How Which !

Part Two : Cell Phone Technologies

Many people use their Cell Phone everyday, talking, browsering, listerning, watching ... But how do cellular phones connect to other phones, and how the cell phones work?

Maybe you have noticed “cell phone towers” appearing in many locations. When you make a call through your cell phone, your cell phone will link to the cellphone network via the nearby cell phone tower. Then, the cell phone tower send the signals to seek the phone you dialed. After the one you called answered your phone, the cell phone network connect both sides by a wireless channel, to transfer your voice or digital data.

In the mid-1940s, the first mobile wireless phone services appeared in the United States, using one tower in each metropolitan area for it was expensive and there were only a few users. But the demand for mobile phone services growed rapidly. With the cell phone technology improving, cell phones seems morw and more smaller and cheaper. In the 1960s engineers developed the "cellular" technology what is now been used on cellular phone services.

It sounds reasonable to use a single centrally-located antenna to provide cell phone services. But the radio frequency (RF) spectrum seems too limited for its great number of cell phone users. Then the cell phone regulatory agencies (in the United States, the Federal Communications Commission or FCC), provides a limited range of RF spectrum available to cell phone services. These regulations limits the maximum number of simultaneous phone calls to mere thousands in each city —- far below the expected demand , and forced cell phone providers to adopt the cellular approach, where multiple cell phone towers are sited to cover a large geographic area.

In the cellular system each tower —- or Base Station -— covers a roughly circular area called a cell, splitting a large region into some smaller aeras. Then different base stations can use the same frequencies, or channels, for communication links as long as a sufficient distance separates them, known as frequency re-use. This technique allows thousands or even hundreds of thousands of cell phone users in a metropolitan area to share far fewer channels. When you make a call, the system may assign a channel to you that is already in use by dozens of other people. At the same time, the other subscribers are assigned to other cells.

The other advantage of the cellular concept is that as each base station covering a smaller area, phones need less transmit power to reach a base station. It is important for reducing the size of the battery and the weight of the phone. The improvements in miniaturization have been so astonishing —- few people using today’s palm-sized phones are aware of the “bag” or “briefcase phones” of the early and mid-1980s.

But what happens when you make a call and then keep talking as you move about a large area and leave your original cell? The cell phone network automatically keeps track of the strength of the signal from your phone at multiple base stations. As you move, the base station receiving the strongest signal changes, and the network “hands off” your call from one base station to another.