Giz Explains: Why Your Call Dropped [Giz Explains]

The call just died, and for no apparent reason. You were just walking down the street, for God’s sake. So, let’s discuss it: What happened?

To you, this case was simple, and nothing really changed: You walked a number of feet and your phone stopped working. On your phone, though, the scene was rather a lot more interesting.

To understand the arena as seen by a cellphone, it helps to imagine a large grid. On this grid, there are cell towers, some tall and conspicuous, others hidden. These towers each carry calls placed within a definite radius. In an open, rural environment, this radius is mostly a few kilometers. In a city, it may be well under 500 yards.

These areas of coverage generally overlap, so that there’s nowhere a phone can also be where it doesn’t have a tower to chat to. A phone keeps track of these cells, as they’re called, and notes what number are strong enough to put a decision on. When one fades, in theory, the phone could have another to which it could hand off the call.

But these areas aren’t a similar size as one another, or maybe a consistent size. They fluctuate wildly, by reason of a phenomenon call ” cell breathing.”

On on the subject of any 3G network, carriers transmit voice signals with CDMA, or code division multiple access. (Yep, this includes HSPA 3G, that is sometimes called GSM.) What this suggests is that multiple phones can transmit over an analogous radio frequencies, and their signals are differentiated by code. (Disclaimer: it is a brutal simplification.) As one network engineer told me, sharing a cell tower is like sharing a room with a group of folks that speak different languages. Different people can hold concurrent conversations, but everyone can understand what they must-their brains block out anything else 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 quantity starts to rise. The more everyone speaks, the louder one has to speak to be understood. Likewise, the more those that are using a cell tower, the more power each phone should be ” heard” by the tower. This actually leads to a contraction of the cell’s coverage area.

In other words, the more people using a tower without delay, the less its range. Cell breathing actually explains a couple of frustrating scenarios. The five-bar call drop, as an instance, 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 might possibly 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 could cause dropped calls, nevertheless it’s also something carriers are well privy to, and might plan for-generally, they’ve. There still shouldn’t be that many gaps in coverage, and in a populated area, your phone will usually have a minimum of another active cell to fall back to.

So what was it?

Think back to that grid, with the entire overlapping cell towers’ coverage areas. They’re different diameters, based on their individual powers, tower heights and locations. They’re expanding and contracting based in keeping with what number individuals are using them at a given time. But they’re also all shaped differently, because any coverage area-be it in a virtually empty rural area or a dense city-has traits so that it will upset an electromagnetic field.

As a network engineer explained to me, in an urban environment peculiarly these cells’ coverage areas assume weird shapes, thanks to reflection and refraction. A city-or anywhere where humans live, really-is a hostile, or at the very least action-packed, place for radio frequency communications. To your street, thick and varied buildings, built from concrete and steel and laced with wires and current, redraw the bounds of a cell’s coverage, pulling it out of form and filling it with pockets and weak spots. So while that grid of cells in theory leaves no spot uncovered, in point of fact these vibrating fields of coverage have strange shapes which are difficult to calculate, and subject to constant change.

So maybe it was that. But wait-you made a choice in this area yesterday, and another several days before. Your phone works here, usually, and you may’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 an absence of coverage, as far as you will find.

So, again, what was it?

Well, maybe it was that bus that drove by. Or among the cars in traffic. Or a kind of old-looking power tools at the construction site you walked past. Or that dude who brushed by you on the sidewalk. Or you.

Electromagnetic fields are fickle things, and interference can come from almost anywhere. Nearly any form of electronic device will probably be an electromagnetic emitter, from another cellphone to a car to a few decrepit old power tool, spitting unintended frequencies as it slowly grinds itself to death. Granted, most emitters don’t share frequencies with modern cellphones-legally, they’re not allowed to-however it still happens.

Worse, though, is that while most objects on your surroundings don’t emit radio frequencies, nearly any object can affect how they reach you. A bus driving by, as an instance, could knock your signal strength down by 50%, just by entering into the best way of your particular transmissions. A human crossing your path at the incorrect angle could do an identical. Sometimes, 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 in case you happen to be on the brink of a cell’s coverage area, and the subsequent 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!

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This post is tagged: code division multiple access, consistent size, different languages, network carriers, network engineer, radio frequencies, walking down the street