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Unformatted text preview: rounded by a PVC insulation over which a sleeve of copper
mesh is placed. The metal sleeve is again shielded by an outer shield of thick PVC
material. The signal is transmitted by the inner copper wire and is electrically
shielded by the outer metal sleeve.
Coaxial cables offer much higher bandwidths than UTP cables and are capable of
transmitting digital signals at rates of 10 mega bits per second. They are
extensively used in long distance telephone lines and as cables for cable TV. They
are also used by telephone companies to transmit data. In many cases, several
coaxial cables are packaged into a very large cable that can handle over 40,000
telephone calls simultaneously. Furthermore, coaxial cables have much higher
noise immunity and can offer cleaner and crisper data transmission without
distortion or loss of signal.
Another popular transmission media is microwave. This is a popular way of
transmitting data since it does not I incur the expense of laying cables. Microwave
systems use very high frequency radio signals to transmit data I through space.
However, at microwave frequencies, the electromagnetic waves cannot bend or pass obstacles like I hill. Hence it is necessary that microwaves transmission be in
a line-of-sight. In other words, the transmitter and I receiver of a microwave
system, which are mounted on very high towers, should be in a line-of-sight. This
may I not be possible for very long distance transmission. Moreover, the signals
become weaker after traveling a certain distance and require power amplification.
In order to overcome the problem of line-of-sight and power amplification of weak
signals, microwave systems use repeaters at intervals of about 25 to 30 kms in
between the transmitting and receiving stations (see Figure 17.5). The first
repeater is placed in line-of-sight of the transmitting station and the last repeater is
placed in line of-sight of the receiving station. Two consecutive repeaters are also
placed in line-of-sight of each other. The data signals are received, amplified, and
retransmitted by each of these stations.
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- Spring '14