LESSON
3
WIRELESS COMMUNICATIONS
1. What means of wireless communications do you
know?
2. What are the main principles of wireless
communications?
3. What opportunities do wireless
communications systems provide?
4. What are the merits and demerits
of the wireless communications sys-tems?
5. Can you describe the perspectives
of using wireless communications systems in
II.
Read the text below and
check
its comprehension.
Wireless
Communications
Wireless
сommunications are various
telecommunications systems that use radio waves to carry signals and messages
across distances. Wireless
communications systems use devices called transmitters to generate radio waves.
A microphone or other mechanism converts messages, like sounds or other data,
into electronic impulses. The
transmitters change, or modulate, the radio waves so they can carry the
impulses, and then transmit the modu-lated radio signals across distances. Radio receivers pick up these signals and decode
them back into original messages. Commercial
radio and televi-sion are also wireless
telecommunications system, but radio and television are mainly public broadcast
services rather than personal communications systems.
Wireless communications allow people
greater
flexibility while communica-ting, because they do not need to remain at a
fixed location, such as a home or office. Wireless technologies make
communications services more readily available than traditional wire-based
services (such as ordinary telephones), which require the installation of wires. This is useful in places where only
tem-porary communications services are
needed, such as at outdoor festivals or large sporting events. These
technologies are also useful for communicating in remote locations, such as
mountains, jungles, or deserts, where telephone service might not exist.
Wireless services allow people to communicate while in a car, airplane, or
other moving vehicle. Police, fire, and other emergency departments use two-way radio to communicate
information between vehicles
that are already responding to
emergency calls, which saves valuable
time. Construction and
utility workers frequently use hand-held radios for short-ran-ge communication
and coordination. Many business people
use wireless com-munications,
particularly cellular radio telephones,
to stay in contact with col-leagues
and clients while traveling.
All wireless communications devices use radio waves to
transmit and rece-ive signals. These devices operate on different
radio frequencies so that sig-nals from one device will not overlap and
interfere with nearby transmissions
from other devices.
Principles of Wireless
Communications
Wireless communications begin with a
message that is converted into an electronic signal by a device called a
transmitter. The transmitter uses an os-cillator to generate radio waves. The transmitter modulates the radio
wave to carry the electronic signal and then sends the modified
radio signal out through space, where it is picked up by a
receiver. The receiver decodes, or demodu-lathes the radio wave and plays the decoded message over a speaker. Wire-less communications provide more flexibility
than wire-based means of com-munication.
However, there are some drawbacks. Wireless communications are limited by the
range of the transmitter (how far a signal can be sent), and since radio waves travel through the atmosphere,
they can be disturbed by electrical
interferences (such as lightning) that cause static.
Wireless communications systems
involve either one-way transmissions, in which a person merely receives notice of a message, or two-way transmis-sions,
such as a telephone conversation between two people.
An example of a device that sends one-way
transmission is a pager, which
is a radio receiver.
When a person dials a pager number, the pager company sends a radio signal to
the desired pager. The encoded signal triggers the pager circuitry and notifies
the customer carrying the pager of the incoming call with a tone or a
vibration, and often the telephone number of the caller. Advanced pagers can
display short messages from the caller, or provide news updates or sports
scores.
Two-way transmissions require both a
transmitter and a receiver for sending and receiving signals. A device that
functions as both a transmitter and a receiver is called a transceiver.
Cellular radio telephones and two-way radios use transceivers, so that
back-and-forth communication between two people can be maintained. Early
transceivers were very large, but they have decreased in size due to advances
in technology. Fixed-base transceivers, such as those used at police stations,
can fit on a desktop, and hand-held transceivers have shrunk in size as well.
Several current models of hand-held transceivers weigh less than
Modes of Wireless Communications
Wireless communications systems have grown and
changed as technolo-gy has improved. Several different systems are
used today, all of which ope-rate on different radio frequencies. New
technologies are being developed to
provide greater service and reliability.
Air Transceivers
Radio operators still monitor
distress channels, but maritime and aviation telecommunications systems now use
high-frequency radios and satellites capable of transmitting speech, rather than wireless telegraphy, to send mes-sages. Aircraft pilots use radios to communicate with air traffic controllers
at airports and also to communicate with other pilots. Navigation beacons are
equipped with transmitters that send automated signals to help ships and air-craft in distress determine their positions.
While high-frequency radio can
transmit signals over long distances, the quality of these signals can be dimi-nished
by bad weather or by electrical interference in the atmosphere, which is often
caused by radiation from the sun.
Hand-Held Radio Transceivers
Police, fire, and other emergency organizations, as well as the
military, ha-ve used two-way wireless radio
communication since the 1930s. Early vehicle-based radios were large,
heavy units. After the invention of the transistor in 1948, radios shrank in
size to small hand-held radio transceivers, which civil authorities now use to
communicate with each other directly. Public two-way radios with several
frequency options are widely available as well. Usually li-mited in range to a few miles, these units are
great aids for such mobile pro-fessionals as construction workers, film crews,
event planners, and security personnel. Simpler two-way radios, called
walkie-talkies, have been popular children’s toys for years.
