Cellular Communications

Wireless telephone sets are used in a specialized communications system called cellular. Originally, the cellular communications network was used mainly by traveling business people. Nowadays, many people regard cell phones as necessities, and some of them come equipped with extra features such as text messaging, Web browsing, and digital cameras.

How Cellular Systems Work

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In an ideal cellular system, a moving vehicle is always within range of at least one base station
A cell phone looks like a cross between a cordless telephone receiver and a walkie-talkie, but smaller. The unit contains a radio transmitter and receiver combination called a transceiver. Transmission and reception take place on different frequencies, so you can talk and listen at the same time, and easily interrupt the other party if necessary. This capability is known as full duplex.
 
In an ideal cellular network, all the transceivers are always within range of at least one repeater. The repeaters pick up the transmissions from the portable units and retransmit the signals to the telephone network and to other portable units. The region of coverage for any repeater (also known as a base station) is called a cell.
 
When a cell phone is in motion, say in a car or on a boat, the set goes from cell to cell in the network. This situation is shown in above figure. The curved, dashed line is the path of the vehicle. Base stations (dots) transfer access to the cell phone. This is called handoff. The hexagons show the limits of the transmission/reception range for each base station. All the base stations are connected to the regional telephone system. This makes it possible for the user of the portable unit to place calls to, or receive calls from, anyone else in the system, whether those other subscribers have cell phones or regular phones.
 
Older cellular systems occasionally suffer from call loss or breakup when signals are handed off from one repeater to another. This problem has been largely overcome by a technology called codedivision multiple access (CDMA). In CDMA, the repeater coverage zones overlap significantly, but signals do not interfere with each other because every phone set is assigned a unique signal code. Rather than abruptly switching from one base-station zone to the next, the signal goes through a region in which it is sent through more than one base station at a time. This make-before-break scheme gets rid of one of the most annoying problems inherent in cellular communications.
 
In order to use a cellular network, you must purchase or rent a transceiver and pay a monthly fee. The fees vary, depending on the location and the amount of time per month you use the service. When using such a system, it is important to keep in mind that your conversations are not necessarily private. It is easier for unauthorized people to eavesdrop on wireless communications than to intercept wire or cable communications.

Cell Phones and Computers

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A cell phone can be equipped with a modem, allowing portable or mobile access to online computer networks.
You can connect a personal computer (PC) to a cell phone with a portable modem that converts incoming computer data from analog to digital form, and also converts outgoing data from digital to analog form. In this way, you can access the Internet from anywhere within range of a cellular base station. Above figure is a block diagram of this scheme.
 
Most commercial aircraft have telephones at each row of seats, complete with jacks into which you can plug a modem. If you plan to access the Internet from an aircraft, you must use the phones provided by the airline, not your own cell phone, because radio transceivers can cause interference to flight instruments. You must also observe the airline’s restrictions concerning the operation of electronic equipment while in flight.