Journal: News & Comment

Friday, May 17, 2002
# 12:45:00 AM:

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Why do they call them cell phones?

Hey Jo, at the end of a long blog, asks: "By the way, why do they call them cel phones anyway? Other countries refer to them as 'mobiles', which is a much better descriptor. Cellular, digital, whatever -- they're mobile."

[Diagram from University of Washington]They're called cell(ular) phones because the radio network they work on is broken up into cells (often represented as interlocking honeycombs in diagrams, but roughly round on flat ground in reality), and that makes them different from other kinds of mobile phones. Each transmitter antenna on a building or tower is the base station, or hub, at the centre of a cell. A city has hundreds or thousands of these invisible cells mapped across it.

The reason the network is set up that way is simple: there's only so much radio spectrum (so many "channels") to go around, and breaking a given land area into small cells means:
  1. that the individual base stations can be relatively low power, since they don't have to send their signals very far, and

  2. that base stations a few cells apart can reuse the same radio frequencies for different calls, permitting tens or hundreds of thousands of simultaneous transmissions across a city using only a few hundred different radio channels. (That explains why phone companies have trouble at the Olympics or other big stadium sporting events, when so many people are packed into an area covered by only one or two cells.)

If you're moving (in a bus or car, or walking), your cell phone moves between cells as you talk, and your phone and the transmitters on the network seamlessly "hand you off" from cell to cell, even switching radio channels on the fly, in order to keep the conversation connected, sort of like a trapeze artist grabbing a new rope before letting go of the old one.

Here's a pretty good explanation of how cellular phones work, with some excellent diagrams, such as the one above.

Before cell phones, there were mobile phones, but they were expensive and limited, because there were only one or two big transmitters in a city, so only a few dozen people could possibly use that kind of phone at the same time. Plus they were single-duplex -- only one side of the conversation could talk at a time, like CB radio, so you had to say "over" and such to end a statement.

So, calling them cellular phones serves a useful distinction: they are a particular type of mobile phone using a cellular-style radio network. But no one sees much of the other style anymore, unless you're on a boat and use marine radiophones. For that matter, in theory, cordless phones are mobile too, just not very far, while satellite phones are super-mobile but super-expensive.

Finally, digital and analog cellular phones use the same kinds of networks, but with different technology. They're both cellular (and there are a few different kinds of digital service). Analog is more expensive, has slightly better sound quality when reception is good, and is more widespread because it's been around longer. Digital is cheaper and more flexible. Some phones do both, so you can use them in areas without digital coverage.

Okay, Derek's spent too much time contracting for a wireless modem company.


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