SIMS-201
Wire and Fiber Transmission
Systems
Overview
Chapter 15
Wire and Fiber Transmission Systems
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Wire as a transmission medium
Fiber optics as a transmission medium
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Introduction
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There are fundamentally two mediums for
information transmission:
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Guided electromagnetic (EM) waves - wire, fiber
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Unguided EM waves - air
optics, etc.
The past two lectures have concentrated on
radio communications using air as the
transmission medium
Next, we will learn about some important
aspects of the forms of wire and fiber optics
used for information transmission
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Unguided
Air
Cable
Fiber optics
Guided
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Wire as a Transmission Medium
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Wire is currently the most common and versatile medium of
transmission
All wire-based transmission media are called cables
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
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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)
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Parallel
wires
UTP
STP
Coax
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Cable characteristics
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A cable moves EM waves by providing a channel. The EM waves
traverse the cable moving through the conductors. The EM waves are
confined in this way, as they interact with the free electrons in the
conductor, which are responsible for guiding the waves.
While traversing through the cable however, due to physical effects, the
wave loses energy and the intensity of the wave diminishes, the farther
it goes.
This results in a decrease of the signal amplitude at the receiving end –
called attenuation
In other words, 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 the conductor in the cable (radius), 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
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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
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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
Power: unit of measurement is watts (W)
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dB = 10 log10 P1/P2 (power ratio)
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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 times, or in other words, by 30 dB
2mW
output
power
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Logarithm (log)
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How do we know that
10 log10 1000 = 10 x 3 ?
Logarithmic and Exponential Functions
Logarithmic and exponential functions are inverses of each other:
If y = logbx then x = by
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In words, logb x is the exponent you put on base b to get x. lol
So,
If x = log10 1000 then 1000 = 10x
and how much is x here? 10 to the power of what is equal to
1000? It would be 10 to the power of 3 (10x10x10) = 1000.
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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
Logarithm:
if b = logax then x = ab
1.5 = log10 P1/P2
then P1/P2 = 101.5
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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
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Exercises
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If the input signal power is 10mW (milliWatts) 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
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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
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Cross section of optical fiber cable
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Core and cladding with
different indices of refraction
Core-cladding boundary
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Let us look at some useful fiber optics
demos:
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http://www.eas.asu.edu/~palais/demos/demos.htm
http://electronics.howstuffworks.com/fiberoptic2.htm
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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.
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