"Hi, I'd like a large barbecue chicken pizza with extra—*fzzt* oni *asjkhwakj* no *kssshh*" CALL FAILED. What just happened here? With all of today's modern wireless technology, why do mobile phones still pull this crap?
For all the miraculous things we're able to do with phones now—tell 600 Twitter followers unpleasantly intimate details of our lives, for instance—it's amazing what's still missing: Universally excellent reception. Without enough bars, your phone becomes a shiny, useless brick. We've already explained how cell towers basically work. Now, we're gonna talk about how the invisible fairies who carry your voice and data between the cell site and your Moto RAZR actually do their jobs.
Okay, there are no fairies. Everything is actually carried on radio waves—ultra high frequency (UHF, as in the Weird Al movie) radio waves, to be precise. UHF refers to any frequencies between 300MHz and 3GHz, so Wi-Fi, your mom's cordless phone, your lame Bluetooth headset and other stuff all run on the broad UHF band. The thing about radio waves is that they're pretty easy to screw with, and UHF is no exception, despite the fact it has "ultra" in the name. Maybe if we had like, Chuck Norris Frequency, things would be different.
The Interference You're walking around with a glorified walkie talkie. It's emitting radio waves, and trying to catch others that come from a tower. So a huge reason that you sometimes get a signal weaker than bodega coffee is stuff getting in the way. And the farther away from the cell tower your are, the more likely it is that stuff is gonna get in your way, even if the radio waves are strong enough to reach you.
Since the frequencies for cell service essentially travel in a straight line, you're screwed if you roll behind a big hill or building. Big obstacles are obviously trouble, but little obstacles cause huge problems too. Different materials have different effects on the radio waves, since they are subject to things like reflection and absorption. A building with lots of reflective metal on the outside is gonna have a crummy signal inside. Conductive materials have a tendency to absorb and weaken, (or "attenuate") the signal. (This is why you can't forge a phone completely out of aluminium.) Plants, while friendly to the earth, are not friendly to cell signals since they absorb the signal.
The Frequency It also depends on what frequency you're rolling on. Today, Verizon and AT&T use 850 and 1900MHz. T-Mobile uses the 1700, 1900 and 2100MHz bands, mostly 1900. Nextel's iDEN network uses 900MHz, while Sprint's main network runs on 1900MHz with roaming on 800MHz. The Sprint/Clearwire WiMax network is higher up, at 2500MHz, aka 2.5Ghz.
The reason the 700MHz is such hot property for AT&T and Verizon's upcoming high-speed 4G LTE deployment is that lower-frequency signals travel farther and allegedly penetrate some materials better using the same power as a signal on a higher frequency. This is why LTE is suitable for rural broadband deployments.
(Significantly higher frequencies actually do penetrate certain things better at close range and with more power, so this whole discussion can quickly turn into a headache factory if you let it.)
AT&T is currently shifting a lot of their 3G to 850MHz for better penetration after everyone with an iPhone cried about their crappy reception. So being on a lower frequency potentially poses a better chance to have a more solid signal.
Needless to say, the more noisy a particular frequency, the more trouble calls can encounter as well, so carriers have to manage their bandwidth like freeway designers, to avoid the congestion that also contributes to dropped calls.
The Handoff Handoffs (aka handovers) are another reason your order for a pizza with extra cheese might turn into one for extra grease, especially if you're flying down the highway at 90km/h and your phone is wirelessly bouncing from cell tower to cell tower. All kinds of wonky things, like ping-ponging between the two towers, can happen that'll result in a dropped call, especially if it's between two stations that aren't in line of sight.
The Backhaul Okay, you say, but I have full bars goddammit. Explain that. Well, for one, full bars doesn't necessarily mean anything. There's no industry yardstick that translates the bars into actual information, but they all basically represent averages of signal strength over small chunks of time. But just because the signal strength is good, that doesn't mean the call's going through.
For starters, there are only a finite number of calls a cell tower can handle, which varies depending on the demands of the phone or wireless modem. As calls come in, they are juggled by the tower, then routed through a wired connection (or occasionally a powerful wireless connection) to the greater telecom infrastructure. This is called backhaul, and it can be a bottleneck.
Assuming you're all good, with a fantastic signal and a lock on the tower, but still, your Yelp reviews aren't flowing like they should. Don't forget, at some point, like any dial-up or cable modem, your cell data requests have to compete with everything else on the internet. Even calls are so digitalised these days, their "packets" of data can be interrupted by unknown internetty forces.
The final lesson there? The fatter the hard pipe, the more data it can handle—fibre is the best, obviously, but most towers still use an array of T1 lines. And that bottleneck, as Om pointed out last year, could be mobile broadband's biggest roadblock. Assuming you get past all those other roadblocks.