IT-101
Section 001
Introduction to Information
Technology
Lecture #19
Overview
Chapter 15
Wire and Fiber Transmission Systems
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Wire as a transmission medium
Fiber optics as a transmission medium
Introduction
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There are fundamentally two mediums for
information transmission:
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Wire, fiber optics, etc. (guided electromagnetic
(EM) waves)
Air (unguided EM waves)
The past two lectures have concentrated on
radio communications using air as the
transmission medium
Now, we will learn about some important
aspects of the various forms of wire and fiber
optics for information transmission
Unuided
Air
Cable
Fiber optics
Guided
Wire as a Transmission Medium
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Wire is currently the most common and versatile medium of
transmission
Wire based transmission schemes guide electromagnetic waves
either between a pair of separate wires or inside a coaxial
(coax) arrangement
A coax cable has both a center conductor and a second shield
conductor
These conductors are separated by an insulating material, such
that the shield conductor entirely surrounds the center
conductor
All of these wire-based transmission media are called cables, not
just the coaxial cable
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In the case of non coaxial transmission, the pair of
wires may be held either parallel to each other by a
stiff insulating material, or individually insulated and
twisted around each other
A surrounding shield may be placed around the
resulting twisted pair to form a shielded twisted pair
(STP)
If a surrounding shield is not placed around the
twisted pair, then this arrangement is called an
unshielded twisted pair (UTP)
Parallel
wires
UTP
STP
Coax
Cable characteristics
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A cable moves EM waves by providing a channel in which the pair of
conductors act like mirrors between which the wave bounces back and
forth
While traversing through the cable however, the wave loses energy and
the intensity of the wave diminishes due to physical effects
This results in a decrease in signal amplitude at the receiving end
called attenuation
In other terms, the magnitude of the signal diminishes as it reaches the
end of the cable
Original signal
Attenuated signal
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The longer the cable, the larger the attenuation
The larger (radius) the conductor in the cable, the lower
the attenuation (up to some extent)
It is desirable to use larger, more expensive cables in situations
that require high transmission quality over long distances
High transmission quality means that the receiver is able to
detect correctly if a 1 or a 0 is transmitted
If a signal is highly attenuated at the receiving end, the receiver
will not be able to distinguish between the levels of 1 and 0,
and this will lead to erroneous transmission of information
(remember threshold?)
Typical attenuation figures for various
cables:
Cable type
Signal attenuation
per 1000 ft @100
MHz
UTP
56 dB
STP
37.5 dB
Coax (thin ethernet)
60 dB
Coax (thick ethernet)
20 dB
Cheap
Expensive
The Decibel
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What is a decibel?
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In electrical engineering, the decibel (abbreviated as dB)
is a logarithmic unit used to describe the ratio between
two power levels (or voltage/current levels provided same resistance)
Power: unit of measurement is watts (W)
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dBP = 10 log10 P1/P2 (power ratio)
Example
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If the input signal power is 2 W and the output signal power is
measured to be 2 milliWatts, calculate the power attenuation in dB of
the cable
Original signal
2W input
power
Attenuated signal
Length of cable
The input signal power is: P1=2 W
The output signal power is: P2=2x10-3 W
The Power attenuation is: dBP = 10 log10 P1/P2
=10 log10 2/(2x10-3)
= 10 log10 1000
=10 x 3
=30 dB
The signal power has attenuated by 30dB while passing through the cable
Note: Since P1/P2 = 1000, we can say that the signal has suffered a power
attenuation of 1000 fold, or in other words, by 30 dB
2mW
output
power
Example
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If the power attenuation of a length of cable is given to be 15 dB, find the ratio
of the input/output power
15=10log10 P1/P2
1.5=log10 P1/P2
P1/P2 =101.5 =31.62
P1/P2 =31.62
The calculation above illustrates that signals passing through this length of cable
suffer a power attenuation by 31.62 times. Note that this is a ratio! There are no
units.
If, for example, the input power is 1W, the power at the output of the cable would
be 1/31.62=0.0316 W
General formula: if b=logax, then x=ab
1.5=log10 P1/P2, then P1/P2 =101.5
Note..
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Note that the dB scale is a
logarithmic scale, and is a
convenient method to express
large ratios
In the first example, the ratio
1000 was expressed as 30 dB
For example, if the ratio is:
800,000,000, then this
expressed in dB is: 89 dB (a
much smaller number)
Ratio
dB
1
0
10
10
100
20
1000
30
10,000
40
100,000
50
1,000,000
60
Exercises
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If the input signal power is 10mW and the output
signal power is measured to be 5 μW (micro watts),
calculate the power attenuation in dB of the cable
If the power attenuation of a length of cable is given
to be 65 dB, find the ratio of the input/output power
Fiber Optics as a
Transmission Medium
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Information is carried through a fiber optic cable by transmitting
pulses of light (which is also an EM wave)!
A fiber optic cable is a coaxial arrangement of glass or plastic material
of immense clarity (i.e., highly transparent)
A clear cylinder of optical material called the core is surrounded by
another clear wrapper of optical material called the cladding
These two materials are selected to have different indices of refraction
The fiber is surrounded by a plastic or teflon jacket to protect and
stiffen the fiber
Light is guided through the optical fiber by continual reflection from the
core-cladding boundary
This is made possible due to the different refractive indices of the core
and cladding materials
The index of refraction (n) of a material affects the angle by which a
light ray is bent while passing through the material
If the light incident on the core-cladding boundary is at a suitable
angle, then the light will be totally reflected from the boundary. This is
called total internal reflection
Cross section of optical fiber cable
Core and cladding with
different indices of refraction
Core-cladding boundary
Advantages of fiber optics
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Much Higher Bandwidth (Gbps) - Thousands of channels can be multiplexed
together over one strand of fiber
Immunity to Noise - Immune to electromagnetic interference (EMI).
Safety - Doesn’t transmit electrical signals, making it safe in environments like a
gas pipeline.
High Security - Impossible to “tap into.”
Less Loss - Repeaters can be spaced 75 miles apart (fibers can be made to have
only 0.2 dB/km of attenuation)
Reliability - More resilient than copper in extreme environmental conditions.
Size - Lighter and more compact than copper.
Flexibility - Unlike impure, brittle glass, fiber is physically very flexible.
Disadvantages include the cost of interfacing equipment necessary to convert
electrical signals to optical signals. (optical transmitters, receivers) Splicing fiber
optic cable is also more difficult.