Speaker 1: Welcome to brain Stuff from how stuff Works dot com
Speaker 1: where smart Happens. Hi Am Marshall Brain with today's question,
Speaker 1: how does a cell phone work in your pocket? You
Speaker 1: have an amazing device. It's a communicator that allows you
Speaker 1: to talk instantly to almost anyone on the planet. Plus
Speaker 1: it may also give you instant stock quotes, help you
Speaker 1: read your email, and let you play games. Your cell
Speaker 1: phone truly is an achievement. But how does it do it?
Speaker 1: A little history can help here. Remember when you were
Speaker 1: a kid and you played with a walkie talkie. A
Speaker 1: walkie talkie is an incredibly simple toy that can send
Speaker 1: your voice to a friend who's a quarter of a
Speaker 1: mile away or so. It does this with about twenty
Speaker 1: five cents worth of parts. The parts take your voice,
Speaker 1: change it from sound waves to electric waves, and then
Speaker 1: amidst those electronic waves with the antenna at the other end,
Speaker 1: your friends walkie talkie receives the waves and converts them
Speaker 1: to sound waves with the speaker your friend, here's your voice,
Speaker 1: and vice versa. What if you want to transmit for
Speaker 1: more than a quarter of a mile, you increase the
Speaker 1: power A toy walkie talkie transmits at about a tenth
Speaker 1: of a watch. A CB radio, which can transmit several miles,
Speaker 1: uses four watts. Before there were cell phones, there were
Speaker 1: car radios, which were essentially very very large walkie talkies
Speaker 1: using twenty five watts or more. These things were huge.
Speaker 1: But you can't stick at wat radio in your pocket.
Speaker 1: It would need a gigantic battery. So how did engineers
Speaker 1: solve the problem and create the tiny cell phones we
Speaker 1: have today. The genius of the cell phone system is
Speaker 1: the idea of a cell At the time it was conceived,
Speaker 1: the idea was incredibly audacious. The idea for poses that
Speaker 1: cell phone companies would put up cell phone towers, which
Speaker 1: cost on the order of a million dollars each, on
Speaker 1: a grid approximately four miles apart. A large city would
Speaker 1: need dozens and dozens of these towers. But if you
Speaker 1: put up all those towers, it means that the radio
Speaker 1: and the cell phone only has to transmit at most
Speaker 1: about two miles or so. The radio doesn't have to
Speaker 1: be much more powerful than the radio in a walkie talkie,
Speaker 1: which keeps it and its battery requirements small. There was
Speaker 1: another part of the original cell phone idea that was
Speaker 1: also audacious. A normal Wakie talkie can only transmit on
Speaker 1: one frequency. Cell phones can transmit on hundreds of frequencies.
Speaker 1: That means that one tower can handle dozens of simultaneous
Speaker 1: calls without people interfering with each other. Each cell phone
Speaker 1: uses a different frequency to talk to the tower, so
Speaker 1: the different people can't hear each other, and each cell
Speaker 1: phone actually uses too frequencies simultaneously for a call, so
Speaker 1: you and your friend can talk simultaneously without the walkie
Speaker 1: talkies push to talk button. And there is a third
Speaker 1: radio in your phone that's always talking to the tower
Speaker 1: even if you aren't on a call. This radio handles
Speaker 1: the control channel. When a call comes in, the tower
Speaker 1: talks to your phone on the control channel to set
Speaker 1: the call up. The tower tells your phone what frequencies
Speaker 1: do you use for the call, and also sends stuff
Speaker 1: like the caller I D to the phone. The control
Speaker 1: channel also handles one other important feature of cell phones.
Speaker 1: Let's say that you're in your car driving down the
Speaker 1: road while talking on the phone. Your phone is connected
Speaker 1: to one tower, but it's driving away from it. As
Speaker 1: you get closer to the next tower, the towers communicate
Speaker 1: and arrange a handoff your phone gets a message on
Speaker 1: the control channel telling it to change frequencies as your
Speaker 1: driveing along. Your phone might switch between ten different towers
Speaker 1: on a single call and you don't even know it.
Speaker 1: The original cell phones were called analog cell phones. That
Speaker 1: was the first generation. These were replaced by digital cell
Speaker 1: phones because several digital calls can fit in the space
Speaker 1: of one analog call. Digital cell phones were the second generation.
Speaker 1: Now we're in the third generation, or three G, where
Speaker 1: the towers and phones can handle calls as well as
Speaker 1: high speed data connections. The data connection allows a phone
Speaker 1: to browse web pages, get stock quotes, read email, watch videos,
Speaker 1: and so on. When you think about it, it's truly
Speaker 1: amazing that in your pocket you can have a mini
Speaker 1: laptop computer that's always connected to the Internet, combined with
Speaker 1: the ability to talk to anyone in the world. Be
Speaker 1: sure to check out our new video podcast, Stuff from
Speaker 1: the Future. Join how Stuff Work staff as we explore
Speaker 1: the most promising and perplexing possibilities of tomorrow. The Houstufforks
Speaker 1: iPhone app has arrived. Download it today on iTunes.