More on Propagation
Module B
Copyright 2003 Prentice HallPanko’s Business Data Networks and Telecommunications, 4th edition
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Modulation
Modulation converts an digital computer signal into a form that can travel down an ordinary analog telephone line
There are several forms of modulation Amplitude modulation Frequency modulation Phase modulation
Quadrature amplitude modulation (QAM), which combines amplitude and phase modulation
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Waves
Frequency of a wave The number of complete cycles per second
Called Hertz
kHz, MHz, GHz, THz
Frequency (Hz)
Cycles in One Second
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Figure B.1: Frequency Modulation (FM)
LowFrequency
(0)
HighFrequency
(1)
FrequencyModulation
(1011)
Wavelength
Wavelength
1
0
1
1
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Amplitude Modulation (AM) (from Chapter 3)
LowAmplitude
(0)
HighAmplitude
(1)
AmplitudeModulation
(1011)
Amplitude (low)
Amplitude (high)
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Phase
Two signals can have the same frequency and amplitude but have different phases--be at different points in their cycles at a given moment
BasicSignal
180 degreesout of phase
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Figure B.2: Phase Modulation (PM)
In Phase(0)
180 degreesout of phase
(1)
FrequencyModulation
(1011)
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Phase Modulation (PM)
Human hearing is largely insensitive to phase So harder to understand than FM and AM
But equipment is very sensitive to phase changes PM is used in all recent forms of modulation for
telephone modems
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Figure B.3: QAM
Quadrature Amplitude Modulation (QAM) Uses two carrier waves: sine and cosine (out of
phase), both amplitude-modulated
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Figure B.3: QAM
Suppose each carrier wave has four possible amplitude levels In each clock cycle, there are 16 combined
possibilities In each clock cycle, can send 4 bits (2^4=16)
Sine Wave
Cosine(Quadrature)
Wave
High/High1111
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Figure B.4: OFDM
Orthogonal Frequency Division Multiplexing Send signal is a large channel Divide the channel into many subchannels Send part of the signal in each subchannel
Don’t use impaired channels or transmit more slowly
ChannelBandwidth
Subchannel withPart of Signal
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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 (From Figure B.5)
Frequency Spectrum is Divided into Major Bands
Ultra High Frequency (UHF) Signals still bend around objects and pass through
walls Cellular telephony
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Major Bands (From Figure B.5)
Frequency Spectrum is Divided into Major Bands
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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Figure B.6: 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
Broadcasts to an area called its footprint
Uplink is to satellite; downlink is from satellite
UplinkDownlink
Footprint
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Some Popular Satellite Frequency Bands
Band Downlink Frequency
(From Satellite)
Uplink Frequency (To
Satellite)
C 4 GHz 6 GHz
Ku 12 GHz 14 GHz
Ka 20 GHz 30 GHz
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Infrared Transmission
Uses light instead of radio for transmission
Like a television remote control but diffused to reduce line-of-sight limitations
Relatively low speeds
Bright sunlight and light fixtures can interfere with it
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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
LightSource
Cladding
Core
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
LightSource
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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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
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Frequency
Signal Frequency Determines the Propagation Distance before Mode Problems Become Serious
Long Wavelength (low frequency) Signals travel farther but light sources cost more
Within large buildings and between buildings
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Frequency
There are 3 frequency windows where optical fiber attenuation is very low
850 nm: Shortest distance but lease expensive optical transceivers
1300 nm: Good balance of cost and distance
1550 nm: Longest distance and most expensive
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Fiber Quality
Some fiber is higher quality in terms of internal construction
Measured as bandwidth x distance product 160 MHz x km is the most common
1 km with a light signal bandwidth of 160 MHz (highest bandwidth so highest speed)
4 km with a light signal bandwidth of 40 MHz
200 MHz x km is becoming more popular
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SC and ST Connectors
For UTP, there is only a single connector, RJ-45
For optical fiber, several connectors are popular
SC: squarish, snaps into port, recommended in TIA/EIA-568
ST: tubular, bayonnette connection, popular
Several popular small form factor connectors
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SC and ST Connectors
STConnectors(Popular)
SCConnectors
(Recommended)
Two fiber cords for full-duplex (two-
way) transmission