When you strip it all away -- the e-mail and texting, the voice navigation, and Flappy Bird -- your smartphone isn't a smartphone at all. It's a radio.
The job of a radio is to detect, receive, and maintain the signal that leashes the cell phone to the network. When you turn on your phone, it chirps out data that lets the network know another cell phone has hopped on board.
The network authenticates the phone, and the phone is free to send and receive data or calls. In other words, more fundamentally than software and the processor, it's those radios that bring the cell phone its spark of life.
Like all things in tech, connecting a hunk of plastic and metal to an invisible network is easier said than done.
First, there are the practical issues to consider: Is this a GSM or CDMA network; is it LTE, HSPA+, or 3G -- or all three? Which radio frequency bands will the phone use to draw network resources?
Then there are the detailed technical issues, the standards and specifications that the FCC requires all handset makers to meet.
Follow this with the carrier's own to-do list, supported by weeks of testing and tweaking to get each handset just right, and you start getting a sense of what it takes to get a new phone up and running on an established carrier.
Not optimized, validated
On my side of the industry, we often use the word "optimize" to describe the relationship between a phone and its network.
More often than not, the term takes on a negative tone, warning consumers when a phone is not optimized; when, although you can use it, it was never intended for U.S. markets. As in, "This phone may not be as fast as you want; it hasn't been optimized for a US carrier."
On the other side of the industry -- that space occupied by wireless carriers and handset makers -- "optimize" doesn't regularly enter into daily parlance. Instead, "validation" is the word most often tossed around.
Getting the stamp of approval
In the US, where relationships between carriers and device makers are particularly strong, the two engage in a long validation and certification period to ensure that a phone is compatible with the network in question.
2012's Samsung Galaxy S3 was a perfect example of this since it was first introduced globally, without LTE, then later released with LTE on five US carriers. Samsung needed to be sure as shooting that its phone would work consistently well on all five networks, and this is where the complexity comes in.
Each variation of the Galaxy S3 had to first operate within the industry standard for the major puzzle pieces: the FCC guidelines, then GSM or CDMA standards, LTE specifications, and others.
AT&T, for instance, is part of the 3GPP (3rd Generation Partnership Project) for LTE, GSM, and UMTS, so the phone had to meet those requirements before AT&T's own testing, a final hurdle for the Galaxy S3 and any other handset on a release schedule.
Each carrier also issues a list of conformance and performance tests to make sure that the fundamental capabilities work according to spec -- can you place a call and send a text? Do apps behave as expected? How quickly does data flow under different scenarios for each megahertz band?
Field testing is also involved, with real live humans checking for app and user experience defects. You know all those third-party apps preinstalled on your device? A chunk of the testing process centers around making sure those apps work correctly, according to Lee Rudolph, AT&T's area vice president of subscriber product engineering.
The entire testing and vetting process can take weeks. "We find lots and lots of issues with every phone we test," Rudolph told CNET in an interview.
Ask Rudolph and he'll tell you that keeping phones from freezing and hanging is one of the hardest things to get right. "One of the areas we really look at is the stability of the device, or resets. How often does it freeze or get hung?" he said.
Interesting phones you can't buy from a US carrier (pictures) See full gallery
"We have an iterative process with the OEMs where we give them feedback," Rudolph continued.
Underpinning the testing efforts are a bug system and procedures for charting test results, videos, logs, and screenshots.
Speed bumps and blocking bands
Even though the vast majority of phones are already "optimized," seeing what you lose when bringing an unlocked phone onto the network is another way of showing what it takes to make individual devices and the network play nice.
LTE is on the rise around the world, but even when global phones support the standard, they may not work equally fast in every country. Just as voice and 3G data travel over different frequencies, so does LTE.
If your network in question doesn't support the same bands as the phone, you'll have to accept slower data speeds, or plan to connect to a lot of Wi-Fi.
Synchronization secrets, roaming, and more
A lot of synchronization goes into play between the network and the phone, and if this intricate dance skips a beat, your experience ever so slightly suffers.
Let's take roaming, for example. In addition to the network speed limitations you may see, the quality you get when roaming isn't guaranteed. An unlocked device could be left out of carriers' domestic agreements, which means leaving a carrier's home coverage area could cut off your network connection completely.
The handset's radio frequency, or RF, also plays a major role. A phone with sensitive RF transmission helps control how fast the browser opens, a standard that carriers enforce when approving phones for their networks.
Then there's the question of battery life, which is dependent on the efficiency of both the device and the software that runs it, but also on the network. In order to save their batteries incrementally, and on a small scale, devices go to sleep for milliseconds at a time. But they must also constantly wake up to check the network for incoming calls, messages, and other events.
Let's say your phone is downshifting from a LTE data connection to a 3G connection. If the timing of these sleep cycles is off, as they could be with nonvalidated phones, you'll lose data every time your data speed jumps a level. Uncertified phones are also more susceptible to dropped calls, slow or lost data connections, and a spottier signal depending on the placement of the radio inside.
Your Bluetooth connection may not work well, either, and battery life could take a hit as the unlocked phone spins its wheels, inefficiently searching for voice and data networks.
Skip global phones altogether?
The point of this discussion isn't to scare you away from buying an unlocked phone originally intended for another market. Go ahead, be intrepid!
For the most part, sticking a SIM card into an unlocked smartphone will work just fine. You may notice that you're not getting blazing LTE speeds, that call quality isn't quite as good as you'd like, or any of a dozen other little bobbles. Or, more likely, you won't notice terribly much at all. Do be mindful of the compatibility between the phone's bands and what your carrier uses; those data speeds and voice bands will affect you the most.
There's a big world out there of devices we typically don't see within our domestic borders. But if you do experience bizarre connection issues, you're largely on your own. There's no carrier to shoulder the responsibility if you hit a bump.
Smartphones Unlocked is a monthly column that dives deep into the inner workings of your trusty smartphone.