Transcript Fundamental Limits of Transmission Media

IT2101: Communication
Technology
5.Transmission/ Network
Media
Program: BSCS I (January Semester – 2014)
Lecturer: Rebecca Asiimwe
Email: [email protected]
Network Media
• Network media is the actual path over
which an electronic signal travels as it moves
from one component to another or one
device to another.
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Transmission Media
• Numerous transmission media types are
used for Data Communication
• Generally categorized as:
1. Guided
2. Unguided (Wireless)
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The Basics of Transmission
• The Transmitter generates and encodes
data as energy which is transmitted
through a medium.
• Transmitted energy is carried through
some medium
• The medium can be copper, glass, air,
etc
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• At the destination the energy is decoded
back into data that can be used by the
destination device.
• Energy can be electrical, light, radio etc.
• Note: Each form of energy has different
properties
and
requirements
for
transmission.
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Fundamental Limits of Transmission Media
i) All channels allow only a limited set of
frequencies to be passed. This limit is called
bandwidth of that channel.
• Bandwidth: Is the data transfer rate - the
amount of data that can be carried from one
point to another in a given time period (usually
a second).
• I.e., How much data is allowed to move
through a medium usually per second.
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Fundamental Limits of Transmission Media
ii) All channels are subject to background
noise (noise is unwanted electrical or
electromagnetic energy that degrades the
quality of signals and the data received).
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Fundamental Limits of Transmission Media
• NB: Sources of noise include:
a) crosstalk: A signal on one line is
picked up by adjacent lines as a small
noise signal
– For crosstalk, both cables / wires have
the same strength.
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b) Near-End Crosstalk
• Caused when a strong transmitter output
signal interferes with a much weaker
incoming receiver signal.
A strong wire Vs a weak energy wire, so
the weak /lower strength cable is
affected.
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c) Impulse noise
• Caused by external activity or
equipment which generates electrical
impulses.
• Can be caused by voltage spikes in
equipment, lightening flashes, during
thunderstorms and a wide variety of
other phenomenon
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Thermal noise (white noise):
• Caused by the thermal agitation of electrons
associated with each atom in the device or
transmission line material.
• Eg, electrical (“UMEME wire”) running
parallel but near with the telephone wire. So,
the thermal signals from the electrical wire
might disorganize the telephone wire.
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Additional Limitations of Transmission Media
iii) Signal Attenuation:
Is the phenomenon where the Amplitude/
strength of a signal decreases as it propagates
along a transmission line.
-Attenuation is a function of distance and
frequency of signal
-Repeaters are used to increase the power of
the signal at appropriate intervals.
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Media Types
Two categories
a) Guided media
b) Unguided
a) Guided Transmission Media uses a "cabling"
system that guides the data signals along a
specific path. The data signals are bound by
the "cabling" system.
 Also known as Bound
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Media Types
• Guided Transmission Media include:
– Fiber optics
– Twisted pair cables
– Coaxial cables
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1) Twisted-Pair Cable
• Twisted-pair cable: Two of wires form a circuit that
can transmit data. Cables are twisted to provide
protection against crosstalk.
• When two wires in an electrical circuit are placed
close together, the magnetic fields are the exact
opposite of each other.
• The two magnetic fields cancel each other out. They
also cancel out any outside magnetic fields. Twisting
the wires can enhance this cancellation effect.
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Twisted pair cable
• Can be
a) Unshielded Twisted Pair (UTP)
b) Shielded Twisted Pair (STP)
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a) UTP
• UTP cable is a medium that is composed
of pairs of wires.
• Used in a variety of networks.
• Each of the eight individual copper wires
in UTP cable is covered by an insulating
material.
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UTP Cont’d
• UTP cable relies solely on the cancellation
effect produced by the twisted wire pairs to
limit signal degradation caused by
electromagnetic interference (EMI) and
radio frequency interference (RFI).
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b) Shielded Twisted-Pair (STP)
• Combines the techniques of shielding,
cancellation, and wire twisting. Each pair
of wires is wrapped in a metallic foil. The
four pairs of wires then are wrapped in an
overall metallic braid or foil.
• Has a cancellation effect and a shield to
prevent it from attacks like EMI.
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STP
UTP
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Merits of the twisted pair technology
• It is rather cost
effective
in
comparison to other
technologies like the
fiber optic.
• It is extremely easy to
terminate.
• Its thin and flexible
and can be strung
around walls
• More lines can be run
through the same
connection ducts
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Demerits of the twisted pair technology
• It is prone to external
interference.
• They require many
repeaters for
relatively long
distances which
makes it expensive.
• THE RJ-45 connector
• NB: The twisted pair
technology uses A
Registered Jack (RJ
45) in connections.
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2) Coaxial Cable
• Consists of a hollow outer cylindrical
conductor that surrounds a single inner
wire made of two conducting elements.
One of these elements, located in the
center of the cable, is a copper
conductor.
• NB: e.g. those used on TV sets
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Coaxial cable
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Coaxial Cable
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3) Fiber-Optic Cable
• Uses glass (or plastic) threads (fibers) to
transmit data.
• A fiber optic cable consists of a bundle of
glass threads, each of which is capable
of transmitting messages as light waves
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Parts of fiber optics
 Core - Thin glass center of the fiber where the
light travels
 Cladding - Outer optical material surrounding the
core that reflects the light back into the core
 Buffer coating - Plastic coating that protects the
fiber from damage and moisture
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How fiber optics operate
• The light in a fiber-optic cable travels through the
core (hallway) by constantly bouncing from the
cladding
(mirror-lined
walls),
a
principle
called total internal reflection. Because the
cladding does not absorb any light from the core,
the light wave can travel great distances.
