Чтение общенаучной литературы. Кытманова О.А. - 22 стр.

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23
Another problem engineers faced was that a signal gets weaker the further it
travels. Items of electrical equipment called repeaters , placed at suitable points
along the course of a cable, can boost, or strengthen, the signal and send it on its
way. Yet reception can still be poor because of interference, loss of current, or
other factors.
These problems have been overcome in the second half of the 20th century by
combining conventional cable links with radio and microwave links. Calls still go
part of the way by cable, but most of their journey is through space. Satellites play
a vital part in this area of telecommunications today. Using microwaves and
satellites means that less of the signal is lost, and the quality of reception, even for
a call between, say. New York and Tokyo, is generally excellent.
The problem of providing for a large number of users all at the same time has
been tackled in several ways. Wrapping several wires within one cable has been an
obvious approach. Each wire can carry several calls at once, each call being
handled on a time-share basis. Modern methods of transmitting the signal digitally
help in this way and also provide a better quality of reception. Perhaps the most
revolutionary method of transmitting thousands of messages over one line all at the
same time has come from the field of fiber optics .In this form of
telecommunications technology, speech sounds are converted into pulses of laser
light and directed along glass tubes as thin as a human hair. Because the laser beam
is reflected many times off the inside of the tube, there is hardly any loss of
signal. Fiber-optic cables can carry up to 80.000 telephone conversations at once.
Among the latest developments in telecommunications has been that of the mobile
telephone. This system, in which short-wave radio transmitter-receivers can be
linked to he ordinary telephone network, allows people access to the telephone
service while on the move. In a ''cellular radio" network, a metropolitan area is
split up into sections, or cells, -o that a person can keep in contact with his or her
call, even at long range, because the link can be kept open as the caller moves from
one "cell" to another. Radio-paging services also exist in which short pieces of text
can be sent over the radio-telephone network.
It is now possible to send copies of documents over the telephone lines using a
"fax", or facsimile machine, which scans the page and converts its contents into
electric signals that can be sent through the telephone system.
Computers can also be linked through the telephone system. Information stored
on a central computer can be shown on the screen of a "terminal" anywhere in the
world through the telephone system. In this way transactions can be made between
banks, airline and theater tickets can be booked, library catalogs and indexes can
be consulted, and information retrieved from agencies about such things as stock
market prices. The computer is linked to the system through a device called a
modem that converts the information to be sent into a form that can be transmitted
by telephone.
Messages transmitted this way are sometimes known as electronic mail, They are
entered at a computer terminal and can be sent to any other computer "mail box" in
the system. Messages are picked up by computer through the modern and can then
be printed out at the terminal.

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