"Radio-phone" just didn't stick
A big secret that the cell phone companies have been keeping from the world is that a cell phone is nothing more than a radio. It is a complex radio, but still a radio. In order to really understand the way a cell phone works, we must discuss some of the cell phones history for just a moment.
Back around the early 1950s', cell phones were really only used in automobiles. But these mobile-radio-phones were about as common as cruise control in post World War II cars. They were literally like driving around with an entire telephone company in one's car. And to make things worse, they only worked in cities.
In select urban areas, there were large, central antennas that were specifically allocated for these radio-phones. Each car that had a radio-phone required a big antenna that could transmit at least 40 or 50 miles. Since radio technology itself was only in the building phase, only about 25 channels were available for private use. So basically only 25 people could be talking on their radio-phones at the same time.
And in cities like New York and San Francisco, this was a problem. For there were more than just 25 people who had radio-phones in their cars.
The Cell Approach
The solution to this problem was to divide each city up into small divisions, or "cells". The technology behind cells have changed dramatically over the years, just as cell phones have, but now most standard cells are about 10 square miles large. They are usually in the shape of a hexagon. Nowadays, every individual cell has its own base station, rather than only one for an entire city.
And now cell phones are made to be low-power transmitters (either 0.3 watts or 6 watts), which is much lower wattage than in past decades. This means that the same frequency can be used in the same city, at the same time, but in different cells.
Look at the diagram below to get a visual picture of how city is divided into cells. The yellow cells are transmitting at the same frequency.
Think of it like an ice cube tray. The cell phone transmitting towers don't spill their transmissions that far out of their own cells. They may spill slightly into the most adjacent cells, but not into cells more than one cell away. Usually each separate carrier, (Verizon, AT&T, Cingular, etc) have their own control office in each major urban area called the Mobile Telephone Switching Office (MTSO). This is where they control their respective towers. This office also connects all of the cell-phone calls to the land-line phones.
But what happens when one moves from cell to cell?
Relax, this was all taken into consideration. Each modern cell phone (meaning it was created in the last 20 years or so) has special codes programmed into them.
The most important code is the system identification code or the (SID). It's a five digit code that the FCC gives to each different cell phone carrier. When a cell phone is turned on, whether it's making a call or not, it's picking up the SID that is being transmitted from the closest cell phone tower. The phone's personal carrier is also transmitting its SID to the phone on specific channels that the phone programmed to listen for.
But if it can't pick any of them up, the dreaded "NO SERVICE" message appears on the display. However, when the carrier can pick up the phone, you're in luck. It's connected!
Once You're Connected
Now that the phone is connected, calls can be sent and received. In addition to the SID being sent back and forth between phones and towers, there is a registration signal that is also being sent. This is so the carrier's MTSO always knows where its customers are, should someone dial their phone. When someone calls a cell phone, the MTSO finds where the phone is at and connects to it by finding a common frequency, in the cell, that the phone is in. It verifies the SID number and then, your girlfriend can finally ask you "What are you doing?"
When in Roaming
When a cell phone is not found on its carrier's MTSO, it may still be close enough to a different carrier's tower that can use the same channels (most of them can). The cell phone realizes that it is connecting to a different SID (meaning its carrier has no towers in the area or that none of them are on local network) that means the phone is, gulp, roaming. But this is most important when you are moving from cell to cell, such as when you are ridding in car. The tower notices that your signal is dying as you move to the boarder of its cell. In the same instance the tower in the cell you are traveling to realizes that the signal is getting stronger. With a little help from the MTSO, the two towers switch call to a different frequency in just milliseconds. And VOLIA, you can keep talking! In the Cell phone business this is called a hand-off.
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Copyright © 2004 Robert D. Keith All rights reserved