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Speaker 1: slash stuff. Welcome to Brainstuff from house stuff works dot
Speaker 1: com where smart Happens. Hi, I'm Marston Brain with today's question.
Speaker 1: Why is a cell phone called a cell phone? One
Speaker 1: of the most interesting things about a cell phone is
Speaker 1: that it's really a radio. It's really an advanced walkie talkie,
Speaker 1: if you want to think about it. Before cell phones,
Speaker 1: people who needed mobile communications installed radio telephones in their car.
Speaker 1: In the radio telephone system, there was one big central
Speaker 1: antenna per city and maybe a handful of channels that
Speaker 1: were available on that antenna. You had to put this big,
Speaker 1: high powered radio in your car. And when I say big,
Speaker 1: I mean like it filled half your trunk. And it
Speaker 1: had to be big because if you were twenty miles
Speaker 1: away from that central tower, you had to transmit at
Speaker 1: thirty watts or something. You had a radio station in
Speaker 1: your car to use this system. The idea of a
Speaker 1: cell phone changed all that completely. The cellular phone system
Speaker 1: divides this city into a bunch of small cells. This
Speaker 1: allows extensive frequency reuse across the city, so that millions
Speaker 1: of people can use cell phones simultaneously. Here's how it works.
Speaker 1: The carrier chops up an area such as the city,
Speaker 1: into these cells. Each cell is typically sized at about
Speaker 1: ten square miles perhaps three miles by three miles. Cells
Speaker 1: are normally thought of as hexagons on a big hexagonal grid.
Speaker 1: Each cell has a base station that consists of a
Speaker 1: tower and a small building containing the radio equipment. Cell
Speaker 1: phones have low power transmitters in m and the base
Speaker 1: station is also transmitting at low power. Low power transmitters
Speaker 1: have two advantages. First, the power consumption of the cell phone,
Speaker 1: which is normally battery operated, is relatively low. Low power
Speaker 1: means small batteries, and this is what has made handheld
Speaker 1: cellular phones possible. Second, the transmissions of the base station
Speaker 1: and the phones within its cell do not make it
Speaker 1: very far outside that cell. Therefore, the same frequencies can
Speaker 1: be reused extensively across the city. The cellular approach requires
Speaker 1: a large number of base stations in a city of
Speaker 1: any size. A typical large city can have hundreds of towers,
Speaker 1: but because so many people are using cell phones, costs
Speaker 1: remain fairly low per user. Each carrier in each city
Speaker 1: also runs one central office called the Mobile Telephone Swishing
Speaker 1: Office or MTSO. This office handles all the phone connections
Speaker 1: to the normal land base phone system and controls all
Speaker 1: the base stations in the region. As you move towards
Speaker 1: the edge of your cell, so you're in a car
Speaker 1: and you're driving away from the tower for your cell,
Speaker 1: your cells base station will note that your signal strength
Speaker 1: is diminishing. Meanwhile, the base station in the cell you're
Speaker 1: moving toward, which is listening and measuring the signal strength
Speaker 1: on all frequencies, not just its own one seventh of
Speaker 1: the frequency spread, we'll be able to see that your
Speaker 1: phone signal strength is increasing. The two base stations coordinate
Speaker 1: themselves through the mt s O, and at some point
Speaker 1: your phone gets a signal on a control channel telling
Speaker 1: it to change frequencies. This handoff switches your phone to
Speaker 1: the new cell, and you don't even know it's happening.
Speaker 1: In most cases, as you're driving down Interstate Highway, for example,
Speaker 1: your phone might switch towers every five minutes, and that's
Speaker 1: completely invisible to someone who's making an hour long call.
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