Shortwave
Long-range broadcast services and
frequencies, in what is known as the shortwave radio band (with frequencies of
3 to 30 megahertz), are available for amateur or ham radio operators. Shortwave
radio broadcasts can travel long distances because of the concentration of
ionized, or electrically charged, particles in the
layer of the atmosphere known as the
ionosphere. This layer reflects radio signals, sending signals that are
transmitted upward back to earth. This
skipping of waves against the ionosphere can greatly increase the range of the
transmitter. The degree of
reflectivity of the ionosphere depends on the time of day.
Cellular Radio
Telephones
Cellular radio telephones, or cell
phones, combine their portable radio ca-pability with the wired, or wire line, telephone
network to provide mobile users with access to the rest of the public telephone
system used by non-mobile callers.
Modern cellular telephones use a network of several short-range an-tennas that connect to the telephone system.
Because the antennas have a shorter
range, frequencies can be reused a
short distance away without in-terference.
Satellite Communications
Satellite communications services
connect users directly to the telephone network from almost anywhere in the
world. Special telephones are available to consumers that communicate directly
with communications satellites orbi-ting the earth.
The satellites transmit these signals to ground stations that are connected to
the telephone system. These satellite services, while more expensive than cellular
or other wireless services, give users access to the telephone network in areas
of the world where no telephone service exists.
The number of companies
offering wireless communications
services have grown steadily in
recent years. In 1988 about 500 companies offered cellular radio telephone
services. By 1995 that number had grown to over 1500 companies serving
millions of subscribers. Wireless communication is becoming increasingly
popular because of the convenience
and mobility it affords, the expanded availability of radio frequencies for
transmitting, and improvements in technology.
Comprehension Check Up
1. What functions do transmitters
perform? 2. What functions do receivers perform? 3. What
are the advantages of wireless communications? 4. Has
the number of companies offering wireless communications services
grown
steadi-ly in recent years? 5. What are the
main principles of wireless
communications? 6. How does a pager work?
7. What devices use transceivers? 8.
What can diminish the quality of high-frequency
radio signals? 9. What devices are great aids for mobile professionals?
10. Why can shortwave radio
broadcasts travel long distances? 11.
What are cellular radio telephones
used for? 12. What are the merits of satellite
communications services?
III.
Choose the best answer.
1. The
word message means
…
a) a written request; b) a piece of news or a request sent to smb.; c)
move-ment of the hand, head, etc. used with or instead of words; d) a signal.
2. The
best
explanation of the word transmitter might be …
a) a part of an apparatus for receiving broadcast signals; b) a person who receives; c) a part
of a telegraph or radio apparatus for sending out signals, messages, etc.; d) an
instrument for recording oscillations.
3. The
word frequency implies …
a) excitement; b) one swing of
an electric charge; c) a vibrating movement;
d) rate of occurrence; number of repetitions (at a given time).
4. The
word range refers to …
a) a variation between limits; b) a line of persons
or things; c) a position
in a scale; d) a category or class.
5. The
word wire means
…
a) a radio set;
b)
metal drawn out into
the
form of a thread;
c)
a rope; d) elec-tric current.
6. The
best explanation of the word broadcast might be…
a) To give or pass; b) to send out in all
directions, esp. by radio or TV;
c) to
help with the hand; d) to pass by tradition,
inheritance.
7. The
verb oscillate means …
a) to swing backwards and forwards as the pendulum of a clock does; b) to move regularly to and fro; c) to cause
smb. to move in a certain direction
by waving; d) to distribute.
IV.
Use the
right word from those given below.
cellular, overlap,
to decode, flexibility, converts, dials, to communicate, band, circuitry, charged.
1. One
of the functions of radio
receivers is … signals back into
the original messages. 2 .Wireless
communications provide people with greater … while communicating. 3. Many businessmen
prefer to use … radio
telephones to stay in contact with colleagues and clients. 4.
Signals of wireless
communications devices mustn’t … and interfere with nearby transmissions
from other device. 5. Transmitter … a message into an electronic signal.
V.
Are the following statements True or False?
1. The transmitters transmit the
modulated radio signals across distances. 2. Commercial radio and television aren’t wireless
communications systems. 3. A person should
remain at a fixed location, such as
home or office while using wireless
communications. 4. Traditional
wire-based services make com-munications services
more
readily available than wireless technologies. 5. Wire-less communications are useful in remote
locations where telephone service might not
exist. 6. The
number of companies offering wireless communications services have decreased in recent years. 7. The
transmitter decodes, or demo-dulates the radio wave
and plays the decoded message over a speaker.
VI.
Express the main idea of
each
paragraph
in one sentence.
VII.
Summarize the
text and be ready to retell it.
VIII.
Tell us about
the advantages and disadvantages of using wireless
communications.