Transmission Media

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Chapter 7 Transmission Media
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Figure 7.1 Transmission medium and physical layer
Transmission media are located below the physical
layer and are directly controlled by the physical
layer.
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Figure 7.2 Classes of transmission media
signals electromagnetic
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7-1 GUIDED MEDIA
Guided media, which are those that provide a
conduit from one device to another, include
twisted-pair cable, coaxial cable, and
fiber-optic cable.

• Uses a conductor such as a wire or a fiber optic
  cable to move the signal from sender to receiver.
• Transmission capacity depends on the distance and
  on whether the medium is point-to-point or
  multipoint
• A signal traveling along any of these media is
  directed and contained by the physical limits of
  the medium.
• Twisted-pair and coaxial cable use metallic
  (copper) conductors that accept and transport
  signals in the form of electric current. Optical
  fiber is a cable that accepts and transports
  signals in the form of light.

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Figure 7.3 Twisted-pair cable

• Telephone line in your home Low frequency
  transmission medium
• Consists of two conductors (normally copper),
  each with its own plastic insulation, twisted
  together.
• One is used to carry signals to the receiver,
  the other is used only for ground reference. The
  receiver uses the difference between the two.
• Interference (noise) and crosstalk may affect
  both wires and create unwanted signals.
• What if the two wires are parallel?
• Why twisted? To minimize the electromagnetic
  interference between adjacent pairs.
• Twising the pairs, a balance is maintained. One
  twist one is closer, then next twist, the other
  is closer. So both wires are equally affected by
  external influences. The unwanted signals are
  mostly canceled out.

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Figure 7.4 UTP and STP cables
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Table 7.1 Categories of unshielded twisted-pair
cables
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Figure 7.5 UTP connector
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Figure 7.6 UTP performance
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Twisted Pair

• Advantages
• Inexpensive and readily available
• Flexible and light weight
• Easy to work with and install
• Disadvantages
• Susceptibility to interference and noise
• Attenuation problem
• For analog, amps needed every 5-6km
• For digital, repeaters needed every 2-3km
• Relatively low bandwidth

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Figure 7.7 Coaxial cable

• Used for cable television, LANs, etc
• Conductors share a common center axial, hence the
  term co-axial

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Table 7.2 Categories of coaxial cables
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Figure 7.9 Coaxial cable performance
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Coax

• Advantages
• Higher bandwidth
• Can be tapped easily
• Much less susceptible to interference than
  twisted pair
• Disadvantages
• High attenuation rate makes it expensive over
  long distance
• Bulky

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Figure 7.10 Bending of light ray

• Relatively new transmission medium used by
  telephone companies in place of long-distance
  trunk lines
• Also used by private companies in implementing
  local data communications networks

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Figure 7.11 Optical fiber
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Figure 7.12 Propagation modes
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Figure 7.13 Modes
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Table 7.3 Fiber types
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Figure 7.16 Optical fiber performance
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Fiber Optic

• Advantages
• greater capacity
• smaller size and lighter weight
• lower attenuation
• immunity to environmental interference
• highly secure due to tap difficulty and lack of
  signal radiation
• Disadvantages
• expensive over short distance
• requires highly skilled installers
• adding additional nodes is difficult

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7-2 UNGUIDED MEDIA WIRELESS
Unguided media transport electromagnetic waves
without using a physical conductor. This type of
communication is often referred to as wireless
communication.
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Figure 7.17 Electromagnetic spectrum for
wireless communication
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Figure 7.18 Propagation methods
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Table 7.4 Bands
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Figure 7.19 Wireless transmission waves
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Figure 7.20 Omnidirectional antenna
Radio waves are used for multicast
communications, such as AM, FM, maritime radio,
cordless phones, television, and paging systems.
Ranged from 3kHz 1GHz
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Figure 7.21 Unidirectional antennas
Microwaves are used for unicast communication
such as cellular telephones, satellite
networks,and wireless LANs. Ranged from 1-300GHz
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Infrared

• Infrared signals can be used for short-range
  communication in a closed area using
  line-of-sight propagation.
• Ranged from 300 GHz to 400 THz.
• Cannot penetrate walls prevents interference
  between two systems.
• Useless for long-range communication
• Cannot be used outside of a building
• Applications?