More on Propagation

Module B

Copyright 2003 Prentice HallPanko’s Business Data Networks and Telecommunications, 4th edition

Modulation

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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)

Amplitude is the intensity of the signal Loud or soft

Amplitude(power)

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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

Radio Propagation

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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

Optical Fiber

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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

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SC Port

Full-Duplex, So Need Two Plugs per Port

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ST Port

Full-Duplex, So Need Two Plugs per Port