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• Guided Transmission Media


  Provides a conduit in which electromagnetic signals are contained
• Unguided Transmission Media


  Emitted then radiated through air or a vacuum
• Cable Transmission Medium


  Used to propagate electromagnetic signals between two locations in a communications system
• Cable Transmission Systems


  Most common means of interconnecting devices in local area networks
• Transmission line


  Metallic conductor system used to transfer electrical energy from one point to another using electrical current flow
• Longitudinal and transverse waves


  Two basic kinds of waves
• Frequency


  The rate at which the periodic wave repeats
• Metallic circuit currents


  Currents that flow in opposite directions in a balanced wire pair
• Longitudinal currents


  Currents that flow in the same direction
• Common mode rejection


  Cancellation of common mode signals
• Single-ended or unbalanced


  One wire is at the ground potential, whereas the other is at signal potential
• Balun


  A circuit device used to connect a balanced transmission line to an unbalanced load
• Most common metallic cables

  • Parallel-conductor transmission lines
  • Coaxial transmission lines
• Twisted-pair


  Formed by twisting two insulated conductors around each other
• Types of twisted pair

  • Unshielded twisted pair
  • Shielded twisted pair
• Near-end crosstalk


  Coupling that takes place when a transmitted signal is coupled into the received signal at the same end of the cable
• Standard color code for CAT-5 cable

  • Pair 1: blue/white stripe and blue
  • Pair 2: orange/white stripe and orange
  • Pair 3: green/white strip[e and green
  • Pair 4: brown/white stripe and brown
• Braid


  Woven into a mesh
• Plenum


  Name given to the area between the ceiling and the root in a single-story building or between the ceiling and the floor of the next higher level in a multi-story building
• Coaxial


  Used for high data transmission rates to reduce losses and isolate transmission path
• Shielding


  Refers to the woven stranded mesh that surrounds some types of coaxial cables
• Dual shielded


  One layer of foil insulation and one layer of braided shielding
• Types of coaxial cables

  • Rigid air-filled
  • Solid flexible
• Distributed parameters


  Uniformly distributed throughout the length of the line
• Secondary constants


  Transmission characteristics of a transmission line
• Surge impedance


  Impedance seen looking into an infinitely long line
• Propagation constants


  Expressed the attenuation and the phase shift per unit length of a transmission line
• Velocity factor


  Ratio of the actual velocity of propagation of an electromagnetic wave through a given medium to the velocity of propagation through a vacuum
• Dielectric constant


  Relative permittivity of a material
• Ways in which signal power is lost

  • Conductor loss
  • Radiation loss
  • Dielectric heating loss
  • Coupling loss
  • Corona
• Incident voltage


  Voltage that propagates from the source toward the load
• Reflected voltage


  Voltage that propagates that propagates from the load toward the source
• Flat or non resonant line


  Transmission line with no reflected power
• Reflection coefficient


  Vector quantity that represents the ratio of reflected voltage to incident voltage or reflected current to incident current
• Matched line


  Incident power is absorbed by the load
• Unmatched or mismatched line

  Incident power returned (reflected) to the source
• Standing wave


  Two travelling waves set up an interference pattern
• Standing-wave ratio


  Ratio of the maximum voltage to the minimum voltage or the maximum current to the minimum current of a standing wave on a transmission line
• Quarter-wavelength transformers


  Used to matched transmission lines to purely resistive loads whose resistance is not equal to the characteristic impedance of the line
• Time domain reflectometry (TDR)


  A technique that can be used to locate an impairment in a metallic cable