Transcript Carrier Trunk Lines

Carrier Trunk Lines
Carrier Trunk Lines
• Trunk lines are high-speed lines that connect the
switches of carriers
• There are several kinds of trunk lines
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Optical fiber
Radio transmission
Microwave transmission
Satellite transmission
• LEOs
• VSATs
Optical Fiber
• Thin Core of Glass
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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
• Modes
Optical Fiber
– 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
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
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
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
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
Wave Division Multiplexing
• Use multiple light sources of different
frequencies
– Place a separate signal on each
– Increases the capacity of the optical fiber
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
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
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)
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Needs line-of-sight view of receiver
Rain attenuation is strong, especially at the higher end
High channel capacity
Used in microwave, satellites
Microwave Transmission
• Terrestrial (Earth-Bound) System
• Repeaters can relay signals around obstacles
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
Footprint
Downlink
Some Popular Satellite
Frequency Bands
Band
C
Downlink
Uplink
Frequency
Frequency (To
(From Satellite)
Satellite)
4 GHz
6 GHz
Ku
12 GHz
14 GHz
Ka
20 GHz
30 GHz