Unformatted text preview: ber optic cables is not so simple and easy as for
twisted copper wire pairs or coaxial cables. It requires special equipment to do so.
Optical fibers are already being widely used for data transmission among
computers. Their advantage is making them the transmission medium of choice for
many applications involving telephones, televisions and computers.
Digital and Analog Data Transmission
Data is propagated from one point to another by means of electrical signals, which
may be in digital or analog form. As shown in Figure 17.8(a), in an analog signal,
the transmitted power varies over a continuous range, for example sound, light and
radio waves. The amplitude (v) of analog signals is measured in volts and its
frequency (f) in hertz (Hz). The higher is the frequency of the signal, the more
number of times it crosses the time axis. However, a digital signal is a sequence of
voltage pulses represented in binary form (see Figure 17.8(b)).
Computer generated data is digital whereas the telephone lines used for data
communication in computer networks are usually meant for carrying analog
signals. When digital data is to be sent over an analog facility, the digital signals
must be converted to analog form. The technique by which a digital signal is
converted to analog form is known as modulation. The reverse process, that is the
conversion of analog signal to its digital form at a destination device, is called
Modulation is most often used for superimposing digital data on analog waves.
There are three forms modulation - amplitude, frequency and phase modulation.
They are discussed below.
1. Amplitude Modulation (AM). Two binary values (0 and 1) of digital data are
represented by two different amplitudes of the carrier signal keeping the frequency
and phase constant. On voice grade lines, it is used up to 1200 bits per second.
However, amplitude modulated signals are sensitive to impulse noises that arise
due to electrical sparks near the transmission line.
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This document was uploaded on 04/07/2014.
- Spring '14