All over the world, people use and communicate with cell phones. Cell phones range in sizes from as small as a thumb, to as large as a regular telephone if needed. Cell phones can flip, fold, compact and store just about anything. But how do they work?
Imagine...A Radio?
In a sense, a cell phone is just like a radio. It picks up signals from towers But if you wonder how the cell phone works and what makes it different from a regular household phone, read below as we go through the basics.
Each cell phone has a cellular system. When we say a cellular system, it means a division of a city into small cells. Each cell has a base station that consists of a tower and a small building containing the radio equipment this allows widespread frequency reuse across a city, so that millions of people can use cell phones concurrently. Each cell is typically sized at and covers about a 10 square miles radius.
There is a requirement to have a large number of base stations in a city of any size to make cell phone use function conveniently. A typical large city can have hundreds of towers placed in certain regions to cover most of the areas completely. Central offices called the Mobile Telephone Switching Office (MTSO) handles all of the phone connections to the normal land-based phone system, and controls all of the base stations in the region. Each carrier in each city runs one.
Special Codes - All Cell Phones Have 'Em
All cell phones have special codes related with them. These codes are used to identify the phone's owner, phone, and the service provider that they use. Here is what happens when you use your phone:
When a person first turns on their phone, it listens for a System Identification Code (SID) on the control channel. An SID is a unique 5-digit number that is assigned to each carrier by the FCC. This is a unique frequency that the phone and base station use to send signals to another about things like call set-up and channel changing. If the phone can't find any control channels to listen to, then it's out of range and will display on the phone a "no service" message.
When it receives the SID, the phone matches up to the SID programmed into the phone. If the SIDs match, the phone realizes that the cell it is corresponding with is part of its home system.
The phone also transmits a registration request, along with the SID and the MTSO keeps track of your phone's location in a database -- this way it is known what cell you are in when it wants to ring your phone.
The MTSO gets the call that is calling you and it tries to find you by looking in its system to see which cell you are in. The call is sent to you at that time. You are now talking by two-way radio to a friend!
As you travel and move near the end of your cell, your cell's base station sees that your signal strength is diminishing. In the meantime, the base station in the cell you are moving closer to sees your phone's signal strength increasing. The two base stations coordinate with each other through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies. This hand off switches your phone to the new cell with out interruption to you and your call. As you travel, the signal is passed from cell to cell. And the amazing thing is that all of this happens within seconds!
Small devices, amazing technology. Who would have thought something so small could be so powerful and complex? All of the above is transparent to a customer, which makes a cell phone even more enjoyable!
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