CWNA Guide to Wireless LANs, Second Edition
Chapter 3
How Wireless Works
At a Glance
Instructor’s Manual Table of Contents

Overview

Objectives

Teaching Tips

Quick Quizzes

Class Discussion Topics

Additional Projects

Additional Resources

Key Terms
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Lecture Notes
Overview
In this chapter, the students will explore the fundamentals of how wireless technology
works. Although wireless transmissions can take place using infrared or radio waves as
discussed in Chapter 2, radio wave transmission is the preferred method used today and
is the focus of this chapter. The students begin by looking at the principles behind
sending voice and data through radio wave transmissions. Next, they learn how radio
frequency waves actually behave and how this behavior can be measured. Finally, the
students will find out about one of the most important elements of a radio wave system,
namely the antenna.
Chapter 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
Teaching Tips
Radio Wave Transmission Principals
1. Explain that understanding the principles of radio wave transmission is important not
only for troubleshooting wireless LANs but also to create a context for understanding
wireless terminology.
What Are Radio Waves?
1. Define the term electromagnetic wave. Stress that in wireless communication, data
travels on radio waves, which are a form of electromagnetic wave. Mention the
similarity between electromagnetic waves and the waves emitted from a hose,
illustrating with Figure 3-1.
2. Discuss light and heat waves briefly, mentioning their physical limitations.
3. Introduce the concept of radio waves. Explain how radio waves are generated and how
they propagate. Discuss the physical differences between radio waves and light or heat
wave, using Table 3-1 to facilitate the discussion.
Teaching
Tip
All forms of electromagnetic energy, from gamma rays to radio waves, travel
through space in waves.
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Teaching
Tip
Guglielmo Marconi first used radio waves to transmit a Morse code signal across
the Atlantic Ocean in 1901. This type of transmission was originally called
“wireless.” An international conference in Berlin in 1906 officially changed the
name “wireless” to “radio,” from the Latin word radius meaning a ray or beam.
Teaching
Tip
The electromagnetic spectrum is divided in octaves, the natural way to represent
frequencies. An octave represents eight diatonic degrees or a gradual
frequency increasing of a 10-factor. The spectrum is also divided into bands.
Each band represents around 3 octaves.
Analog vs. Digital Transmissions
1. Discuss the form of analog and digital transmissions, illustrating with Figure 3-2 and 34 respectively. Stress that analog signals are continuous, while digital signals are
discrete.
2. Explain the concepts of modulation and demodulation. Explain how a modem is used in
the process of the transmission of digital signals over an analog medium.
Teaching
Tip
Make sure that the students realize that computers operate using digital signals
(binary code is discrete, thus it is digital). When analog data, such as a video
image or an audio sound, needs to be stored on the computer, it must be first
converted into a digital format.
3. Explain that wireless transmissions are digital transmissions.
Frequency
1. Explain the concept of frequency as it relates to waves. Illustrate long waves and short
waves with Figures 3-5 and 3-6 respectively.
2. Explain that the term hertz is used as the unit of measurement for frequency. Mention
the terms kilohertz, megahertz, and gigahertz.
3. Explain the concept of a sine wave, illustrating with Figure 3-7. Discuss the
characteristics of a sine wave.
Teaching
Tip
In electrical terms the cycle produces what is known as an alternating current
(AC) because it flows between positive (+) and negative (-). AC is the type of
current that runs to the electrical outlets in a house. Direct current (DC) is found
in batteries. With DC the current flows only from one terminal (+) to the other (-)
and does not alternate.
4. Using Table 3-2 as a guide, discuss the various terminology used for electricity.
CWNA Guide to Wireless LANs, Second Edition
Teaching
Tip
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The formula for voltage (V=I*R) was first proposed in 1827 by German
physicist George Ohm and is called Ohm’s Law.
5. Explain that the frequency of the radio wave can be changed by modifying the voltage.
Discuss the concept of a carrier signal. Illustrate with Figure 3-8.
Teaching
Tip
It is important that students think about radio waves, and waves in general, in
three dimensions.
Modulation
1. Explain that the carrier signal sent by radio transmissions is simply a continuous
electrical signal, which itself carries no information.
