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Transmission Media
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Types of Transmission Media
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  • Guided:
    • The waves are guided along a physical path
  • Unguided:
    • The medium transmits the waves but does not guide them
Some Important Terminology
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  • Direct Link:
    • The signal is propagated directly from transmitter to receiver (perhaps through amplifiers/repeaters)
  • Point-to-Point:
    • A guided medium that provides a direct link between two and only two devices
  • Multipoint:
    • A guided medium that is shared by more than two devices
Multiplexing
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  • The Concept:
    • Simultaneously transmit multiple channels of information on a single direct link
  • Common Approaches:
    • Frequency Division Multiplexing (FDM) - Each signal, \(m_i\) is modulated onto a different frequency (called a subcarrier and denoted by \(f_i^c\))
    • Time Division Multiplexing (TDM) - Multiple signals are interleaved (in bits or blocks) over time
Guided Media - Twisted Pair Cable
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  • Description:
    • Two insulated copper wires that are twisted together (which results in destructive interference and, hence, reduces radiation and, hence, energy loss)
    • The individual pairs can be shielded (STP) or unshielded (UTP)
    • The cable can also be shielded (S/STP)
  • Example:
    • images/twisted-pair-cable.gif
Twisted Pair Cable (cont.)
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  • Categories:
    • 1 - primarily for voice
    • 2 - 4Mbps
    • 3 - 16MHz of bandwidth; 10Mbps
    • 4 - 20MHz of bandwidth; 16Mbps
    • 5/5e - 100MHz of bandwidth; 100Mbps (4 pairs/cable)
    • 6 - 400MHz of bandwidth; 1Gbps (4 pairs/cable)
  • Common Uses:
    • Voice Telephony (including private branch exchanges)
    • Digital Subscriber Line (DSL) in many flavors
Twisted Pair Cable (cont.)
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  • Advantages:
    • High availability
    • Low installation cost
  • Disadvantages:
    • Usable frequency spectrum is small
    • Limited data rates
    • Limited range
    • High susceptibility to electromagnetic and radio frequency interference
Guided Media - Coaxial Cable
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  • Description:
    • A (thick) copper wire surrounded by an insulator and an outer conductor
    • The current flowing on the inner conductor is equal and opposite to the current flowing on the outer conductor so little energy is lost to radiation
  • Example:
    • images/coaxial-cable.gif
Coaxial Cable (cont.)
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  • Common Types:
    • RG-6 - 1.00mm core, 75\(\Omega\)
    • RG-59 - 0.81mm core, 75\(\Omega\)
    • RG-58 - 0.90mm core, 50\(\Omega\)
    • RG-8 - 2.17mm core, 50\(\Omega\)
  • Common Uses:
    • Cable TV (Old: RG-59; New: RG-6)
    • Ethernet (RG-58: 10Base2; RG-8: 10Base5)
    • Amateur radio
Coaxial Cable (cont.)
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  • Advantages:
    • High capacity (370-1000 times more than twisted pair)
    • Low susceptibility to noise and crosstalk (because of shielding)
  • Disadvantages:
    • Noise on the return path
    • High installation cost
Guided Media - Fiber Optic Cable
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  • Description:
    • A thin glass core surrounded by cladding that reflects light into the core, surrounded by a protective plastic buffer/jacket
  • Example:
    • images/fiber-optic-cable.gif
Fiber Optic Cable (cont.)
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  • Operation:
    • Light travels through the core
    • It bounces off of the cladding to "make turns"
    • The signal degrades because of impurities in the glass
  • Illustration:
    • images/fiber-optics_total-internal-reflection.gif
  • Total Internal Reflection:
    • Light passing from one medium (\(m_1\)) to another (\(m_2\)) with lower index of refraction (\(r_i\)) bends away from the normal line
    • At the critical angle, defined by \(\sin(\theta) = r_2 / r_2\), the refracted light will not go into the second medium but will travel along the border
Fiber Optic Cable (cont.)
