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About This QuizIn line at the corner deli, weaving through traffic, right next to you at a movie theater ... cell phones are everywhere. But are they really that different from walkie-talkies?
A cell phone is basically a sophisticated:
When you get down to it, even the fanciest phone, with every conceivable bell and whistle, is really just a souped-up two-way radio.
The cellular phone gets is name from:
the division of a geographic area into cells
You might imagine the cell phone network as blanket of radio waves spread over a large region. It's really more like a quilt made of small, hexagonal areas called cells. Each cell has its own tower and base station.
cellular membrane technology
the grouping of different frequencies into cells
Unlike CB radios, cell phones use two frequencies, one for talking and the other for listening. This is:
In full-duplex communication, you and the person at the other end of the line can talk simultaneously. In half-duplex communication, it's one at a time -- if you try to talk over your chatty neighbor, he won't hear you.
The range served by each cell phone base station is about:
5 square miles (13 square kilometers)
10 square miles (26 square kilometers)
The signals from each tower don't travel far beyond their range of 10 square miles. This means that cellular providers can use the same frequencies over and over in different cells, so more people can talk at the same time.
25 square miles (65 square kilometers)
How do cell phones let you keep talking as you move in and out of different cells?
Your phone's transmitter sets up relays to get your signal to your initial location.
The Mobile Telephone Switching Office coordinates when your phone changes frequencies from one tower to another.
Imagine the process of moving a call from tower to tower as handing a beach ball from one person to another. If it all goes well, the MTSO lifts the ball from one hand and places it in another. If it doesn't, the MTSO drops the ball -- and your call.
Your phone sends a stronger signal to reach the tower you were using when you started your call.
Your cell phone knows when you're roaming because:
The Mobile Telephone Switching Office (MTSO) checks your SID code in its database.
The MTSO isn't Big Brother -- it doesn't necessarily have any idea where you are. It does know whether your SID matches the one in its database. If it doesn't, the MTSO checks in with your home system to make sure your SID is real. Then, it lets you go on talking -- but at roaming rates.
The global positioning system (GPS) chip in your phone informs the cell towers of your location.
The cell towers triangulate your position, and your phone's processor compares it to your service area.
How do digital phones let more people within a cell talk at once?
They transfer data quicker, so the frequencies aren't clogged.
They convert voices into binary code, compressing the calls.
If you're talking on a digital phone over a digital network, your voice becomes a stream of 1s and 0s. These 1s and 0s take up a lot less room than an analog signal, which is a constantly varying wave of information.
They use higher frequencies than analog phones.
Cell phone cloning involves:
buying a phone and using someone else's account information to set it up
retrieving ID numbers from a phone and using them to eavesdrop or make fraudulent calls
You may have heard of cell phone cloning on TV shows like "The Wire." To clone a phone, someone -- either police or a criminal -- has to have your phone's identification information.
transferring a SIM chip from an old phone to a new phone
Cell phones use lower-power radio signals because:
The signals don't travel very far.
Sending a signal doesn't drain the battery.
both A and B
It may seem like a far-reaching signal would be better for carrying on a conversation, but a cell phone's signal needs to go only as far as the nearest station. A high-powered signal would mean no battery life and interference with faraway phone calls.
A locked phone:
can work with only one specific provider's service
When the Apple iPhone hit the market, it was a locked phone -- it worked only with AT&T service. But inventive users quickly found hacks that could bypass the lock, making their phones usable on other networks.
can work only in a limited range within a network
has no SIM card
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