The cellular telephone moves
among various antennas during a call. This technology, proposed by Bell Laboratories in
1971 and introduced commercially in 1983, is quite different from the cordless telephone,
which receives and transmits only to a single fixed base.
A cellular telephone uses a different set of radio
frequencies than a cordless telephone and is linked to a much more powerful transceiver.
The transceiver sends a strong radio signal and is sensitive enough to receive
radiotelephone signals from miles away. A series of antenna towers, each covering an
assigned area called a cell, services the cellular telephone's built-in antenna. Wire
links each antenna tower to a central switch of special design. If the cellular telephone
is used from a stationary position, as from a parked automobile, the transmission involves
only one cellular antenna tower. If the automobile is moving, however, the cellular
phone's link to the first antenna handling the call is transferred automatically to the
next antenna as the automobile leaves one cell and enters another. The
"hand-offs" from cell to cell continue as the automobile moves through the
service area, until the call ends.
Buildings or mountains that come between the antenna and
the moving automobile, however, may interrupt the reception by the cellular telephone.
Each of the cellular antennas can simultaneously handle
hundreds of separate two-way telephone conversations (or fax transmissions, when wireless
fax units are used). A call seizes one of 666 frequency bands in a cell and is
automatically handed off to any available frequency band in the next cell. This frees up
the frequency band in the previous cell, so another customer can use it.
Cellular technology replaced earlier radio-telephone
systems with far less capacity. These systems were limited to a single antenna in a given
city and provided a maximum of 25 two-way voice channels, a limitation that resulted in
long waiting lists of customers requesting that kind of mobile service. A cellular
telephone system, by contrast, can have thousands of channels. Each city is allowed by the
FCC to have two cellular-service providers, one of which is a subsidiary of the local
telephone Company.
Calls made on citizens band (CB), government, and
commercial radio systems (such as police and taxi networks) are not true telephone
communications. Other persons tuned in to the same radio frequencies can overhear them.
Also, because only one transmission at a time can be made in either direction, each person
must signal with the word "over" a readiness to receive the other person's next
transmission. Cellular telephony, on the other hand, is private and provides simultaneous
two-way calling. (See also Citizens Band [CB] Radio.)
Cellular telephones are available in three basic
configurations. A mobile cellular telephone is installed in an automobile and powered by
the latter's battery. Similar systems are used in passenger aircraft. A transportable
cellular telephone is powered by a separate portable battery and can be carried about or
plugged into an automobile's cigarette lighter. The portable or pocket cellular telephone
has its own internal battery and may weigh as little as four ounces (110 grams).
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