Air Interface

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Air Interface

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Analog Transmission

• In analog transmission, the state of line can
  vary continuously and smoothly among an infinite
  number of states
• States can be signal strengths, voltages, or
  other measurable conditions
• Human voice is analog telephone mouthpiece
  generates analogous electrical signal

Strength
Time
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Digital Transmission

• Time is divided into fixed-length clock cycles
• Modems a few thousand clock cycles per second
• LANs millions of clock cycles per second
• The line is kept in one of only a few possible
  states (conditions) during each time period
• this is why the signal must be kept constant
• At the end of each time period, the line may
  change abruptly to another of these few states

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Digital Versus Binary Transmission

• Digital transmission a few states
• Binary transmission exactly two states (1 and 0)
• Binary is a special case of digital

Two States
Few States
1
0
Digital
Binary
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Digital Versus Binary Transmission

• Sender and Receiver associate one or more bits
  with each state
• Simplest case High state 1, Low state 0
• If four states, might have the following
• Highest 11
• Second highest 10
• Next highest 01
• Lowest 00

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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
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Wire Propagation Effects Attenuation

• Attenuation Signal Gets Weaker as it Propagates
• May become too weak for receiver to recognize
• Distortion Signal changes shape as it propagates
• Adjacent bits may overlap
• May make recognition impossible for receiver

Signal Strength
Distance
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Wire Propagation Effects Noise

• Noise Thermal Energy in Wire Adds to Signal
• Noise floor is average noise energy
• Noise spikes are random energy affecting bits

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
• Smaller spikes can harm the signal
• So noise errors increase with distance, even if
  the average noise level is constant
• Want a high Signal-to-Noise Ratio (SNR)
• Signal strength divided by average noise strength
• As SNR falls, errors increase

Noise Floor
SNR
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
• External signal converted to electrical energy
• Adds to signal, like noise
• Often intermittent (comes and goes), so hard to
  diagnose
• Often called electromagnetic interference (EMI)

Signal
Signal Strength
Interference
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Wire Propagation Effects Cross-Talk Interference

• Cross-Talk Interference
• Multiple wires in a bundle each radiates its
  signal
• Causes cross-talk interference in nearby
  wires
• Wire Usually is Twisted
• Several twists per inch
• Interference adds to signal over half twist,
  subtracts over other half

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Practical Issues in Propagation Effects

• Distance limits in standards prevent serious
  propagation effects
• Usually 100 meters maximum for ordinary copper
  wire
• Problems usually occur at connectors
• Crossed wires
• Poor connections
• Cross-talk interference

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Radio Propagation

• Broadcast signal
• Not confined to a wire

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Radio Waves

• When Electron Oscillates, Gives Off Radio Waves
  (electromagnetic 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

• 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
• Signals can be blocked by dense objects
• Creates shadow zones with no reception

Shadow Zone
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Radio Propagation Problems

• 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 called multipath interference
• Can be constructive or destructive interference
• Very different reception characteristics with in
  a few meters or centimeters

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Radio Propagation Waves

• Waves

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Frequency in hertz (Hz) Cycles per Second
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One Second 7 Cycles
Wavelength (meters)
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Amplitude (strength)
2
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
• 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

0 Hz
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Radio Propagation Service Bands

• Service Bands
• Divide frequency 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
• 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 FM Radio
Channel 2
Channel 1
0 Hz
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Radio Propagation Channels and Bandwidth

• Shannons Equation -- W B Log2 (1S/N)
• W is maximum possible (not actual) transmission
  speed in channel
• B is bandwidth of channel highest frequency -
  lowest frequency
• S/N is the signal-to-noise ratio
• The wider the channel bandwidth (B), the faster
  the maximum possible transmission speed (W)

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Broadband vs. Baseband

• Baseband Inject signal into medium propagates
• Broadband Different signals sent different
  channels
• Begin with baseband signal
• Modulate to fit in radio frequency signal (RF)
• Channel bandwidth is wide broadband
  transmission
• Channel bandwidth is narrow narrowband
  transmission