CWNA guide to wireless LANs 2nd ch03

CWNA Guide to Wireless
LANs, Second Edition
Chapter Three
How Wireless Works


Objectives
• Explain the principals of radio wave transmissions
• Describe RF loss and gain, and how it can be
measured
• List some of the characteristics of RF antenna
transmissions
• Describe the different types of antennas

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Radio Wave Transmission Principles
• Understanding principles of radio wave
transmission is important for:
– Troubleshooting wireless LANs
– Creating a context for understanding wireless
terminology

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What Are Radio Waves?
• Electromagnetic wave: Travels freely through
space in all directions at speed of light
• Radio wave: When electric current passes through
a wire it creates a magnetic field around the wire
– As magnetic field radiates, creates an
electromagnetic radio wave
• Spreads out through space in all directions

– Can travel long distances
– Can penetrate non-metallic objects

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What Are Radio Waves? (continued)

Table 3-1: Comparison of wave characteristics

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Analog vs. Digital Transmissions

Figure 3-2: Analog signal

Figure 3-4: Digital signal

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Analog vs. Digital Transmissions
(continued)
• Analog signals are continuous
• Digital signals are discrete
• Modem (MOdulator/DEModulator): Used when
digital signals must be transmitted over analog
medium
– On originating end, converts distinct digital signals
into continuous analog signal for transmission
– On receiving end, reverse process performed

• WLANs use digital transmissions
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Frequency

Figure 3-5: Long waves

Figure 3-6: Short Waves

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Frequency (continued)
• Frequency: Rate at which an event occurs
• Cycle: Changing event that creates different radio
frequencies
– When wave completes trip and returns back to
starting point it has finished one cycle

• Hertz (Hz): Cycles per second
– Kilohertz (KHz) = thousand hertz
– Megahertz (MHz) = million hertz
– Gigahertz (GHz) = billion hertz

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Frequency (continued)

Figure 3-7: Sine wave

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Frequency (continued)

Table 3-2: Electrical terminology

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Frequency (continued)
• Frequency of radio wave can be changed by
modifying voltage
• Radio transmissions send a carrier signal
– Increasing voltage will change frequency of carrier
signal

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Frequency (continued)

Figure 3-8: Lower and higher frequencies

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Modulation
• Carrier signal is a continuous electrical signal
– Carries no information

• Three types of modulations enable carrier signals
to carry information
– Height of signal
– Frequency of signal
– Relative starting point

• Modulation can be done on analog or digital
transmissions
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Analog Modulation
• Amplitude: Height of carrier wave
• Amplitude modulation (AM): Changes amplitude
so that highest peaks of carrier wave represent 1
bit while lower waves represent 0 bit
• Frequency modulation (FM): Changes number of
waves representing one cycle
– Number of waves to represent 1 bit more than
number of waves to represent 0 bit

• Phase modulation (PM): Changes starting point of
cycle

– When bits change from 1 to 0 bit or vice versa
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Analog Modulation (continued)

Figure 3-9: Amplitude

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Analog Modulation (continued)

Figure 3-10: Amplitude modulation (AM)

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Analog Modulation (continued)

Figure 3-11: Frequency modulation (FM)

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Analog Modulation (continued)

Figure 3-12: Phase modulation (PM)

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Digital Modulation
• Advantages over analog modulation:
–
–
–
–

Better use of bandwidth
Requires less power
Better handling of interference from other signals
Error-correcting techniques more compatible with
other digital systems

• Unlike analog modulation, changes occur in
discrete steps using binary signals
– Uses same three basic types of modulation as
analog
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Digital Modulation (continued)

Figure 3-13: Amplitude shift keying (ASK)

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Digital Modulation (continued)

Figure 3-14: Frequency shift keying (FSK)

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Digital Modulation (continued)

Figure 3-15: Phase shift keying (PSK)

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Radio Frequency Behavior: Gain
• Gain: Positive difference in amplitude between two
signals
– Achieved by amplification of signal
– Technically, gain is measure of amplification
– Can occur intentionally from external power source
that amplifies signal
– Can occur unintentionally when RF signal bounces
off an object and combines with original signal to
amplify it

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Radio Frequency Behavior: Gain
(continued)

Figure 3-16: Gain

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