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Transmission Media

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Transmission Media – PowerPoint PPT presentation

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Title: Transmission Media


1
Transmission Media
2
Transmission Media
  • Transmission environments
  • Copper
  • Fibre
  • Air
  • Transmission media
  • Electromagnetic waves
  • Radio waves
  • Microwaves
  • Infrared
  • Laser light (including visible light)
  • Electric charge

3
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

4
Electric Networks
  • All electrically charged objects generate
    electromagnetic fields (and interference)

N S
5
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

6
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

7
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)

8
Coaxial Cable
Outer Insulating Jacket
Inner Insulating Layer
Braided Metal Shield (Ground)
Transmission Wire
9
Unshielded Twisted Pair Cable
10
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

11
Airwave Transmission
  • Electromagnetic waves are used (and varied) to
    represent binary 0 or 1

12
Airwave Transmission
  • A satellite might also be used to facilitate
    transmission over longer distances

13
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

?
?
14
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

15
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

?
?
16
Controlled Refraction
  • This could be used to bend light inside a cable,
    which allows the cable to be bent around corners
    (to some degree)

17
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

18
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

19
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

20
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

21
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)

22
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

23
Amplitude Modulation
4v
2v
-2v
-4v
0.2 µs
0.2 µs
0.2 µs
0.2 µs
0
1
0
1
24
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

25
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
26
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

27
Phase Shift Modulation
4v
-4v
0.2 µs
0.2 µs
0.2 µs
0.2 µs
0
1
0
1
28
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

29
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

30
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)

31
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
32
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

33
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
34
Multiplexing and Demultiplexing
  • Multiplexing
  • Combining multiple signals into one (complex)
    signal
  • Demultiplexing
  • Restoring multiple signals from one (complex)
    signal
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