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 Transmission
is
through
electroluminescence
(an optical phenomenon and electrical phenomenon in
which a material emits light in response to the passage).
 The emitted light is incoherent with a relatively wide
spectral width of 30-60 nm.
 Receivers- the main component of an optical receiver
is a photo-detector that converts light into electricsignal through the photoelectric effect .
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Transmitter-the
most commonlyused optical
transmitters are
semiconductor
devices such as
light-emitting diodes
(LEDs) and laser
diodes.
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Fiber-Optic Cable
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Two types of fiber-optic cables
a) Single-mode
Allows only one mode of light to
propagate through the fiber.
Capable of higher bandwidth, and it is
often used as a backbone - (10kms).
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Multimode
Multimode fiber cable allows multiple
modes of light to propagate through the
fiber.
It uses light-emitting diodes (LEDs) as a
light-generating device (2km)
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Advantages
• Fibers do not leak light and are quite difficult to
tap (Secure).
• Handles much higher bandwidth than copper
wires.
• The loss of signal in optical fiber is less than in
copper wire. Repeaters are needed at 30 km Vs 5
for copper.
• Not affected by electromagnetic interference.
Unlike electrical signals in copper wires, light
signals from one fiber do not interfere with those
of other fibers in the same cable.
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Disadvantages
• They are quite difficult to install
• Its an expensive technology - expensive to
install.
• Very delicate / fragile.
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b) Unguided Media-Wireless Communication
• Unguided Transmission Media refers to
data signals that flow through the air. They
are not guided or bound to a channel to
follow.
• Unguided media provide a means for
transmitting electromagnetic waves but do
not guide them; examples are propagation
through air, vacuum and sea-water.
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Wireless communication
• Is the transfer of information over a
distance with the use of electromagnetic
waves.
• Electromagnetic waves are formed when
an electric field couples with a magnetic
field.
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Example of unguided media
1. Radio wave transmission
2. Micro wave transmission
3. Light wave transmission
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1. Radio transmission
• The magnetic and electric fields of an
electromagnetic wave are perpendicular to
each other and to the direction of the wave
• There are 3 types of RF (Radio
Frequency) Propagation:
– Ground Wave,
– Ionospheric and
– Line of Sight (LOS) Propagation.
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Characteristics of radio transmission
•Easily generated
•Omni-directionally travel a long distance
•Can penetrate building
•Frequency dependant
•Relatively low bandwidth for data
communication
•Tightly licensed by governments
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i) Ground Wave Propagation
• Follows the curvature of the Earth. Ground
Waves have carrier frequencies up to 2
MHz. AM radio is an example of Ground
Wave Propagation.
• Radio, television and micro-waves are
types of electromagnetic waves. They
differ from each other in wavelength.
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ii) Ionospheric Propagation
• Bounces off of the Earths Ionospheric
Layer in the upper atmosphere.
• It is sometimes called Double Hop
Propagation. It operates in the frequency
range of 30 - 85 MHz
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iii) Line of Sight Propagation
• Line of Sight Propagation transmits exactly
in the line of sight. The receive station
must be in the view of the transmit station.
It is sometimes called Space Waves or
Tropospheric Propagation.
• Examples of Line of Sight Propagation
are: FM Radio, Microwave and Satellite.
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2. Microwaves Transmission
Frequency of above 100MHz,
Travel in straight lines
Can be narrowly focused.
Use a parabolic antenna.
Both the transmitting and the receiving
antennas should be accurately aligned
with each other.
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 Its used for long distance communication,
 Can not easily pass through strong
buildings and can easily be absorbed by
the rain.
 Affected by rain, vapor, dust, snow, clouds,
mist and fog, heavy moisture, depending
on chosen frequency.
 It’s used for long distance communication.
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3. Bluetooth and infrared
• Bluetooth uses radio to transmit between
microchip devices like mobile phone and a Hands
free device.
• Infrared (IR) uses electromagnetic waves for
transmission as a smaller wavelength than
radio. A TV remote control is an example of an
Infrared application.
• Wireless Personal Area Networks (WPANs) are
able to communicate using technologies like
Bluetooth (IEEE 802.15.1) and infrared.
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Infra-red
• Infrared signals can be used for short range
communication. Infrared signals, having high
frequencies, cannot penetrate walls. This helps to
prevent interference between one system and
another.
•
• There are a number of computer devices which are
used to send the data through infrared medium e.g.
keyboard mice, PCs, phones and printers. There are
some manufacturers who provide a special part
called the IrDA (Infrared Data Association) port that
allows a wireless keyboard to communicate with the
pc.
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4.Light wave transmission
• Unguided optical signal such as laser.
• Connect 2 LANS in two buildings via a laser
mounted on their roof.
• Unidirectional, easy to install, don’t require
license.
• Laser beams can be used for communication
but can not penetrate rain or thick fog.
• Laser beams can easily be diverted by turbulent
air.
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Advantages of wireless communication
– It’s ideal for non-reachable places.
– It’s ideal for temporary network
setups.
– It supports more users unlike the wired
ones.
– It’s neat and easy to install.
– It’s flexible/can be moved easily.
– It’s cheaper to maintain.
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Disadvantages
•?
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The choice of medium depends on:
• Distance to be covered.
• Desired Bit Rate (in bits per second, bps)
• Cost Considerations
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Review Questions
• Discuss the different types of network media
with an in depth study of fiber optics and
shielded twisted pair giving their advantages
and disadvantages.
• Explain the advantages associated with
wireless communication .
• How do guided differ from unguided media?
• What is the purposes of cladding in an optical
fiber?
• What are the applications of Radio wave?
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Q&A