Transmission Media

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Transmission Media
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Transmission Media

• Transmission environments
• Copper
• Fibre
• Air
• Transmission media
• Electromagnetic waves
• Radio waves
• Microwaves
• Infrared
• Laser light (including visible light)
• Electric charge

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Electric Networks

• In these networks, copper wires are used to
  transmit an electric charge
• In rare cases, aluminium, platinum or gold can be
  used instead of copper
• A electric current is pushed through the wire in
  a controlled manner
• In power cords, the amount of electricity is
  determined by how much the appliance requires
• In network wires/cables, the amount of
  electricity (or charge) can vary in a wave
  pattern, called a carrier wave

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Electric Networks

• All electrically charged objects generate
  electromagnetic fields (and interference)

N S
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Electric Media

• One of the goals of electrostatic networks is to
  reduce the effect of electromagnetic interference
• There are several techniques, including
• Shielding around the cables/wires
• Twisting the wires within the cable

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Electric Networks

• Electric networks work by varying the electric
  charge of the wire in some way
• Techniques for varying signals is somewhat
  media-independent, and will be discussed
  separately

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Electric Cables

• A cable is a collection of wires
• A wire is typically a single strand of copper,
  with a small protective coating around it
• Two common types of cables used in networks are
• Coaxial cables
• Used by cable/satellite television, older
  Ethernet, 802 Token Bus networks
• Unshielded twisted pair (UTP)
• Used by newer Ethernet (4 pair), and telephone
  networks (2 or 3 pair)

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Coaxial Cable
Outer Insulating Jacket
Inner Insulating Layer
Braided Metal Shield (Ground)
Transmission Wire
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Unshielded Twisted Pair Cable
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Electromagnetic Networks

• These fall into two common types
• Airwave transmission
• Typically, this involves radio waves or
  microwaves
• Optical transmission (or fibre optics)
• The use of laser light passed/refracted through
  glass fibres

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Airwave Transmission

• Electromagnetic waves are used (and varied) to
  represent binary 0 or 1

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Airwave Transmission

• A satellite might also be used to facilitate
  transmission over longer distances

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Physics Primer Reflection

• An incidental wave of light approaching a
  reflective surface (e.g. silver) will bounce off
  the surface
• The angle of incidence is equal to the angle of
  reflection

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Reflective Optical Cable
light
reflective surface

• This type of fibre optic cable is made of glass,
  with a highly reflective surface on the outside
• This surface must be coated, to make it more
  reflective, which prevents significant light loss

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Physics Primer Refraction

• An incidental wave of light approaching a
  refractive surface (e.g. glass) will enter the
  surface
• The angle of incidence is not equal to the angle
  of refraction
• The angle inside a material of higher density
  will decrease, until exiting the material

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Controlled Refraction

• This could be used to bend light inside a cable,
  which allows the cable to be bent around corners
  (to some degree)

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Refractive Optical Cable
light
lower density glass
higher density glass
reflective surface

• Light refracts as it passes from a material of
  one density into a material of another density
• This type of fibre optic cable is made of
  variable density glass
• The further toward the inside of the fibre, the
  higher density the glass
• The higher density glass refracts the light beams
  toward the centre

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Carrier Waves

• The media just described all serve the same
  purpose to transmit data
• Data is transmitted on a carrier wave
• A carrier wave is some waveform
• This waveform is typically sinusoidal (like a
  sine or cosine curve)
• The carrier wave is varied (modulated) in some
  way to represent bits
• Typically an unmodified wave represents no data

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Carrier Waves

• For a wireless networks, the radio waves
  themselves are the carrier waves
• For electric networks, the voltage is varied
  between a positive value and a negative value in
  a wave pattern
• For optical networks, often the intensity
  (brightness) of the light is varied in a wave
  pattern
• The light waves are not often used as carrier
  waves, as the frequency of visible light is very
  high, and difficult to control