2. Discuss the three types of modulation that allow a carrier signal to carry information.
Explain that modulation can be done on either analog or digital transmissions.
Teaching
Tip
Make sure the students understand that, although WLAN transmissions are
digital, understanding analog modulation helps in the understanding of digital
modulation.
Analog Modulation
1. Discuss the concepts of amplitude and amplitude modulation, illustrating with Figure 39 and 3-10 respectively. Mention that the height of the carrier can be changed so that a
higher wave represents a 1 bit while a lower wave represents a 0 bit.
Teaching
Tip
Amplitude modulation is most frequently used by broadcast radio stations.
However, AM is often susceptible to interference from outside sources such as
lightning from a thunderstorm, and is not generally used for data transmissions.
2. Discuss the concept of frequency modulation, illustrating with Figure 3-11. Explain
that, when using frequency modulation, the number of waves needed to represent a 1 bit
is more than the number of waves needed to represent a 0 bit.
Teaching
Tip
Like amplitude modulation, frequency modulation is often used by broadcast
radio stations. However, unlike AM, FM is not as susceptible to interference
from outside sources.
3. Discuss the concept of phase modulation. Illustrate with Figure 3-12. Mention that the
change in the starting point of a cycle takes place only when the bits being transmitted
change from a 1 bit to a 0 bit or vice versa.
CWNA Guide to Wireless LANs, Second Edition
Teaching
Tip
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Although radio broadcasts use either amplitude modulation (AM) or frequency
modulation (FM), television broadcasts actually use AM, FM, and phase
modulation (PM). Television video uses AM, the sound uses FM, and the color
information uses PM.
Digital Modulation
1. Using the list on page 84 of the text as a guide, discuss the advantages of digital
modulation over analog modulation. Mention that almost all wireless transmission uses
digital modulation.
2. Explain that, with digital modulation, the changes to a carrier signal are in discrete steps
using binary signals. Explain that digital modulation uses the same three types of
modulation that analog modulation uses.
3. Discuss the amplitude shift keying technique for digital modulation. Illustrate with
Figure 3-13.
Teaching
Tip
Digital (binary) modulation is still shown as a standard sine wave.
4. Discuss the frequency shift keying technique for digital modulation. Illustrate with
Figure 3-14.
5. Discuss the phase shift keying technique for digital modulation. Illustrate with Figure 315.
Radio Frequency Behavior and Measurement
1. Explain that one of the key elements of managing a wireless LAN is the understanding
of how radio frequency (RF) signals behave.
RF Behavior
1. Explain that the behavior of an RF signal can be categorized by whether something adds
power to the signal or takes power away from the signal.
Gain
1. Define the term gain. Explain that gain is achieved by an amplification of the signal.
Illustrate with Figure 3-16.
2. Mention that gain can occur intentionally or unintentionally, and describe generally how
this occurs.
Loss
1. Define the terms loss and attenuation. Illustrate with Figure 3-17.
2. Explain the reasons why intentional loss may be implemented.
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3. Explain that loss is usually unintentional, and that there are many factors that can result
in signal loss. Using the list on pages 86 through 89 of the text as a guide, discuss in
detail the phenomena that can result in signal loss. Illustrate with Figures 3-18 through
3-23.
Teaching
Tip
A useful tool for illustrating some of the types of loss is an animation. Some
animations of these phenomena can be found online. For instance, animations
illustrating reflection, refraction, and diffraction can be found at:
http://lectureonline.cl.msu.edu/~mmp/kap13/cd372.htm
RF Measurement
1. Explain that, because RF signals can be affected by the environment, it is sometimes
necessary to be able to calculate the gain or loss of the signal. Mention that, for WLAN
network managers, approximations can often be sufficient, but these measurements are
still important.
RF Math
1. Explain that RF power can be measured by two different units on two different scales.
2. Discuss the milliwatts scale, mentioning that it is a linear scale. Explain that the
reference point on this scale is zero, or the absence of power. Stress that this scale does
not account for the relationship of loss or gain to the whole.
3. Discuss the concept of a relative scale. Stress that the relative scale is not as precise as a
linear scale, but presents a better picture of the loss or gain relative to the whole.
4. Discuss the concept of a logarithm, and explain that relative scales often are
