WEBVTT - How cell phones work

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<v Speaker 1>slash stuff. Welcome to Brainstuff from house stuff works dot

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<v Speaker 1>com where smart Happens. Hi, I'm Marston Brain with today's question.

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<v Speaker 1>Why is a cell phone called a cell phone? One

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<v Speaker 1>of the most interesting things about a cell phone is

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<v Speaker 1>that it's really a radio. It's really an advanced walkie talkie,

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<v Speaker 1>if you want to think about it. Before cell phones,

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<v Speaker 1>people who needed mobile communications installed radio telephones in their car.

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<v Speaker 1>In the radio telephone system, there was one big central

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<v Speaker 1>antenna per city and maybe a handful of channels that

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<v Speaker 1>were available on that antenna. You had to put this big,

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<v Speaker 1>high powered radio in your car. And when I say big,

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<v Speaker 1>I mean like it filled half your trunk. And it

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<v Speaker 1>had to be big because if you were twenty miles

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<v Speaker 1>away from that central tower, you had to transmit at

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<v Speaker 1>thirty watts or something. You had a radio station in

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<v Speaker 1>your car to use this system. The idea of a

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<v Speaker 1>cell phone changed all that completely. The cellular phone system

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<v Speaker 1>divides this city into a bunch of small cells. This

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<v Speaker 1>allows extensive frequency reuse across the city, so that millions

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<v Speaker 1>of people can use cell phones simultaneously. Here's how it works.

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<v Speaker 1>The carrier chops up an area such as the city,

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<v Speaker 1>into these cells. Each cell is typically sized at about

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<v Speaker 1>ten square miles perhaps three miles by three miles. Cells

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<v Speaker 1>are normally thought of as hexagons on a big hexagonal grid.

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<v Speaker 1>Each cell has a base station that consists of a

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<v Speaker 1>tower and a small building containing the radio equipment. Cell

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<v Speaker 1>phones have low power transmitters in m and the base

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<v Speaker 1>station is also transmitting at low power. Low power transmitters

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<v Speaker 1>have two advantages. First, the power consumption of the cell phone,

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<v Speaker 1>which is normally battery operated, is relatively low. Low power

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<v Speaker 1>means small batteries, and this is what has made handheld

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<v Speaker 1>cellular phones possible. Second, the transmissions of the base station

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<v Speaker 1>and the phones within its cell do not make it

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<v Speaker 1>very far outside that cell. Therefore, the same frequencies can

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<v Speaker 1>be reused extensively across the city. The cellular approach requires

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<v Speaker 1>a large number of base stations in a city of

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<v Speaker 1>any size. A typical large city can have hundreds of towers,

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<v Speaker 1>but because so many people are using cell phones, costs

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<v Speaker 1>remain fairly low per user. Each carrier in each city

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<v Speaker 1>also runs one central office called the Mobile Telephone Swishing

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<v Speaker 1>Office or MTSO. This office handles all the phone connections

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<v Speaker 1>to the normal land base phone system and controls all

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<v Speaker 1>the base stations in the region. As you move towards

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<v Speaker 1>the edge of your cell, so you're in a car

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<v Speaker 1>and you're driving away from the tower for your cell,

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<v Speaker 1>your cells base station will note that your signal strength

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<v Speaker 1>is diminishing. Meanwhile, the base station in the cell you're

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<v Speaker 1>moving toward, which is listening and measuring the signal strength

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<v Speaker 1>on all frequencies, not just its own one seventh of

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<v Speaker 1>the frequency spread, we'll be able to see that your

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<v Speaker 1>phone signal strength is increasing. The two base stations coordinate

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<v Speaker 1>themselves through the mt s O, and at some point

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<v Speaker 1>your phone gets a signal on a control channel telling

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<v Speaker 1>it to change frequencies. This handoff switches your phone to

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<v Speaker 1>the new cell, and you don't even know it's happening.

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<v Speaker 1>In most cases, as you're driving down Interstate Highway, for example,

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<v Speaker 1>your phone might switch towers every five minutes, and that's

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<v Speaker 1>completely invisible to someone who's making an hour long call.

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<v Speaker 1>For more on this and thousands of other topics, is

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