Chapter 4, Transmission Media

• From the front of the chapter
  • Transmission used to convey information can be classified as guided or unguided. Guided media provide a physical path along which the signals are propagated; these include twisted pair, coaxial cable and optical fiber. Unguided media employ an antenna for transmitting through air, vacuum or water.
  • Traditionally, twisted pair has been the workhorse for communications of all sorts. Higher data rates over longer distances can be achieved with coaxial cable, and so coaxial cable has often been used for high speed local area network and for high capacity long distance trunk applications. However, the tremendous capacity of optical fiber has made that medium more attractive than coaxial cable, and thus optical fiber has taken over much of the market for high speed LANs and long distance applications.
  • Unguided transmission techniques commonly used for information communications include broadcast radio, terrestrial microwave, and satellite. Infared transmission is used in some LAN applications.
• The characteristics and quality of data transmission are determined by
  • The characteristics of the Signal
  • The characteristics of the Transmission Medium
  • In guided, the medium is the overriding factor
  • In unguided the signal bandwidth is most important.
    • The higher the frequency the more directed the signal.
    • Low signals are omnidirectional.
    • Higher signals can be focused into a directional beam.
  • The characteristics we are concerned with are:
    • Data rate
    • Distance
  • The factors that influence this are
    • Bandwidth - the greater the bandwidth, the higher the data rate
      • Minimum of Nyquest's and Shannon's laws
    • Transmission impairments - as in the previous chapter
    • Interference - competing signals
    • Number of receivers - (in guided media), These introduce noise and attenuation (maximum fiber connections)
  • Figure 4.1 summarizes the frequencies used by many different transmission media.
• Guided Media
  • Twisted Pair
    • This is currently the least expensive and most widely used guided medium
    • Two wires twisted together. A twisted pair.
    • A number of these pairs are run in a single plastic sheath (4) 8 wires total.
    • The twists vary to reduce the crosstalk.
    • This is the main thing local phone systems are built out of.
    • Local networks have gone this way too.
    • Data rates up to 1Gbps have been achieved.
    • Distance is the main problem here
      • Booster every 5-6 k for analog lines
      • Booster every 300 meeters for digital
      • Shorter at higher data rates
    • This medium is very susceptible to noise
    • (Crosstalk from electrical wires)
    • UTP
      • Unshielded Twisted Pair
      • No special shielding
      • Normally extra wires are pulled into a building to facilitate communications needs.
      • Low cost
      • Cat 3 (Category 3)
        • This is an industry standard
        • First datagrade cable
        • Frequencies of up to 16 MHz
        • Up to 16 mbps could be achieved
        • Using multiple pairs, VGAnylan from HP could achieve 100mbps
      • Cat 5 (Category 5)
        • Frequencies up to 100MHz.
        • More Twists per uint than cat 3
        • This is more expensive, but provides better performance.
      • These networks tend to be point to point
      • They have modular plugs.
      • Generally devices have an led to indicate connection.
  • Coax
    • Coax has a much lower loss per distance.
    • Is a single center cable, insulator, outer cable, insulation.
    • Much less interference and crosstalk.
    • Television, LANS, Cable Network
    • Larger frequency range
  • Fiber
    • Single glass tube surrounded a different glass tube
    • Outer tube is called a jacket.
    • Light reflects through the tubes.
    • Greater range of frequencies
    • Much higher frequencies
    • Greater data rate
    • Lower attenuation
    • No effects from electromagnetic radiation
    • Longer Distances.
    • Single mode - no real bouncing through the cable
      • Narrow inner core
      • Outcoming beam
      • More expensive
    • Multimode - multiple bounces
      • More distortion
      • Less expensive.
• Unguided Media
  • Two basic types
    • Omnidirectional - broadcast
    • Directional - point to point
  • Three frequencies
    • 2GHz - 40GHz, microwave
    • 30 MHz - 1 GHz, omnidirectional, Broadcast radio
    • 3x10¹¹ - 2x10¹⁴ are inferred
  • Microwave
    • Parabolic dish. 3 M in diameter.
    • Distance between antennas is given by
    • d = 7.14 sqrt(Kh)
      • d = distance in kilometers
      • h = antenna height
      • K adjusts for refracting of signal = 4/3
    • So two antennas 100m in the air can be 82 Km apart.
    • This assumes an unobstructed view.
    • This type of communication is point to point and a series of towers can strung together.
    • Primarily used as alternative to guided media for long haul transmission lines.
    • Also used to circumvent local telephone charges in a datalink between two sites.
    • Characteristics

      • Frequency (GHz)	Bandwidth (MHz)	Data Rate (Mbps)
        2	7	12
        6	30	90
        11	40	135
        18	220	274

      • Attenuation is the primary source of loss
        • L = 10 log(4*pi*d/gamma)² dB
        • d is distance
        • gamma is the wavelength
        • Above 10GHz rainfall is very disruptive.
      • Transmission overlap is also a problem
        • Higher frequencies are more susceptible to attenuation.
        • They require smaller antennas.
        • They are mostly used for short haul data comm.
        • 4-6GHz is for telecommunications
        • 12GHz is for cable tv.
        • 22GHz is for data comm.
  • Satellite Microwave
    • The satellite is a microwave relay station.
    • It receives a signal (uplink)
    • It rebroadcasts that signal (downlink)
    • It can operate on a number of frequencies called transponder channels or transponders
    • This can be either point to point, or broadcast
    • They must remain at the same spot wrt the earth
      • The orbit is at a height of 35,784 km
    • They must be far enough apart so that the don't interfere with each other.
    • Applications
      • Television
        • Central broadcast facility, many down links for rebroadcast.
        • PBS is almost completely done this way
        • Direct Broadcast Satellite - Direct TV
          • From the Directv website
          • 5 satellites (DBS-1, DBS-2, DBS-3, and DIRECTV1-R, TMPO2)
          • Located 22,300 miles up, (35,880 km)
          • Two broadcast centers , download programs from satellite and cable
          • Uses MPEG-2 compression
          • 30Mbps data rate.
          • They claim a two second delay from Central system to home receiver.
      • Long Distance Telephone Transmission
      • Private Business Networks
    • Transmission Characteristics
      • Below 1GHz noise becomes a real problem
      • Above 10GHz the atmosphere effects the signal
      • 5.925 - 6.425GHz uplink
      • 4.2 - 4.7 GHz downlink
      • This is becoming full.
      • 1/4 second transmission delay between earth stations.