Notes On Chapter Seven -- Transmission Media

• 7.1 Introduction
  
  
  • media: wired, wireless, and optical
  • concepts of electromagnetic propagation
  • shielding
  • capacity
  
  
• 7.2 Guided and Unguided Transmission
  
  
  • wire and optical fiber are guided media - sometimes both referred to as "wired"
  • radio transmission is unguided, referred to as "wireless"
  
  
• 7.3 Taxonomy by Forms of Energy
  
  
  • Energy Types
    • Electrical
      • twisted pair
      • coaxial cable
    • Light
      • optical fiber
      • infra red
      • laser
    • Radio (Electromagnetic)
      • terrestrial radio
      • satellite
  
  
• 7.4 Background Radiation and Electrical Noise
  
  
  • Electromagnetic radiation caused by things like motors and light fixtures can induce current flow in wires used for network communication.
  • Such induced electrical flow is noise that can corrupt the data carried by the wires.
  • Metal absorbs electromagnetic radiation, and so it can be used to shield wires from the noise.
  
  
• 7.5 Twisted Pair Copper Wiring
  
  
  • Two wires twisted together
  • Each wire electrically insulated with plastic
  • If wires are parallel, not twisted then one is likely to shield the other. If one wire in the circuit absorbs more radiation than the other then the radiation is likely to cause a net change to the electrical flow - noise.
  • If the wires are twisted, then each tends to be exposed equally to radiation. Thus external radiation is less likely to have any net effect on the electrical flow - less chance of noise.
  
  
• 7.6 Shielding: Coaxial Cable and Shielded Twisted Pair
  
  
  • Twisted pair wiring has problems when:
    • the noise is especially strong or close, or
    • when the data is transmitted at high frequency. (Much noise is high-frequency.)
  • Coaxial cable is more effective at protecting network wiring in these situations.
  • A braided metal shield protects an inner signal wire from interference, and other wires from the interference the signal wire might cause.
  • Shielded Twisted pair is a variation that is more flexible than coaxial cable.
  
  
• 7.7 Categories of Twisted Pair Cable
  
  
  • The American National Standards Institute (ANSI), the Telecommunications Industry Association (TIA), and the Electronic Industries Alliance (EIA) created specifications for network wiring categories, e.g. CAT 1 through CAT 7. (See page 118.)
  
  
• 7.8 Media Using Light Energy and Optical Fibers
  
  
  • Optical fibers are thin strands of glass or plastic surrounded by a cladding that prevents light from leaking out.
  • A laser or light-emitting diode (LED) at one end transmits.
  • At the other end a device detects the incoming information.
  • Fiber is flexible - the light bounces around inside and keeps going toward the destination, although there is significant dispersion over longer distances.
  
  
• 7.9 Types of Fiber and Light Transmission
  
  
  • Three grades of optical fiber are:
    • Multimode Step Index - high dispersion
    • Multimode Graded Index - less dispersion
    • Single Mode - least dispersion
  • Single Mode fiber is used for long distance, high bit-rate applications.
  
  
• 7.10 Optical Fiber Compared to Copper Wiring
  
  
  • Optical Fiber:
    • is immune to electromagnetic interference
    • has low attenuation
    • has high bandwidth
  • Copper Wire:
    • costs less
    • requires less expertise and work to install
    • is less likely to break
  
  
• 7.11 InfraRed Communication Technologies
  
  
  • IR is just below the lowest colors of visible light that people can see, and its characteristics are quite similar to visible light.
  • IR disperses quickly, can reflect off a smooth hard surface, is easily blocked by anything opaque, and is also blocked by moisture in the air.
  • IR is good for indoor, low bandwidth applications, where there are clear lines of sight between communicating objects.
  • Applications: TV remote controls, connections from computers to printers and other peripherals.
  • Bandwidth ranges from 100Kps to 4Mbps.
  
  
• 7.12 Point-To-Point Laser Communication
  
  
  • Laser connections are similar to IR in some ways
  • It requires clear line of sight and can be blocked by moisture in the air.
  • Unlike IR, the beam of light remains pencil-thin over a long distance.
  • It has relatively high bandwidth
  • Transmitters and receivers must be precisely aligned.
  • It can be used outdoors as a substitute for stringing wires.
  • Initial cost is high but upkeep is low.
  
