LESSON
6
MAKING A TELEPHONE CALL
I.
Consider
the following
questions in
the group
of four. A spokesperson
will report on your discussion
to the
whole group.
1. Do your know how to make a telephone call?
Describe your actions, please.
2. Are there any telephone subscribers served
by manual exchanges in your town?
3. Why do you think that to make a call to
4. What kind of telephone do you consider the
most convenient?
II.
Read the text below and
check
its comprehension.
Making a Telephone Call
A telephone call starts when the
caller lifts a handset off the base. This closes an electrical switch that
initiates the flow of a steady electric current over the line between the
user's location and the exchange. The exchange detects the current and returns
a dial tone, a precise combination of two notes that lets a caller know the
line is ready.
Once the dial tone is heard, the
caller uses a rotary or push-button dial mounted either on the handset or base
to enter a sequence of digits, the telephone number of the called party. The
switching equipment in the exchange removes the dial tone from the line after
the first digit is received and, after receiving the last digit, determines
whether the called party is in the same exchange or a different exchange. If
the called party is in the same exchange, bursts of ringing current are applied
to the called party's line. Each telephone contains a ringer that responds to a
specific electric frequency. When the called party answers the telephone by
picking up the handset, steady current starts to flow in the called party's
line and is detected by the exchange. The exchange then stops applying ringing
and sets up a connection between the caller and the called party.
If the called party is in a
different exchange from the caller, the caller's ex-change sets up a connection over the telephone network to the called
party's exchange. The called
exchange then handles the process of ringing, detecting
an answer, and notifying the calling exchange and billing machinery when the
call is completed.
When the conversation is over, one
or both parties hang up by replacing their handset on the base, stopping the
flow of current. The exchange then initiates the process of taking down the
connection, including notifying billing equipment of the duration of the call
if appropriate. Billing equipment may or may not be involved because calls
within the local calling area may be either flat rate or message rate. In
flat-rate service, the subscriber is allowed an un-limited number of calls for a fixed fee each month. For message-rate
subscri-bers, each call involves a charge
that depends on the distance between the calling and called parties and the
duration of the call. A long-distance call is a call out of the local calling
area and is always billed as a message-rate call.
Telephone switching equipment
interprets the number dialed and then completes a path through the network to
the called subscriber. For long-dis-tance calls with complicated paths through the network, several levels of switching equipment may be needed. The
automatic exchange to which the subscriber's telephone is connected is the
lowest level of switching equip-ment and is
called by various names, including local exchange, local office, central-office
switch, or, simply, switch. Higher levels of switching equipment include tandem
and toll switches, and are not needed when both caller and called subscribers
are within the same local exchange.
Today in the
Today's automatic exchanges use a
pair of computers, one running the program that provides service, and the
second monitoring the operation of the first.
The greatly expanded
information-processing capability of modern com-puters permits Direct Distance Dialing, with which a subscriber can
automati-cally place a call to a distant city
without needing the services of a human operator to determine the appropriate
routing path through the network. Computers in the switching machines used for
long-distance calls store the routing information in their electronic memory. A
toll-switching machine may store several different possible routes for a call. As telephone traffic becomes heavier during the day, some routes may
become unavailable. The toll switch
will then select a less direct alternate route to permit
the completion of the call.
Calling from
Analog transmission, in which speech
or data is converted directly into a varying electrical current, is suitable
for local calls. For long-distance calls, the signal is digitized, or converted
to a series of pulses that encodes the information.
Digital transmission systems are
much less subject to interfering noise than are analog systems. The digitized
signal can then be passed through a digital-to-analog converter (DAC) at a
point close to the receiving party, and converted to a form that the ear cannot
distinguish from the original signal.
There are several ways a digital or
analog signal may be transmitted, inc-luding coaxial
and fiber-optic cables and microwave and long wave radio sig-nals sent along the ground or bounced off
satellites in orbit around the earth.
A coaxial wire is an efficient
transmission system. A coaxial wire has a conducting tube surrounding another
conductor. A coaxial cable contains several coaxial wires in a common outer
covering. The important benefit of a coaxial cable over a cable composed of
simple wires is that the coaxial cable is more efficient at carrying very high
frequency currents. This is important because in providing transmission over
long distances. The combined signal containing hundreds of individual telephone
conversations is sent over one pair of wires in a coaxial cable, so the signal
has to be very clear. Coaxial cable is expensive to install and maintain,
especially when it is lying on the ocean floor.
Another telephone-transmission
method uses fiber-optic cable, which is made of bundles of optical fibers, long
strands of specially made glass enca-sed in a
protective coating. Optical fibers transmit energy in the form of light pulses.
The technology is similar to that of the coaxial cable, except that the optical
fibers can handle tens of thousands of conversations simultaneously.
