1 00:00:00,520 --> 00:00:03,400 Speaker 1: Welcome to brain Stuff from how stuff Works dot com 2 00:00:03,600 --> 00:00:14,440 Speaker 1: where smart Happens. Hi Am Marshall Brain with today's question, 3 00:00:14,760 --> 00:00:18,360 Speaker 1: how does a cell phone work in your pocket? You 4 00:00:18,400 --> 00:00:21,840 Speaker 1: have an amazing device. It's a communicator that allows you 5 00:00:21,880 --> 00:00:25,400 Speaker 1: to talk instantly to almost anyone on the planet. Plus 6 00:00:25,400 --> 00:00:27,920 Speaker 1: it may also give you instant stock quotes, help you 7 00:00:27,960 --> 00:00:31,080 Speaker 1: read your email, and let you play games. Your cell 8 00:00:31,120 --> 00:00:34,200 Speaker 1: phone truly is an achievement. But how does it do it? 9 00:00:34,800 --> 00:00:38,080 Speaker 1: A little history can help here. Remember when you were 10 00:00:38,120 --> 00:00:40,720 Speaker 1: a kid and you played with a walkie talkie. A 11 00:00:40,800 --> 00:00:44,400 Speaker 1: walkie talkie is an incredibly simple toy that can send 12 00:00:44,400 --> 00:00:46,600 Speaker 1: your voice to a friend who's a quarter of a 13 00:00:46,680 --> 00:00:49,920 Speaker 1: mile away or so. It does this with about twenty 14 00:00:50,000 --> 00:00:53,800 Speaker 1: five cents worth of parts. The parts take your voice, 15 00:00:54,360 --> 00:00:57,440 Speaker 1: change it from sound waves to electric waves, and then 16 00:00:57,600 --> 00:01:02,240 Speaker 1: amidst those electronic waves with the antenna at the other end, 17 00:01:02,400 --> 00:01:06,319 Speaker 1: your friends walkie talkie receives the waves and converts them 18 00:01:06,360 --> 00:01:10,600 Speaker 1: to sound waves with the speaker your friend, here's your voice, 19 00:01:10,680 --> 00:01:13,720 Speaker 1: and vice versa. What if you want to transmit for 20 00:01:13,880 --> 00:01:16,880 Speaker 1: more than a quarter of a mile, you increase the 21 00:01:17,000 --> 00:01:20,880 Speaker 1: power A toy walkie talkie transmits at about a tenth 22 00:01:20,880 --> 00:01:25,080 Speaker 1: of a watch. A CB radio, which can transmit several miles, 23 00:01:25,160 --> 00:01:28,760 Speaker 1: uses four watts. Before there were cell phones, there were 24 00:01:28,920 --> 00:01:33,520 Speaker 1: car radios, which were essentially very very large walkie talkies 25 00:01:33,640 --> 00:01:37,000 Speaker 1: using twenty five watts or more. These things were huge. 26 00:01:37,440 --> 00:01:41,160 Speaker 1: But you can't stick at wat radio in your pocket. 27 00:01:41,520 --> 00:01:45,199 Speaker 1: It would need a gigantic battery. So how did engineers 28 00:01:45,240 --> 00:01:48,400 Speaker 1: solve the problem and create the tiny cell phones we 29 00:01:48,480 --> 00:01:52,480 Speaker 1: have today. The genius of the cell phone system is 30 00:01:52,520 --> 00:01:56,160 Speaker 1: the idea of a cell At the time it was conceived, 31 00:01:56,200 --> 00:02:00,840 Speaker 1: the idea was incredibly audacious. The idea for poses that 32 00:02:01,040 --> 00:02:04,760 Speaker 1: cell phone companies would put up cell phone towers, which 33 00:02:04,840 --> 00:02:07,560 Speaker 1: cost on the order of a million dollars each, on 34 00:02:07,680 --> 00:02:11,880 Speaker 1: a grid approximately four miles apart. A large city would 35 00:02:11,960 --> 00:02:15,320 Speaker 1: need dozens and dozens of these towers. But if you 36 00:02:15,400 --> 00:02:18,280 Speaker 1: put up all those towers, it means that the radio 37 00:02:18,320 --> 00:02:21,400 Speaker 1: and the cell phone only has to transmit at most 38 00:02:21,720 --> 00:02:24,960 Speaker 1: about two miles or so. The radio doesn't have to 39 00:02:25,000 --> 00:02:28,080 Speaker 1: be much more powerful than the radio in a walkie talkie, 40 00:02:28,280 --> 00:02:32,760 Speaker 1: which keeps it and its battery requirements small. There was 41 00:02:32,800 --> 00:02:35,480 Speaker 1: another part of the original cell phone idea that was 42 00:02:35,560 --> 00:02:39,840 Speaker 1: also audacious. A normal Wakie talkie can only transmit on 43 00:02:39,840 --> 00:02:44,440 Speaker 1: one frequency. Cell phones can transmit on hundreds of frequencies. 