How cell phones work

Today's cell phones are amazingly compact, complex devices that provide a wide array of services. Discover the origins of these technological wonders, and the technology that makes them work, in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:00):
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(00:30):
Why is a cell phone called a cell phone? One
of the most interesting things about a cell phone is
that it's really a radio. It's really an advanced walkie talkie,
if you want to think about it. Before cell phones,
people who needed mobile communications installed radio telephones in their car.
In the radio telephone system, there was one big central

(00:52):
antenna per city and maybe a handful of channels that
were available on that antenna. You had to put this big,
high powered radio in your car. And when I say big,
I mean like it filled half your trunk. And it
had to be big because if you were twenty miles
away from that central tower, you had to transmit at
thirty watts or something. You had a radio station in

(01:14):
your car to use this system. The idea of a
cell phone changed all that completely. The cellular phone system
divides this city into a bunch of small cells. This
allows extensive frequency reuse across the city, so that millions
of people can use cell phones simultaneously. Here's how it works.

(01:35):
The carrier chops up an area such as the city,
into these cells. Each cell is typically sized at about
ten square miles perhaps three miles by three miles. Cells
are normally thought of as hexagons on a big hexagonal grid.
Each cell has a base station that consists of a
tower and a small building containing the radio equipment. Cell

(01:57):
phones have low power transmitters in m and the base
station is also transmitting at low power. Low power transmitters
have two advantages. First, the power consumption of the cell phone,
which is normally battery operated, is relatively low. Low power
means small batteries, and this is what has made handheld
cellular phones possible. Second, the transmissions of the base station

(02:21):
and the phones within its cell do not make it
very far outside that cell. Therefore, the same frequencies can
be reused extensively across the city. The cellular approach requires
a large number of base stations in a city of
any size. A typical large city can have hundreds of towers,
but because so many people are using cell phones, costs

(02:44):
remain fairly low per user. Each carrier in each city
also runs one central office called the Mobile Telephone Swishing
Office or MTSO. This office handles all the phone connections
to the normal land base phone system and controls all
the base stations in the region. As you move towards

(03:05):
the edge of your cell, so you're in a car
and you're driving away from the tower for your cell,
your cells base station will note that your signal strength
is diminishing. Meanwhile, the base station in the cell you're
moving toward, which is listening and measuring the signal strength
on all frequencies, not just its own one seventh of

(03:25):
the frequency spread, we'll be able to see that your
phone signal strength is increasing. The two base stations coordinate
themselves through the mt s O, and at some point
your phone gets a signal on a control channel telling
it to change frequencies. This handoff switches your phone to
the new cell, and you don't even know it's happening.

(03:46):
In most cases, as you're driving down Interstate Highway, for example,
your phone might switch towers every five minutes, and that's
completely invisible to someone who's making an hour long call.
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(04:07):
online shopping destination for dudes. Everything on the side is
up to eight off, All styles are curated, so buyer's
remorse just doesn't happen. What's more is, if your brain stuff,
you can skip the membership wait list and get instant
access at jack threads dot com. Slash stuff