logarithmic.
5. Explain that RF power gain and loss on a relative scale are measured in decibels.
6. Using Table 3-3 as a guide, discuss the 10’s and 3’s Rules of RF Math.
Teaching
Tip
Make sure that the students understand that the Certified Wireless Network
Administrator (CWNA) exam does not require you to perform logarithmic
calculations but does require you to understand the concepts of the 10’s and 3’s
Rules of RF Math.
7. Explain that the reference point that relates the logarithmic relative decibel (dB) scale to
the linear milliwatt scale is known as the dBm.
8. Discuss the EIRP scale. Explain that EIRP uses the dBi unit of measurement. Mention
that the reference point of dBi is a theoretical antenna with 100 percent efficiency.
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WLAN Measurements
1. Explain that in the United States, the Federal Communications Commission (FCC)
defines power limitations for wireless LANS. Mention that the reason for these
limitations is to limit the distance that they can transmit.
2. Explain that the TPO is a measure of the power being delivered to the transmitting
antenna. Mention that the TPO limitation set by the FCC for WLANs is 1,000 mW or
30 dBm.
3. Using Table 3-4 as a guide, discuss the EIRP for 802.11b and 802.11g wireless
networks.
4. Explain the purpose of the RSSI. Mention that vendors use an algorithm or a look-up
table to convert RSSI values to dBm, mW, or signal strength percentage.
Quick Quiz 1
1. When an electric current passes through a wire, it creates a magnetic field in the space
around the wire. As this magnetic field radiates or moves out, it creates an
electromagnetic ____________________ wave that spreads out through space in all
directions.
Answer: radio
2. A(n) ____________________ signal is a continuous signal with no “breaks” in it.
Answer: analog
3. The electrical pressure on a wire is measured in ____________________.
Answer: volts
4. True or False: Modulation can only be performed on analog signals.
Answer: False
5. ____________________ modulation changes the starting point of a cycle.
Answer: Phase
6. ____________________ is a binary modulation technique similar to amplitude
modulation.
Answer: Amplitude shift keying (ASK)
7. ____________________ is defined as the positive difference in amplitude between two
signals.
Answer: Gain
8. When an RF signal moves from one medium to another of a different density the signal
actually bends instead of traveling in a straight line, in a process known as
____________________.
Answer: refraction
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9. ____________________ is bending caused by an object in the path of the transmission.
Answer: Diffraction
10. RF power gain and loss on a relative scale are measured in ____________________.
Answer: decibels (dB)
11. The chipset in 802.11 devices provides a value called the ____________________
which is used to determine different factors, such as when transmission is allowed,
when a roaming device should be associated with a different access point, and what data
rate (transmission speed) should be used for transmission
Answer: Receive Signal Strength Indicator (RSSI)
Antennas
1. Provide an overview of antennas. Explain that without antennas, radio waves would be
unable to travel long distances.
Antenna Concepts
1. Explain that radio waves are transmitted and received by using antennas. Describe the
general structure of an antenna.
2. Discuss how radio waves are generated, transmitted, and received by antennas. Illustrate
with Figure 3-24.
3. Describe the purpose of an intentional radiator.
Characteristics of RF Antenna Transmissions
1. Provide a brief overview of the characteristics of RF antenna transmissions.
Polarization
1. Introduce the concept of polarization. Explain that waves follow the plane of their
electrical fields, and the electric field is parallel to the radiating elements.
2. Discuss vertical and horizontal polarization. Illustrate vertical polarization with Figure
3-25. Stress that devices with antennas that are not polarized in the same way are not
able to communicate with each other effectively.
Teaching
Tip
Polarization is typically referred to as being horizontal or vertical, but the actual
polarization can be at any angle. Circular polarization is also possible.
Wave Propagation
1. Introduce the concept of wave propagation. Illustrate sky wave propagation and RF
LOS propagation with Figures 3-26 and 3-27, respectively. Stress that RF LOS
propagation is associated with WLANs.
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Multipath Distortion
1. Discuss the concept of multipath distortion. Explain that some signals sent via RF LOS
propagation encounter obstructions that refract or diffract the signal, causing some of
the signal to reach the receiving antenna later.
Teaching
Tip
