Physical: Transmission Media

• A van full of tapes
• Wires
  • Twisted pair
    • used in LAN, telephones
    • twists reduce radiation (and interference)
    • shielded (STP) or unshielded (UTP, like Cat 3/5/5e/6/6A/7)
    • connectors: RJ11 (4 wires), RJ45 (8 wires, modern buildings)
    • link types
      • simplex — only one fixed direction at all times
      • half-duplex — both directions, but not at the same time, senders take turns
      • full-duplex — both directions at once (so use different twisted pairs for each direction)
  • coaxial
    • better shielding, more bandwidth, longer distances, higher rates
    • copper core surrounded by insulation and covering
  • power lines
    • e.g. household electrical wiring
    • convenient to use, bad for data transmission
  • cheap and easy, but short-distance with moderate bandwidth and low security
• Fibers
  • utilise total internal reflection in a thin strand of glass (silica fiber)
  • high rates and long distances, enormous bandwidth in THz with little loss
  • types
    • single-mode — narrow, 10 µm core where light can’t even bounce. used with lasers for long distances
    • multi-mode — light can bounce in 50 µm core, used with LEDs
  • an example is TAT-14 TransAtlantic, with two main and two backup fiber pairs on the ocean floor (3 Tbps capacity)
  • very secure and fast, but less convenient and more expensive
• Wireless
  • compared to wires, deployment is easy and inexpensive, with natural support for broadcasting
  • however, transmissions interfere and data rates vary with signal strength
  • Electromagnetic spectrum — carefully divided and regulated, except for WiFi ISM bands
  • radio waves
    • λf = c in vacuum (freq f, wavelength λ)
    • radio/light travels at around 1 foot/nsec
    • penetrate buildings, propagate long distances
      • Very Low Frequency, LF, MF bands — waves follow curvature of earth
      • HF band — waves bounce off ionosphere
  • microwaves
    • high bandwidth, used in WiFi and 3G/satellites
    • strength varies with mobility due to multipath fading (delayed waves arriving out of phase with direct waves & cancelling out)
  • light transmission
    • line-of-sight can be used for links
    • highly directional, lots of bandwidth
    • use LEDs/cameras and lasers/photodetectors
• Satellites
  • Geostationary (GEO)
    • 36,000 km, delay ~250msec up/down
    • does not need tracking
    • VSAT can use it to communicate through a hub
  • Low (LEO)
    • good for coverage, e.g. Iridium’s 66 satellite network for comms routing
    • can relay to each other, or on ground using a bent pipe