When you strip it all away -- the e-mail and texting, the voice navigation, and Angry Birds -- 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+, 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 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 U.S. 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 U.S., 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.
Samsung's Galaxy S3 is a perfect example, since it was first introduced globally, without LTE, then later released with LTE on five U.S. 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 people 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.
"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.
Samsung's Tim Rowden, vice president of product management, agrees. "One by one you fix each of those issues," he told me during a call.
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.
Let's think again about that Samsung Galaxy S3. The global version is GSM, so you'll be limited to using it on AT&T and T-Mobile. It doesn't have an LTE radio, so you'll miss out on those fastest 4G speeds.
In fact, its radio goes up to HSPA+ 21, which is only half the theoretical speed of the fastest HSPA+ 42 network.
But there's more. The global version of the GS3 supports HSPA+21 on the 850/900/1,900/2,100 bands, but T-Mobile uses the 1,700MHz and 2,100MHz bands for 4G HSPA+.
Although T-Mo plans to release 4G on 1900 band by the year's end, for now, the band mismatch means slow speeds. Of course, Samsung and T-Mobile worked together on the U.S. version of the GS3 built specifically for the Magenta network. The "optimized" variant in question fully rides T-Mobile's network, doing so with a slightly different processor-and-radio chipset combination than the LTE-ready versions of the same phone.
Synchronization secrets, roaming, and more
In addition to the network speed limitations you may see, bringing an unlocked phone to a locked network could get in the way of global roaming, said Samsung's Rowden.
Your unlocked phone could be left out of domestic roaming agreements, too, which means leaving a carrier's home coverage area cuts off your network connection completely.
The handset's radio frequency, or RF, also plays a major role, Rowden describes. A phone with sensitive RF transmission helps control how fast the browser opens, a standard that AT&T's Rudolph also says his carrier stipulates.
Battery life is dependent on device and software efficiency, but also on the network. Rowden explains that to save battery 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.
"You have a tremendous amount of synchronization," Rowden said. "It sounds very simple, but when you think about milliseconds, and the device going to sleep...if it doesn't wake up at the right time, it'll miss a message from the network."
Here's an example you wouldn't want to experience.
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, you'll lose data every time your data speed shifts. This very specific example is handset validation at work, and another area where "unoptimized" phones could stumble.
Here are even more examples of what you might get with uncertified phones that aren't "optimized": dropped calls, slow or lost data connections, and a spottier signal depending on the radio placement inside.
Your Bluetooth device 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.
The bottom line is that on top of the basics, each network has its own rules. Verizon and Sprint both use CDMA cellular technology, but they implement it differently. If a handset isn't attuned to a network's particulars, it could still work fine most of the time; however, there's no guarantee, and there's nobody to shoulder the responsibility when and if you hit a bump.
Smartphones Unlocked is a monthly column that dives deep into the inner workings of your trusty smartphone.