ISDN, which
stands for integrated services digital network, is a system of digitizing phone
networks which has been in the works for over
a decade. This system allows audio, video, and text data to be transmitted simultaneously
across the world using end-to-end digital connectivity.The original
telephone system used analog signals to transmit a signal across telephone
wires. The voice
was carried by modulating an electric current with a waveform from a
microphone. The
receiving end would then vibrate a speaker coil for the sound to travel back
to the ear
through the air. Most telephones today still use this method. Computers,
however,
are digital
machines. All information stored on them is represented by a bit, representing
a
zero or a one.
Multiple bits are used to represent characters, which then can represent
words, numbers,
programs, etc. The analog signals are just varying voltages sent across the
wires over time.
Digital signals are represented and transmitted by pulses with a limitednumber of
discrete voltage levels. [Hopkins]
The modem was certainly a big breakthrough in
computer technology. It allowed computers to communicate with each other by converting
their digital communications into an analog format to travel through the public phone
network. However, there is a limit to
the amount of information that a common analog telephone
line can hold. Currently, it is about 28.8 kbit/s. [Hopkins] ISDN allows multiple digital
channels to be operated simultaneously through the same regular phone jack in a home
or office. The change comes about when the telephone company's switches are upgraded to
handle digital calls. Therefore, the same wiring can be used, but a different signal is
transmitted across the line. [Hopkins] Previously, it was necessary to have a phone
line for each device you wished to use simultaneously. For example, one line each for
the phone, fax, computer, and live video conference. Transferring a file to someone
while talking on the phone, and seeing their live picture on a video screen would require
several expensive phone lines. [Griffiths] Using multiplexing (a method of combining
separate data signals together on one channel such that they may be decoded again at the
destination), it is possible to combine many different digital data sources and have the
information routed to the proper destination. Since the line is digital, it is easier to
keep the noise and interference out while combining these signals. [Griffiths] ISDN
technically refers to a specific set of services provided through a limited and standardized
set of interfaces.
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