Air Interface
2
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
Digital Transmission

Time is divided into fixed-length clock cycles
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–

The line is kept in one of only a few possible
states (conditions) during each time period
–

Modems: a few thousand clock cycles per second
LANs: millions of clock cycles per second
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
3
Digital Versus Binary Transmission


Digital transmission: a few states
Binary transmission: exactly two states (1 and 0)
–
Binary is a special case of digital
Few States
Two States
1
0
Digital
Binary
4
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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6
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
Wire Propagation Effects: Attenuation

Attenuation: Signal Gets Weaker as it Propagates
–
May become too weak for receiver to recognize
Signal
Strength
Distance

Distortion: Signal changes shape as it propagates
–
–
Adjacent bits may overlap
May make recognition impossible for receiver
Distance
7
8
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
Signal
Strength
Signal
Noise
Spike
Error
Noise Floor
Time
9
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
Signal
Strength
Signal
SNR
Noise Floor
Distance
Wire Propagation Effects: Noise & Speed

10
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
Noise
Spike
Low Speed
(Long
Duration)
One Bit
Average Noise
During Bit
Error
Noise
Spike
One Bit
High Speed Average Noise
(Short
During Bit
Duration)
Wire Propagation Effects: Interference

11
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
Strength
Signal
Interference
Wire Propagation Effects: Cross-Talk Interference

Cross-Talk Interference
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–

12
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
Interference
- +
Single Twist
Signal
Practical Issues in Propagation Effects

Distance limits in standards prevent serious
propagation effects
–

13
Usually 100 meters maximum for ordinary copper wire
Problems usually occur at connectors
–
–
–
Crossed wires
Poor connections
Cross-talk interference
Radio Propagation

Broadcast signal
–
Not confined to a wire
14
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
15
Radio Propagation Problems

Wires Propagation is Predictable
–
–
–

16
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
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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18
Radio Propagation: Waves

Waves
Frequency in hertz (Hz)
Cycles per Second
3
Wavelength
(meters)
1
One Second
7 Cycles
4
Amplitude
(strength)
2 1 Hz = 1 cycle per second
Radio Propagation: Frequency Spectrum

Frequency Spectrum
–
–
–

19
Frequencies vary (like strings in a harp)
Frequencies measured in hertz (Hz)
0 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
20
Radio Propagation: Service Bands

Service Bands
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–
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Divide frequency spectrum into bands for services
A band is a contiguous range of frequencies
FM radio, cellular telephone service bands etc.
Cellular Telephone
FM Radio
AM Radio
0 Hz
Service
Bands
Radio Propagation: Channels and Bandwidth

21
Service Bands are Further Divided into Channels
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–
Like television channels
Bandwidth of a channel is highest frequency minus
lowest frequency
 Example
–
Highest frequency of a radio channel is 43 kHz
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Lowest frequency of the radio channel is 38 kHz
–
Bandwidth of radio channel is 5 kHz (43-38 kHz)
Channel
Bandwidth
0 Hz
Channel 3
Channel 2
Channel 1
Service
Band– FM Radio
Radio Propagation: Channels and Bandwidth

22
Shannon’s Equation -- W = B Log2 (1+S/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)
–
Maximum
Possible
Speed
Bandwidth
Broadband vs. Baseband
23

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