  
• 7.13 Electromagnetic (Radio) Communication
  
  
  • Radio Frequencies (RF) can travel long distances and go through walls.
  • RF ranges from about 3KHz to 300GHz
  • Government agencies, like the FCC regulate the allocation of electromagnetic frequencies for communication and the setting of allowable power outputs at each range of frequencies.
  • Frequencies:
    • Low Frequencies: 10⁰ to 10⁴ Hz
    • Radio and TV: 10⁴ to 10⁸ Hz
    • Microwave: 10⁸ to 10¹⁰ Hz
    • Infra Red: 10¹⁰ to 10¹⁴ Hz
    • Visible Light: 4 X 10¹⁴ to 7.9 X 10¹⁴ Hz
    • Ultra Violet (UV): 10¹⁵ to 10¹⁶ Hz
    • X-Rays: 10¹⁶ to 10²² Hz
    • Gamma Rays: 10²² to 10²⁴ Hz
    
    
  • 7.14 Signal Propagation
    
    
    • An implication of Nyquist's theorem is that the Baud rate on a communication channel is limited to about twice the frequency.
    • Low Frequency RF can handle about 2 Mbps and can follow the earth's curvature.
    • Medium Frequency RF can handle 2 to 30 Mbps and can reflect off the atmosphere (hence longer range).
    • High Frequency RF can handle more than 30 Mbps, but is "line of sight."
    
    
  • 7.15 Types of Satellites
    
    
    • Low Earth Orbit - low delay, they move in relation to observers on ground.
    • Medium Earth Orbit - elliptical orbit - used for communication at the poles.
    • Geostationary Earth Orbit - fixed position relative to earth, but longer delay.
    
    
  • 7.16 GEO Communication Satellites
    
    
    • GEO satellites orbit at an altitude of 35,785 kilometers (22,236 miles) where the orbital period is exactly one day.
    • Thus a GEO satellite orbiting in the plane of the earth's equator remains stationary directly over one location on the earth.
    • Antenna's don't have to be continually re-aimed at such a GEO satellite.
    • The ground-to-GEO-satellite-to-ground delay for a communication signal is about 0.238 seconds - about 1/4 of one second.
    • For some applications, this delay is significant.
    
    
  • 7.17 GEO Coverage of the Earth
    
    
    • If GEO satellites are too close together, their transmissions will interfere with each other.
    • At current power levels, there is room for 45-90 GEO satellites in orbit above the equator.
    • Three such satellites at a separation of 120-degrees provide coverage of the 'entire earth' -- in other words, from any point on the earth, at least one of them can be observed in the sky. (There are probably locations near the north and south poles where none are visible or where the only ones visible touch the horizon.)
    
    
  • 7.18 Low Earth Orbit (LEO) Satellites and Clusters
    
    
    • Low Earth Orbit Satellites operate at altitudes of 500 to 2000 kilometers.
    • Delays are in the 1 to 5 millisecond range.
    • Tracking from the ground is difficult because the satellites move very fast relative to the ground.
    • Typically LEO satellites work as clusters, for example:
      • ground talks to satellite over Paris
      • satellite over Paris relays to satellite X
      • X relays to satellite Y, Y to Z.
      • This continues until the information is relayed to a satellite currently over the intended destination, say Tokyo.
      • Finally the satellite over Tokyo transmits the information to a receiver on the ground.
    
    
  • 7.19 Tradeoffs Among Media Types
    
    
    • Consideration must be give to:
      • Cost: of materials, installation, operation and maintenance
      • Data Rate: Number of bits per second that can be sent
      • Delay: time required for propagation and processing
      • Effects on Signals: attenuation and distortion
      • Environment: susceptibility to interference/noise
      • Security: susceptibility to eavesdropping/sabotage
    
    
  • 7.20 Measuring Transmission Media
    
    
    • One can derive the following from Nyquist's theorem:
      
      The Max Data Rate on a Communication channel is twice the (analog) bandwidth times the base-two log of the number of possible signal levels.
      
      
    • In symbols: D_max = 2 B log₂K
    
    
  • 7.21 The Effect of Noise on Communication
    
    
    • The Shannon-Hartley Law expresses how the presence of noise puts limitations on maximum data rates:
    • The maximum channel capacity is the product of the hardware bandwidth and the the base-two log of one plus the signal-to-noise ratio.
    • In symbols: D_max = B log₂(1 + S/N)
    • S and N are the power levels of the signal and the noise.
    • For example, if B=1000 Hz, S=70 and N=10, then the Shannon-Hartley Law predicts that no more than 3,000 bits per second can be effectively sent on the channel.
    • Sometimes a signal to noise ratio is converted to decibels:
    • dB = 10 log₁₀(S/N)
    
    
  • 7.22
    
    
    • Nyquist's theorem encourages us to look for clever ways to increase data transmission rates (media with higher bandwidth, better ways to encode more signal levels).
    • The Shannon-Hartley Law reminds us that things such as background radiation present everywhere in the universe will always be there holding us back from achieving ideal data rates.
    
    

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