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Wireless and personal communications systems
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THE TERM WIRELESS WAS COINED IN THE LATE NINETEENTH CENTURY WHEN inventors toyed with the idea of sending and receiving telegraph messages using electromagnetic fields rather than electric currents. In the early twentieth century, the technology became known as radio; video and data broadcasting and communications were added in the middle of the century and the general term electromagnetic communications emerged. The word wireless was relegated to history. In the late 1980s, the term found new life. Today it refers to communications, networking, control devices, and security systems in which signals travel without direct electrical connections.
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Cellular communications
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Radio transceivers can be used as telephones in a specialized communications system called cellular. Originally, the cellular communications network was patchy and unreliable, and was used mainly by traveling business people. Nowadays, cellular telephone units are so common that many people regard them as necessities.
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How cellular systems work
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A cellular telephone set, or cell phone, looks and functions like a cross between a cordless telephone receiver and a walkie-talkie. 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. This capability, which allows you to hear the other person interrupt you if he or she chooses, is known as full duplex.
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Cellular communications 607 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 Fig. 32-1. The curved line is the path of the vehicle. Base stations (dots) transfer access to the cell phone. This is called handoff. Solid lines 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. Path of vehicle
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Cells
32-1 In a cellular system, a moving vehicle is always within range of at least one base station.
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 code-division 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 actually 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. 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
608 Wireless and personal communications systems 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
A personal computer (PC) can be hooked up to the telephone lines for use with online networks such as the Internet. For some people, getting on the Internet is the most important justification for buying a computer. You can connect a laptop or notebook computer to a cell phone with a portable modem that converts incoming computer data from analog to digital and also converts outgoing data from digital to analog. This will let you get on-line from anywhere within range of a cellular base station. Figure 32-2 is a block diagram of this scheme.
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