speeds at 3G and 4G. These are the results.

3G

• AT&T: 2.2 Mbps
• Sprint: 0.64 Mbps
• T-Mobile: 3.5 Mbps
• Verizon: 0.66 Mbps

4G

• AT&T: 7.93 Mbps
• Sprint: 6.56 Mbps
• T-Mobile: 12.26 Mbps
• Verizon: 11.98 Mbps

It’s important to note that 4G coverage uptime plays heavily into what you can expect to get around the clock. This is represented by a percentage of time customers of a given carrier actually have access to 4G speeds. In OpenSignal tests, Verizon came out on top with 87 percent uptime; AT&T rated 83 percent; T-Mobile was third, with 81 percent; and once again Sprint placed last with 70 percent 4G coverage uptime.

What it All Means

Your carrier’s download speeds, as measured in Mbps, determines whether or not you’ll be able to do things like stream high quality video without buffering or experiencing hiccups in performance. The greater the Mbps, the greater your speed. Netflix has specific requirements that your device should meet in order to stream its data without interruption: for SD quality, you need at least 3 Mbps; HD requires 5 Mbps. The Amazon Prime Instant Video requirement for SD is 900 Kbps, and HD is 3.5 Mbps.

This leads to the issue of data limits. To put things into perspective, streaming a 90-minute movie in SD on Netflix will eat up approximately 1.5 GB of data. In HD, that same movie can devour 4.5GB of data. Data usage when streaming music varies, and depends on the quality of the stream. If you stream music at 320kbps, you can burn through some 115MB in an hour. Spread that out over an entire month, and it adds up. Some services, like Pandora, stream at lower quality bit-rates, which makes it possible to get away with more for less – but if you’ve got T-Mobile, you can take advantage of their Music Freedom and Binge On features, which let you stream music and video over certain platforms without eating into your data plan.

Coverage Maps

An accurate determination of which cellular carrier has the “best” coverage depends greatly on where you live. But with respect to pure numbers and greatest overall coverage, Verizon ranks first in the U.S. T-Mobile ranks second; AT&T ranks third; and Sprint comes in fourth.

Want to learn more about buying a smartphone? Read all about no-contract smartphone data plans and pricing.