Page 39 - Intelligent Communication Systems

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ADVANCES IN COMMUNICATION
NETWORKS

The telephone network is used mainly for transmission of voice signals, which
are analog. Frequency modulation or amplitude modulation is used for transmis-
sion of voice signals. The telephone network is an analog network with a trans-
mission capacity of 3.4 kHz, which is necessary to transmit voice signals. About
30 years ago, the data communication system was developed using the telephone
network or dedicated lines. In this system computers are interconnected through
the telephone network or dedicated lines to transmit information. This was fol-
lowed by the facsimile communication system and the videotex communication
system, which were developed to transmit facsimile and video signals, respec-
tively. These systems developed separately; it is therefore costly to construct or
maintain them.
With the data communication system, facsimile communication system, and
videotex communication system, information to be transmitted is digitized. In the
telephone network, the voice is an analog signal. When the voice signal is digi-
tized, all of the foregoing kinds of information can be transmitted on digital lines.
In this way, any kind of communication service can be provided via a single digi-
tal network. This idea is called the integrated services digital network (ISDN).

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