To you, this situation was simple, and nothing really changed: You walked a few feet and your phone stopped working. To your phone, though, the scene was quite a bit more interesting.
To understand the world as seen by a mobile phone, it helps to imagine a massive grid. On this grid, there are cell towers, some tall and conspicuous, others hidden. These towers each carry calls placed within a certain radius. In an open, rural environment, this radius can be a few kilometres. In a city, it can be well under 500m.
These circles of coverage generally overlap, so that there's nowhere a phone can be where it doesn't have a tower to talk to. A phone keeps track of these cells, as they're called, and notes how many are strong enough to place a call on. When one fades, in theory, the phone will have another to which it can hand off the call.
But these circles aren't the same size as one another, or even a consistent size. They fluctuate wildly, due to a phenomenon called "cell breathing".
On just about any 3G network, carriers transmit voice signals with CDMA, or code division multiple access. (Yep, this includes HSPA 3G, which is often referred to as GSM.) What this means is that multiple phones can transmit over the same radio frequencies, and their signals are differentiated by code. (Disclaimer: this is a brutal simplification.) As one network engineer told me, sharing a cell tower is like sharing a room with a bunch of people that speak different languages. Different people can hold concurrent conversations, but everyone can understand what they need to - their brains block out the rest of the conversations, because to them, it's all just gibberish anyway.
Just like in this shared room, though, as a tower gets more crowded, the volume starts to rise. The more everyone speaks, the louder one has to talk to be understood. Likewise, the more people that are using a cell tower, the more power each phone needs to be "heard" by the tower. This actually results in a contraction of the cell's coverage area.
In other words, the more people using a tower at once, the less its range. Cell breathing actually explains a number of frustrating scenarios. The five-bar call drop, for example, can often be attributed to cell breathing. (If a cell is overloaded but you're still within its diminished coverage area, the noise on the phone's operating frequencies can be greater than its signal. Result: CALL FAILED.)
So maybe it was that. Maybe the cell you were on had the breath sucked from it by an influx of callers, and your handset just wasn't prepared with a backup connection.
Or maybe it was something else! Cell breathing can cause dropped calls, but it's also something carriers are well aware of, and can plan for—generally, they have. There still shouldn't be that many gaps in coverage, and in a populated area, your phone will usually have at least one more active cell to fall back to.
So what was it?
Think back to that grid, with all the overlapping cell towers' coverage areas. They're different diameters, based on their individual powers, tower heights and locations. They're expanding and contracting based according to how many people are using them at a given time. But they're also all shaped differently, because any coverage area - be it in a nearly empty rural area or a dense city - has traits that will upset an electromagnetic field.
As a network engineer explained to me, in an urban environment in particular these cells' "circles" assume weird shapes, due to reflection and refraction. A city - or anywhere where humans live, really - is a hostile, or at least action-packed, place for radio frequency communications. On your street, thick and varied buildings, built from concrete and steel and laced with wires and current, redraw the boundaries of a cell's coverage, pulling it out of shape and filling it with pockets and weak spots. So while that grid of cells in theory leaves no spot uncovered, in reality these vibrating fields of coverage have strange shapes that are difficult to calculate, and subject to constant change.
So maybe it was that. But wait - you made a call in this area yesterday, and another a few days before. Your phone works here, usually, and you can't see any recent changes to this little "urban canyon", to borrow the parlance of our cellular technician. Same apartment buildings, same bodega, same pet shop, same road, same sky. No excuse for a lack of coverage, as far as you can see.
So, again, what was it?
Well, maybe it was that bus that drove by. Or one of the cars in traffic. Or one of those old-looking power tools at the construction site you walked past. Or that dude who brushed by you on the footpath. Or you.
Electromagnetic fields are fickle things, and interference can come from almost anywhere. Nearly any kind of electronic device can be an electromagnetic emitter, from another mobile phone to a car to some decrepit old power tool, spitting unintended frequencies as it slowly grinds itself to death. Granted, most emitters don't share frequencies with modern mobile phones - legally, they're not allowed to - but it still happens.
Worse, though, is that while most objects in your surroundings don't emit radio frequencies, nearly any object can affect how they reach you. A bus driving by, for example, could knock your signal strength down by 50 per cent, just by getting in the way of your particular transmissions. A human crossing your path at the wrong angle could do the same. In some cases, a network engineer told me, just turning your head to the side could chop your signal strength by half. It's rare that glancing into a shop window will kill your call, but if you happen to be on the threshold of a cell's coverage area, and the next strongest cell isn't quite close enough to grab onto, it definitely can.
So maybe it was that.
Yeah, it was probably that.
Thanks, anonymous network engineers!