Carrier Trunk Lines

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Carrier Trunk Lines
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Carrier Trunk Lines

• Trunk lines are high-speed lines that connect the
  switches of carriers
• There are several kinds of trunk lines
• Optical fiber
• Radio transmission
• Microwave transmission
• Satellite transmission
• LEOs
• VSATs

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Optical Fiber

• Thin Core of Glass
• Surrounded by glass cladding
• Inject light in on-off pattern for 1s and 0s
• Total reflection at core-cladding boundary
• Little loss with distance

Cladding
Core
Light Source
Reflection
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Optical Fiber

• Modes
• Light entering at different angles will take
  different amounts of time to reach the other end
• Different ways of traveling are called modes
• Light modes from successive bits will begin to
  overlap given enough distance, making the bits
  unreadable

Light Source
Reflection
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Single Mode Fiber

• Single Mode Fiber is very thin
• Only one mode will propagate even over fairly
  long distances
• Expensive to produce
• Expensive to install (fragile, precise alignments
  needed)
• Used by carriers to link distant switches

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Multimode Fiber

• Core is thick
• Modes will appear even over fairly short
  distances
• Must limit distances to a few hundred meters
• Inexpensive to purchase and install
• Dominates LANs

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Graded Index Multimode Fiber

• Index of fraction is not constant in core
• Varies from center to edge
• Reduces time delays between different modes
• Can go farther than if core has only a single
  index of fraction (step index multimode fiber)
• Dominates multimode fiber today

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Multimode Optical Fiber and Frequency

• Signal Frequency Determines the Propagation
  Distance before Mode Problems Become Serious
• Short Wavelength (high frequency)
• Signals do not travel as far before mode problems
  occur
• Uses the least expensive light sources
• Good for LAN use within buildings
• Long Wavelength (low frequency)
• Signals travel farther but light sources cost
  more
• Within large buildings and between buildings

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Wave Division Multiplexing

• Use multiple light sources of different
  frequencies
• Place a separate signal on each
• Increases the capacity of the optical fiber

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Radio Transmission

• Oscillating electron generates electromagnetic
  waves with the frequency of the oscillation
• Many electrons must be excited in an antenna for
  a strong signal

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Radio Propagation

• Propagation Characteristics Depend on Frequency
• At lower frequencies, signals bend around
  objects, pass through walls, and are not
  attenuated by rain
• At higher frequencies, there is more bandwidth
  per major band

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Major Bands

• Frequency Spectrum is Divided into Major Bands
• Ultra High Frequency (UHF)
• Signals still bend around objects and pass
  through walls
• Cellular telephony
• Super High Frequency (SHF)
• Needs line-of-sight view of receiver
• Rain attenuation is strong, especially at the
  higher end
• High channel capacity
• Used in microwave, satellites

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Microwave Transmission

• Terrestrial (Earth-Bound) System
• Repeaters can relay signals around obstacles

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Satellite Transmission

• Essentially, places repeaters in sky
• Idea thought of by Sir Arthur C. Clarke
• Satellite broadcasts to an area called its
  footprint
• Uplink is to satellite downlink is from satellite

Uplink
Downlink
Footprint
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Some Popular Satellite Frequency Bands