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There are several categories of cellular telephone. Mobile
units are mounted in a vehicle. Transportable units can be
easily moved from one vehicle to another. Pocket phones,
weighing in at less than 4 ounces, can be conveniently carried
in a jacket pocket or purse. There are even cellular
telephones that can be worn. Regardless of how they are
packaged, cellular telephones consist of the same basic
elements.
Handset/Keypad The handset and keypad provide the interface
between the user and the system. This is the only component
of the system with which, under normal operation,
the user needs to be concerned. Any basic or enhanced system
features are accessible via the keypad, and once a connection
is established, this component provides similar handset functionality
to that of any conventional telephone. Until a connection
is established, however, the operation of the handset
differs greatly from that of a conventional telephone.
Instead of initiating a call by first obtaining a dial tone
from the network switching system, the user enters the
dialed number into the unit and presses the “Send” function
key. This process conserves the resources of the cellular system,
since only a limited number of talk paths are available
at any given time. The “Clear” key enables the user to correct
misdialed digits.
Once the network has processed the call request, the user
will hear conventional call-progress signals such as a busy
signal or ringing. From this point on, the handset operates
in the customary manner. To disconnect a call, the “End”
function key is pressed on the keypad. The handset contains
a small illuminated display that shows dialed digits and provides
a navigational aid to other features. The keypad
enables storage of numbers for future use and provides
access to other enhanced features, which may vary according
to manufacturer.
Logic/Control The logic/control functions of the phone
include the numeric assignment module (NAM) for programmable
assignment of the unit’s telephone number by
the service provider and the electronic serial number of the
unit, which is a fixed number unique to each telephone.
When a customer signs up for service, the carrier makes a
record of both numbers. When the unit is in service, the cellular
network interrogates the phone for both of these numbers
in order to validate that the calling/called cellular
telephone is that of an authentic subscriber.
The logic/control component of the phone also serves to
interact with the cellular network protocols. Among other
things, these protocols determine what control channel the
unit should monitor for paging signals and what voice channels
the unit should use for a specific connection. The
logic/control component is also used to monitor the control
signals of cell sites so that the phone and network can coordinate
transitions to adjacent cells as conditions warrant
Transmitter/Receiver The transmitter/receiver component of
the cell phone is under the command of the logic/control unit.
Powerful 3-watt telephones are typically of the vehiclemounted
or transportable type, and their transmitters are
understandably larger and heavier than those contained
within lighter-weight handheld cellular units. These more
powerful transmitters require significantly more input
wattage than hand-held units that transmit at power levels
of only a fraction of a watt, and they use the main battery
within a vehicle or a relatively heavy rechargeable battery to
do so. Special circuitry within the phone enables the transmitter
and receiver to use a single antenna for full-duplex
communication.
Antenna The antenna for a cellular telephone can consist of
a flexible rubber antenna mounted on a hand-held phone, an
extendible antenna on a pocket phone, or the familiar curly
stub seen attached to the rear window of many automobiles.
Antennas and the cables used to connect them to radio
transmitters must have electrical performance characteristics
that are matched to the transmitting circuitry, frequency,
and power levels. Use of antennas and cables that
are not optimized for use by these phones can result in poor
performance. Improper cable, damaged cable, or faulty connections
can render the cell phone inoperative.
Power Sources Cell phones are powered by a rechargeable
battery. Nickel-cadmium (NiCd) batteries are the oldest and
cheapest power source available for cellular phones. Newer
nickel–metal hydride (NiMH) batteries provide extend talk
time compared to lower-cost conventional NiCd units. They
provide the same voltage as NiCd batteries but offer at least
30 percent more talk time than NiCd batteries and take
approximately 20 percent longer to charge.
Lithium ion batteries offer increased power capacity and
are lighter in weight than similar-size NiCd and NiMH batteries.
These batteries are optimized for the particular
model of cellular phone, which helps ensure maximum
charging capability and long life.
Newer cellular phones may operate with optional highenergy
AA alkaline batteries that can provide up to 3 hours
of talk time or 30 hours of standby time. These batteries take
advantage of lithium–iron disulfide technology, which
results in 34 percent lighter weight than standard AA 1.5-
volt batteries (15 versus 23 grams per battery) and 10-year
storage life—double that of standard AA alkaline batteries.
Vehicle-mounted cell phones can be optionally powered
via the vehicle’s 12-volt dc battery by using a battery eliminator
that plugs into the dashboard’s cigarette lighter. This
saves useful battery life by drawing power from the vehicle’s
battery and comes in handy when the phone’s battery has
run down. Abattery eliminator will not recharge the phone’s
battery, however. Recharging the battery can only be done
with a special charger.
Lead-acid batteries are used to power transportable cellular
phones when the user wishes to operate the phone away
from a vehicle. The phone and battery are usually carried in
a vinyl pouch.
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