Another approach to long-distance
transmission is the use of
radio. Micro-wave radio uses very high frequency radio
waves and has the ability to handle a large
number of simultaneous conversations
over the same microwave link. Because cable does not have to be
installed between microwave towers, this system is usually cheaper than coaxial
cable. On land, the coaxial-cable sys-tems are often supplemented with
microwave-radio systems.
The technology of microwave radio is
carried one step further by the use of
communications satellites. Most communications satellites are in geosyn-chronous
orbit that is, they orbit the earth once a day over the equator,
so the
satellite is always above
the same place on the earth's surface. Even conside-ring the expense of a satellite, this method is cheaper to install and maintain
per channel than using coaxial cables
on the ocean floor.
Consequently,
satel-lite
links are used regularly in long-distance
calling. But satellite communica-tion does have one serious
shortcoming: because of the satellite's distance from the earth,
there is a noticeable lag in conversational responses. As a re-sult, many calls use a satellite for
only one direction of transmission,
such as from the caller to the receiver, and use a ground microwave or coaxial
link for receiver-to-caller transmission.
A combination of microwave,
coaxial-cable, optical-fiber,
and satellite paths
now link the major cities of the world. The capacity of each type of system
depends on its age and the territory covered, but capacities generally fall
into the following ranges: Frequency modulation over a simple pair of wires
like the earliest telephone lines yields tens of circuits per pair; coaxial
cable yields hundreds of circuits per pair of conductors, and thousands per
cable; microwave and satellite transmissions yield thousands of circuits per
link; and optical fiber has the potential for tens of thousands of circuits per
fiber.
Comprehension Check Up
1. How does a telephone call start? 2. What lets the caller
know the line is ready? 3. How does the switching equipment determine whether
the called party is in the same exchange or different exchange? 4. What
is the difference between flat rate and message rate? 5. What is the
lowest level of switching equipment? 6.
Who operates the automatic exchanges? 7.
What kinds of trans-mission
systems do you know? 8. What are the ways of telephone
transmissi-on? 9. What are the differences between a coaxial
cable and fiber-optic cable? 10.
What are the advantages of the use
of radio? 11.
Does
satellite communica-tion have any serious
shortcomings?
III.
Choose the best answer.
1.
The expression telephone exchange means
…
a) a telephone number; b) a calling area; c) a
switching centre; d) an electrical switch.
2. Billing
machinery refers to a machine …
a) counting the amount
of money you have to pay; b) sending radio signals; c) counting the
amount of money you have to be paid; d) counting your tele-phone calls.
3. The verb
to
handle means…
a) to manage; b) to remove; c) to process; d)
to determine.
4. The
best explanation of the expression analog transmission might be …
a) a system in which each sample is converted
into a binary form; b) a system which controls telephone exchanges; c) a system
which amplifies radio signals; d) a system in which a varying electrical
current transmits the caller’s voice
pattern.
5. The verb to deliver means…
a) to say something to someone; b) to give
smth. to someone; c) to decide to do something; d) to call someone.
6. Fiber-optic
cable refers to long strands
a) of specially made wood, encased in
protective coating; b) of specially made glass encased in a protective coating;
c) of specially made wires encased in a protective coating; d) of specially
made wool encased in a protective coating.
7. The word steady
means …
a) stable; b) startling; c)standard; d)
straight.
8. The word to notify implies…
a) to give good results; b) to give advice; c) to give explanation; d) to give notice.
9. The best explanation of the coaxial cable
might be
…
a) a cable consisting of long strands of
specially made glass encased in a
protective coating; b)a cable consisting of a
number of tubes each with a central copper wire and a shield; c) a link between
two or more communication points; d) an exchange which uses microchip
technology.
10. The verb to distinguish means …
a) to understand clearly; b) to perceive
clearly; c) to answer correctly; d) to supplement with smth.
11. The term automatic exchange refers to a system in which …
a) a small light of a switchboard alters an operator that a subscriber wants a
service; b) sounds are transmitted over considerable distance; c) electricity
from the battery flows through the wire and causes the buzzer to make a noise;
d)the operations are performed by electrically controlled devices without any
intervention of operators.
12. The flat-rate
service suggests that …
a) each call involves a charge that depends on
the distance between the
calling and called
parties; b) the subscriber is allowed an unlimited number of calls for a fixed
fee each mouth; c) each call involves a charge that depends on the duration of the call; d) the
subscriber must pay for each call every day.
IV.
Use the
right noun from those given
below.
switch, satellites, strands, exchange, benefit,
tone,
subscriber, lag, duration, shortcoming, ringer, routes.
1. The
automatic … is called by various names. 2. Once the dial … is heard,
the caller dials the telephone number
of the called party. 3. Each tele-phone contains a … that responds to a specific
electric frequency.
V.
Are the following statements True or False?
1. When the caller lifts a handset
off the base, a steady electrical current starts to flow over the line between
the user’s location and the exchange. 2. When the called party answers the
telephone, by picking up the handset, the flow of current stops. 3. The
automatic exchange is the lowest level of swit-ching equipment.
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 of using an automatic
exchange.