Phones on Wheels—How Do They Work?
IMAGINE being able to place a telephone call to anywhere in the world—Hong Kong, Paris, Melbourne—not while you are at home but while you are traveling down the road in your car! The telecommunications business wants to make a telephone in the car as much an everyday item as the telephone is in many homes today.
Although mobile telephones are not new, three factors have suddenly increased their popularity. For one, competition in the telecommunications industry has driven down the cost of mobile telephone service to a point where more people can afford it.
Also, breakup of the telecommunications monopolies, such as in the United States, and government deregulation have freed local telephone companies to seek new ways for their customers to use phone services more often. One of these ways is to encourage telephones in cars so that calls can be made from previously impractical locations. Telephone companies reason, ‘If a call is easy and convenient to make, more will be made, and so more income for us.’
A third factor is technology itself, which advances at breakneck speed today. One of technology’s key products is the computer, and computers have become the heart of cellular, or mobile, telephone service.
What Is It?
A mobile telephone is simply a two-way radio in the shape of a telephone. In the early days, mobile phones did not even have dials, and if you wanted to telephone someone, you had to wait for a mobile operator to place the call for you. In addition, the limited number of two-way radio channels restricted how many callers could be using their mobile telephones at the same time. Further, the telephone company providing mobile service usually had only one antenna for sending and receiving calls to vehicles all over the city. Therefore, if you were too far away from the central antenna, you could not use your car telephone.
Why are the modern mobile phones called cellular? They are still two-way radios—now with dials or push buttons—but what makes these mobile phones different from past ones is that there are many more antenna sites. Since each area covered by an antenna is called a cell, the name cellular is used.
Besides, there are more antenna sites for cellular mobile phones, so each antenna can operate on many channels, or frequencies, and can cover a hundred square miles (260 sq km). What does all of this mean to you if you have a mobile phone? Usually, you will not have to wait to place your call because each cell can accommodate as many as 40 or 50 simultaneous mobile telephone calls over a much greater area.
How Does It Work?
When you pick up your cellular telephone to make a call, a signal is sent out and picked up by the nearest cell-site antenna, which relays your request to the cellular-telephone switching office. A computer at the office recognizes your approximate location and assigns your telephone call to a two-way radio channel in that cell site, which will receive your voice and transmit the voice of the called party to you. Switching between transmitting and receiving is automatic, so the effect is much like talking from your home telephone.
What happens if you drive out of the range of your cell antenna while on call? Well, the computer in the central switching office keeps track of your radio signal while you are on the telephone. If your signal gets weak because of your location, the computer will automatically assign your telephone to another channel in a new cell site closer to your present location. It is all done automatically—in about a quarter of a second—so you can keep talking, almost without noticing the change, or handoff, as it is called.
Cellular telephones are not limited to cars. Some country clubs have cellular telephones in their golf carts, so that busy executives are never very far from work. Phones are also showing up in city buses, taxicabs, and ferryboats. Companies are already marketing portable phones that can be carried in briefcases, so that calls can be made and received while you are walking down the street.
Cellular telephones are popular now throughout Europe, North America, and in industrialized nations around the globe. The more mobile a society becomes, the more people on the move are going to want to keep in touch with friends and business contacts. Hence, we are likely to see even more uses for phones on wheels.
[Diagram on page 27]
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The area covered by one antenna is a cell. Your call is received by the cell antenna and routed by the central switching station through the local phone system
Central switching station