44 00:02:44,760 --> 00:02:49,079 Speaker 1: That means that one tower can handle dozens of simultaneous 45 00:02:49,120 --> 00:02:53,720 Speaker 1: calls without people interfering with each other. Each cell phone 46 00:02:53,840 --> 00:02:57,000 Speaker 1: uses a different frequency to talk to the tower, so 47 00:02:57,040 --> 00:03:00,120 Speaker 1: the different people can't hear each other, and each cell 48 00:03:00,160 --> 00:03:04,280 Speaker 1: phone actually uses too frequencies simultaneously for a call, so 49 00:03:04,440 --> 00:03:08,200 Speaker 1: you and your friend can talk simultaneously without the walkie 50 00:03:08,240 --> 00:03:12,120 Speaker 1: talkies push to talk button. And there is a third 51 00:03:12,240 --> 00:03:15,160 Speaker 1: radio in your phone that's always talking to the tower 52 00:03:15,480 --> 00:03:18,840 Speaker 1: even if you aren't on a call. This radio handles 53 00:03:18,880 --> 00:03:22,840 Speaker 1: the control channel. When a call comes in, the tower 54 00:03:22,960 --> 00:03:25,880 Speaker 1: talks to your phone on the control channel to set 55 00:03:25,880 --> 00:03:29,440 Speaker 1: the call up. The tower tells your phone what frequencies 56 00:03:29,520 --> 00:03:32,120 Speaker 1: do you use for the call, and also sends stuff 57 00:03:32,160 --> 00:03:35,080 Speaker 1: like the caller I D to the phone. The control 58 00:03:35,200 --> 00:03:39,000 Speaker 1: channel also handles one other important feature of cell phones. 59 00:03:39,440 --> 00:03:41,880 Speaker 1: Let's say that you're in your car driving down the 60 00:03:41,960 --> 00:03:45,600 Speaker 1: road while talking on the phone. Your phone is connected 61 00:03:45,600 --> 00:03:49,160 Speaker 1: to one tower, but it's driving away from it. As 62 00:03:49,160 --> 00:03:52,680 Speaker 1: you get closer to the next tower, the towers communicate 63 00:03:52,760 --> 00:03:56,080 Speaker 1: and arrange a handoff your phone gets a message on 64 00:03:56,120 --> 00:03:59,600 Speaker 1: the control channel telling it to change frequencies as your 65 00:03:59,680 --> 00:04:03,720 Speaker 1: driveing along. Your phone might switch between ten different towers 66 00:04:03,760 --> 00:04:06,000 Speaker 1: on a single call and you don't even know it. 67 00:04:06,760 --> 00:04:10,640 Speaker 1: The original cell phones were called analog cell phones. That 68 00:04:10,720 --> 00:04:14,720 Speaker 1: was the first generation. These were replaced by digital cell 69 00:04:14,760 --> 00:04:18,200 Speaker 1: phones because several digital calls can fit in the space 70 00:04:18,240 --> 00:04:22,680 Speaker 1: of one analog call. Digital cell phones were the second generation. 71 00:04:23,200 --> 00:04:26,560 Speaker 1: Now we're in the third generation, or three G, where 72 00:04:26,560 --> 00:04:29,599 Speaker 1: the towers and phones can handle calls as well as 73 00:04:29,680 --> 00:04:33,480 Speaker 1: high speed data connections. The data connection allows a phone 74 00:04:33,480 --> 00:04:37,839 Speaker 1: to browse web pages, get stock quotes, read email, watch videos, 75 00:04:37,880 --> 00:04:41,040 Speaker 1: and so on. When you think about it, it's truly 76 00:04:41,080 --> 00:04:44,000 Speaker 1: amazing that in your pocket you can have a mini 77 00:04:44,160 --> 00:04:48,720 Speaker 1: laptop computer that's always connected to the Internet, combined with 78 00:04:48,839 --> 00:04:53,640 Speaker 1: the ability to talk to anyone in the world. Be 79 00:04:53,720 --> 00:04:56,360 Speaker 1: sure to check out our new video podcast, Stuff from 80 00:04:56,360 --> 00:04:59,200 Speaker 1: the Future. Join how Stuff Work staff as we explore 81 00:04:59,240 --> 00:05:04,599 Speaker 1: the most promising and perplexing possibilities of tomorrow. The Houstufforks 82 00:05:04,680 --> 00:05:07,919 Speaker 1: iPhone app has arrived. Download it today on iTunes.