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  • Common Wavelengths (in the infrared):
    • 850nm
    • 1300nm
    • 1550nm
  • Common Uses:
    • Cable TV
    • Voice Telephony
    • Ethernet
Fiber Optic Cable (cont.)
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  • Advantages:
    • Low cost
    • High capacity
    • Thin
    • Low interference/degradation
  • Disadvantages:
    • Can be bent
    • Each fiber is one-way
    • High cost of interfaces
Guided Media - Characteristics
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Frequency Attenuation Delay
Twisted Pair 0-3.5kHz 0.2dB/km at 1kHz 50\(\mu\)/km
Coaxial Cable 0-500MHz 7dB/km at 10MHz 4\(\mu\)/km
Optical Fiber 186-370THz 0.2-0.5dB/km 5\(\mu\)/km
Unguided Media - Radio
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  • Einstein on Radio:
    • "You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this?
      And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat."
  • VLF, LF and MF Bands:
    • Range of about 1000km
    • Penetrate buildings
    • Omnidirectional
  • HF and VF Bands:
    • Tend to be absorbed by the Earth
    • Can be bounced off of the ionosphere (called sky wave propagation)
Radio - Some Examples
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  • Pulse Modulation - WWVB in Fort Collins, Co.:
    • Transmits minutes, hours, day of year, etc...
    • 60,000 Hz
    • 0.2 seconds of reduced power is a binary 0; 0.5 sec of reduced power is a binary 1; 0.8 sec of reduced power is a separator
    • Message requires 53 bits and 7 separators
Radio - Some Examples (cont.)
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  • Amplitude Modulation - AM Talk Radio Stations:
    • The wave produced by a person's voice is overlaid onto the transmitter's wave to vary its amplitude
    • AM 680 broadcasts at 680,000 Hz
    • Carrier wave of frequency \(\Omega\)
      \(c(t) = C \sin(\Omega t)\)
    • Signal to broadcast
      \(m(t) = M \sin(\omega t + P)\)
    • AM is \(m(t)+C\)
      \(y(t) = [C + (M \sin(\omega t + P)] \sin(\Omega t)\)
Radio - Some Examples (cont.)
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  • Frequency Modulation - Cordless Telephones:
    • Vary the instantaneous frequency of a carrier wave in accordance with an input signal
    • FM 101.5 broadcasts at 101,500,000 Hz
Unguided Media - Microwave
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  • Properties:
    • Travel in a nearly straight line
    • Can be narrowly focused (by a parabolic antenna)
    • High signal to noise ratio
    • Little interference
    • Low penetration
      • Do not pass through buildings
      • At 4GHz and above, the waves are very short and can be absorbed by water (e.g., rain)
      • At 60GHz the waves can be absorbed by oxygen
  • Range:
    • 2GHz-6GHz: 70km
    • 18GHz-45GHz: 1.6-8km
Unguided Media - Infrared
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  • Mostly directional
  • Cheap
  • Do not pass through solid objects (so no license is required)
Unguided Media - Laser (i.e. Coherent Light)
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  • Unidirectional
  • High bandwidth
  • Cannot penetrate thick fog or rain
The Electromagnetic Spectrum
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International Telecommunication Union (ITU) Bands

Band Frequency Media
Gamma Rays 1022
1021
1020
1019
1018
1017
X-Rays 1016
Ultraviolet 1015
Visible Light 1014 Fiber Optics
Infrared 1013 Fiber Optics
THF 1012 (1THz) Microwave
EHF 1011 Microwave
SHF 1010 PCS, Microwave
UHF 10 9 (1GHz) FM Radio, TV, Cellular Radio
VHF 10 8 TV, Coaxial Cable
10 7 AM Radio, Coaxial Cable
High 10 6 (1MHz) Coaxial Cable
Medium 10 5 Twisted Pair Cables
Low 10 4
Very Low 10 3 (1KHz)
10 2
Extremely Low 10 1
10 0 (1Hz)
There's Always More to Learn
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