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Carrier Waves

• Here is how carrier waves represent data
• A carrier wave can be a flow of electrons in a
  specific pattern through a conductor
• e.g. A copper wire
• When the wave intensity is high, the wave is at a
  crest
• When the wave itensity is low, the wave is at a
  trough

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Carrier Wave Modulation

• Unmodulated carrier waves represent no data
• Carrier waves are modulated in the following
  ways
• Amplitude modulation The intensity
  (amplitude/wave height) is varied
• Frequency modulation The frequency (wave width)
  is varied
• Phase shift modulation The wave is shifted
  (left/right)

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Amplitude Modulation (AM)

• The intensity of the waves (amplitude or height)
  on the communication medium is modified to
  represent 0 or 1
• This is the same as what is applied to radio
  waves for AM radio stations
• This is not typically used with electric networks
• AM waves are sensitive to distances
• As distance lowers intensity of a signal, the
  difference between 0s and 1s may be lost

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Amplitude Modulation
4v
2v
-2v
-4v
0.2 µs
0.2 µs
0.2 µs
0.2 µs
0
1
0
1
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Frequency Modulation (FM)

• The frequency of the waves (how often crests and
  troughs occur per second) on the communication
  medium is modified to represent 0 or 1
• This is the same as what is applied to radio
  waves for FM radio stations
• This is used in most electric networks
• FM waves are less sensitive to signal
  deterioration
• Over distances the intensity of signals decrease
  (electrons get lost)
• Usually, even with deteriorated signals, the
  frequency can still be determined

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Frequency Modulation
4v
-4v
0.2 µs
0.2 µs
0.2 µs
0.2 µs
0
1
0
1
Frequency 1.0107
Frequency 0.5107
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Phase Shift Modulation (PSM)

• The temporal position of the wave is varied
• In other words, the wave is shifted forward or
  backward in time
• PSM is also less sensitive to signal
  deterioration
• Over distances the intensity of signals decrease
  (electrons get lost)
• Usually, even with deteriorated signals, the
  phase shift can still be detected

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Phase Shift Modulation
4v
-4v
0.2 µs
0.2 µs
0.2 µs
0.2 µs
0
1
0
1
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Modem

• Modem modulator/demodulator
• Modulation Given a sequence of bits, creates a
  carrier wave
• The carrier wave is varied (AM, FM, PSM, etc.) to
  represent the data
• Demodulation Given a carrier wave, creates a
  sequence of bits
• The carrier wave variations are detected, and
  turned back into the data they represent

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Standard Telephone Modems

• Modems used in dialup Internet access actually
  use an analog (audio) carrier wave
• This is the weird sound you hear when you pick up
  the telephone when the modem is in operation
• The telephone modem was designed to send digital
  data over the analog phone system

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DSL Modems

• DSL modems are similar to standard telephone
  modems
• Modern phone systems handle a fairly broad audio
  band, most of which is not necessary for
  telephones
• This is because sound at these frequencies is too
  high or low to be heard by the ear, or generated
  by the vocal chords
• These bands are used for digital transmission
• Thus, telephone service is not interrupted
• Multiple signals are sent/received
  simultaneously, using frequency division
  multiplexing (FDM)

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Frequency Division Multiplexing

• With frequency division multiplexing, multiple
  signals are combined into a broadband medium
• The broadband medium is split into narrowband
  channels (frequency ranges)
• Each channel transmits and/or sends one signal at
  any given time

FDM
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Cable Modems

• Cable modems, while similar to telephone modems,
  use coaxial cable
• Coaxial cable uses time division multiplexing
  (TDM) for its channels
• Each TV station uses one channel
• Upstream and downstream use their own channels

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Time Division Multiplexing

• With time division multiplexing, multiple signals
  are combined one signal
• Each signal is given a time slice, during which
  time that signal is put onto the medium
• When the time slice is over, the next signal gets
  transmitted, and the process repeats

TDM
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Multiplexing and Demultiplexing

• Multiplexing
• Combining multiple signals into one (complex)
  signal
• Demultiplexing
• Restoring multiple signals from one (complex)
  signal