Wire Propagation Effects

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Wire Propagation Effects

• Propagation Effects
• Signal changes as it travels
• If change is too great, receiver may not be able
  to recognize it

Original Signal
Final Signal
Distance
2
Wire Propagation Effects Attenuation

• Attenuation Signal Gets Weaker as it Propagates
• May become too weak for receiver to recognize

Signal Strength
Distance
3
Wire Propagation Effects Distortion

• Distortion Signal changes shape as it propagates
• Adjacent bits may overlap
• May make recognition impossible for receiver

Distance
4
Wire Propagation Effects Noise

• Noise Thermal Energy in Wire Adds to Signal
• Noise floor is average noise energy
• Random energy, so noise spikes sometimes occur

Spike
Signal
Signal Strength
Error
Noise
Noise Floor
Time
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Wire Propagation Effects

• Noise and Attenuation
• As signal attenuates, gets closer to noise floor
• So noise errors increase with distance, even if
  the average noise level is constant

Signal Strength
Signal
Noise Floor
Distance
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Wire Propagation Effects SNR

• Want a high Signal-to-Noise Ratio (SNR)
• Signal strength divided by average noise strength
• As SNR falls, errors increase

Signal Strength
Signal
SNR
Noise Floor
Distance
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Wire Propagation Effects Noise Speed

• Noise and Speed
• As speed increases, each bit is briefer
• Noise fluctuations do not average out as much
• So noise errors increase as speed increases

OK
Error
One Bit
One Bit
Noise Spike
Noise Spike
Low Speed (Long Duration)
High Speed (Short Duration)
Average Noise During Bit
Average Noise During Bit
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Wire Propagation Effects Interference

• Interference
• Energy from outside the wire (nearby motors,
  other wires, etc.)
• Adds to signal, like noise
• Often intermittent (comes and goes), so hard to
  diagnose

Signal
Signal Strength
Interference
Time
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Wire Propagation Effects Cross-Talk Interference

• Cross-Talk Interference
• Often, multiple wires in a bundle
• Each radiates some of its signal
• Causes cross-talk interference in nearby wires

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Wire Propagation EffectsCross Talk

• Wire Usually is Twisted
• Usually, several twists per inch
• Interference adds to signal over half twist,
  subtracts over other half
• Roughly cancels out
• Simple but effective

-

Interference
Signal
Single Twist
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Wire Propagation EffectsCross Talk

• Terminal Cross-Talk Interference
• Wire must be untwisted at ends to fit into
  connectors
• So cross-talk interference is high at termination
• Problems severe if untwist more than about 1.25
  cm (1/2 inch)
• Usually the biggest propagation effect

Terminal Cross Talk
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Practical Issues in Propagation Effects

• Distance limits in standards prevent serious
  propagation effects
• For instance, usually 100 meters (328 feet)
  maximum for ordinary copper wire
• Problems usually occur at connectors
• Crossed wires
• Poor connections
• Cross-talk interference

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Wire Media UTP to the Desktop

• UTP
• Dominant for line from desktop to first switch
• Inexpensive to buy and install
• Rugged can take punishment of office work
• Easily 100 Mbps, 1 Gbps with careful insulation

UTP
First Hub or Switch
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Categories of UTP

• Cat 1 - traditional telephone cabling designed
  for voice and not data (2 wire pairs RJ-11)
• Cat 2 - certified for transmissions of 4Mpbs (not
  used in networks where typical transmissions are
  gt10Mbps
• Cat 3 - certified for transmission up to 10Mbps
• in 8 wire pair with RJ-45
• not recommended for new installations because
  transmission speeds are gt 10 Mbps 100 Mbps

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Categories of UTP

• Cat 4 - certified for transmissions up to
  16-20Mbps
• Cat 5 - certified for transmission up to 100 Mbps
• typical cable in use today
• Enhanced Cat 5 - more twists transmission up to
  200 Mbps
• Cat 6 - extra foil insulation
• supports up to 600Mbps

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Wire Media Optical Fiber

• Limited by Distortion
• Light entering at different angles travels
  different distances (different number of
  reflections)
• 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
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Wire Media Optical Fiber
Md B

• Multimode Fiber
• Wide core makes easy to splice (50 or 62 microns)
• Many angles for rays (modes)
• Short propagation distance (usually 200 m to 500
  m)

Light Source
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Wire Media Optical Fiber
Mod B

• Single Mode Fiber
• Narrow core difficult to splice (5 or 8 microns)
• Only one angle for rays (one mode), so (almost)
  no distortion
• Longer propagation distance (usually up to 2 km
  for LAN fiber, longer for long-distance fiber)
• Narrow core makes fiber fragile and difficult to
  splice

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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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Wire Media Optical Fiber

• Optical Fiber
• High speeds over long distances
• 200 m to 2 km
• Costs more than UTP, but worth it on long runs
• Good for all links between hubs and switches
  within and between buildings in a site network

Optical Fiber
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Wire Media UTP and Optical Fiber

• The emerging pattern UTP from first hub or
  switch to desk, Fiber everywhere else on site

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Wire Media Coax

• Coaxial Cable
• Used in cable TV, VCRs
• Central wire, external concentric cylinder
• Outer conductor wrapped in PVC

Inner Wire
Outer Conductor Wrapped in PVC
Screw-On Connector
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Wire Media Coaxial Cable

• Coaxial Cable
• Installed widely today in old 10 Mbps Ethernet
  LANs
• ThinNet and ThickNet
• Not being used in new installations
• Optical fiber more cost-effective for long links
• UTP more cost-effective for desktop links

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Duplex

• Full-duplex transmission both sides can transmit
  simultaneously
• Even if only one sends, still full-duplex line
• Even if neither is sending, still full-duplex line

A
B
A
B
A
B
Time 1 Both can send Both do
Time 1 Both can send Only A does
Time 1 Both can send Neither does
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Duplex

• Half-duplex transmission only one can transmit
  at a time must take turns
• Still half duplex if neither transmits

Time 1 Only one side Can send A does
Time 2 Only one side Can send Neither does
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Duplex

• Duplex is a Characteristic of the Transmission
  System, Not of Use at a Given Moment
• In full duplex, both sides can transmit at once
  in half duplex, only one side can transmit at a
  time
• Still full duplex system if only one side or
  neither side actually is transmitting at a moment
• Still half duplex if neither side actually is
  transmitting at a moment

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Wireless Transmission

• Infrared -uses light beams to send signals
  between pairs of devices
• Direct - transmitter and receiver are within
  line-of-sight of each other
• ex. Laptop and printer in same room
• Indirect - signal bounces off walls , ceilings
  etc.

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Radio Propagation

• Broadcast signal
• Not confined to a wire

33
Radio Waves

• When Electron Oscillates, Gives Off Radio Waves
• Single electron gives a very weak signal
• Many electrons in an antenna are forced to
  oscillate in unison to give a practical signal

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Radio Propagation Problems
Nw

• Radio Propagation is Difficult
• Signals are reflected
• May arrive at a destination via multiple paths
• Signals arriving by different paths can interfere
  with one another
• This is called multipath interference

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Radio Propagation Problems

• Wires Propagation is Predictable
• Signals go through a fixed path the wire
• Propagation problems can be easily anticipated
• Problems can be addressed easily
• Radio Propagation is Difficult
• Signals begin propagating as a simple sphere
• Inverse square law attenuation
• If double distance, only ¼ signal strength
• If triple distance only 1/9 signal strength

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Radio Propagation Waves

• Waves

Frequency in hertz (Hz) Cycles per Second
One Second 7 Cycles
Wavelength (meters)
Amplitude (strength)
1 Hz 1 cycle per second
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Radio Propagation Frequency Spectrum

• Frequency Spectrum
• Frequencies vary (like strings in a harp)
• Frequencies measured in hertz (Hz)
• Frequency spectrum all possible frequencies from
  0 Hz to infinity

0 Hz
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Frequencies

• Metric system
• kHz (1,000 Hz) kilohertz note lower-case k
• MHz (1,000 kHz) megahertz
• GHz (1,000 MHz) gigahertz
• THz (1,000 GHz) terahertz

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Radio Propagation Service Bands

• Service Bands
• Divide spectrum into bands for services
• A band is a contiguous range of frequencies
• FM radio, cellular telephone service bands etc.

Cellular Telephone
Service Bands
FM Radio
AM Radio
0 Hz
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Radio Propagation Channels and Bandwidth

• Service Bands are Further Divided into Channels
• Like television channels
• Bandwidth of a channel is highest frequency minus
  lowest frequency

Channel 3
Service Band
Channel 2
Channel 1
0 Hz
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Radio Propagation Service Bands

• Service Bands
• Divide spectrum into bands for services
• A band is a contiguous range of frequencies
• FM radio, cellular telephone service bands etc.

Cellular Telephone
Service Bands
FM Radio
AM Radio
0 Hz
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Radio Propagation Channels and Bandwidth

• Example
• Highest frequency of a radio channel is 43 kHz
• Lowest frequency of the radio channel is 38 kHz
• Bandwidth of radio channel is 5 kHz (43-38 kHz)

Channel Bandwidth
Channel 3
Service Band
Channel 2
Channel 1
0 Hz
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Radio Propagation Channels and Bandwidth

• Shannons Equation
• W is maximum possible (not actual) transmission
  speed in a channel
• B is bandwidth of the channel highest frequency
  minus lowest frequency
• S/N is the signal-to-noise ratio

W B Log2 (1 S/N)
D 2 B Log2 K (Nyquists Theorem)
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Radio Transmission Broadband

• Speed and Bandwidth
• The wider the channel bandwidth (B), the faster
  the maximum possible transmission speed (W)
• W B Log2 (1S/N)

Maximum Possible Speed
Bandwidth
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Telephony is Narrowband

• Bandwidth in Telephone Channels is Narrow
• Sounds below about 300 Hz cut off to reduce
  equipment hum within telephone system
• Sounds above about 3,400 Hz cut off to reduce the
  bandwidth needed to send a telephone signal

3.1 kHz
20 kHz
300 Hz
3.4 kHz
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Telephony is Narrowband

• Bandwidth in Telephone Channels is Narrow
• A radio channel would have to be from 0 to 3.4
  kHz (3.4 kHz)
• This would mean a maximum possible transmission
  speed of about 35 kbps

Required Radio Channel
3.1 kHz
20 kHz
300 Hz
3.4 kHz
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Broadband

• Two Uses of the Term Broadband
• Technically, the signal is transmitted in a
  single channel AND the bandwidth of the channel
  is large
• Therefore, maximum possible transmission speed is
  high
• Popularly, if the signal is fast, the system is
  called broadband whether it uses channels at all