The “late” multipath signals can actually reduce the power of the straight line
signals.
2. Explain that antenna diversity uses multiple antennas, inputs, and receivers to overcome
multipath distortion.
Teaching
Tip
A wireless NIC can contain multiple embedded antennas.
Fresnel Zone
1. Discuss the concept of the Fresnel Zone, using Figure 3-28 to illustrate. Mention which
zones are strongest and which are weakest.
Teaching
Tip
Calculating the Fresnel zone can be done by hand using a complicated formula or
by using any number of free Web-based calculators. For example, when sending
a signal 8 kilometers (5 miles) with an obstruction at 1.6 kilometers (2 miles) at
2.4 GHz will result in Zone 1 being 9.4 meters (31 feet) at its widest point.
Free Space Path Loss
1. Define the term free space path loss. Explain that this signal dispersion represents the
single greatest source of power loss in a wireless system.
2. Using Table 3-5 as a guide, discuss the approximate free space path loss for IEEE
802.11b and 802.11g WLANs.
Teaching
Tip
The decrease in signal strength due to free space path loss is inversely
proportional to the distance traveled and proportional to the wavelength of the
signal.
3. Explain that the only way for an increase in amplification by the antenna, known as
antenna gain, to occur is by the physical shape of it. Discuss the concept of beamwidth.
Antenna Types and Their Installations
1. Explain the two fundamental characteristics of antennas.
2. Introduce the three basic categories of antennas. Mention that within each category
there are multiple types of antennas, each one with different RF characteristics for
appropriate uses.
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Omni-Directional Antennas
1. Explain that omni-directional antennas are the most common type of antenna. Illustrate
how an omni-directional antenna radiates its signal using Figure 3-29.
Semi-Directional Antennas
1. Explain that semi-directional antennas focus the energy in one direction. Illustrate with
Figure 3-30. Discuss the situations where these types of antennas are most frequently
used.
Highly-Directional Antennas
1. Explain that highly-directional antennas send a narrowly focused signal beam. Mention
that these antennas are used for long distance, point-to-point wireless links, and are
rarely used in WLANs.
WLAN Antenna Locations and Installation
1. Explain that, because WLAN systems use omni-directional antennas to provide the
broadest area of coverage, access points should be located near the middle of the
coverage area. Mention that the antenna should be placed as high as possible, as well.
2. Mention that, if a high-gain omni-directional antenna is used, it is important to
determine that users located below the antenna area still have reception.
Quick Quiz 2
1. Radio waves are transmitted and received by using a(n) ____________________.
Answer: antenna
2. The orientation of the radio waves as they leave the antenna is known as
____________________.
Answer: polarization
3. The type of propagation associated with WLANs is called ____________________.
Answer: RF line of sight (RF LOS)
4. Determining the extent of the “late” multipath signals can be done by calculating the
____________________.
Answer: Fresnel zone
5. The most common type of antenna for a WLAN is a(n) ____________________
antenna.
Answer: omni-directional or dipole
Class Discussion Topics
1. Have the students consider the discrete nature of digital modulation. Can they think of
any issues that must be overcome to achieve digital signal modulation?
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2. Have the students imagine that they are going to install a wireless antenna as part of a
WLAN within the building where the class is being held. What type of antenna would
they install? Where would they install it and why?
Additional Projects
1. Have the students research online to find out how to build a “cantenna” antenna for a
wireless network. It has been claimed that, over a clear line of sight and with short
antenna cable runs, a 12db to 12db “can-to-can” wireless connection should be able to
carry an 11Mbps link well over ten miles. Have the students research the properties of
this type of antenna, and determine why and how it works. Legally, the FCC does not
permit the use of such an antenna, so do not have the students attempt to build and/or
use one (also warn them about the possible ramifications of performing this exercise on
their own). Some articles related to this activity can be found at:
http://www.g4tv.com/screensavers/features/40546/Pringles_Can_WiFi_Antenna.html
http://www.turnpoint.net/wireless/cantennahowto.html
http://www.oreillynet.com/cs/weblog/view/wlg/448
Additional Resources
1. An Introduction to Indoor Radio Propagation:
http://www.sss-mag.com/indoor.html#tutorial
2. The Radio Propagation:
http://www.astrosurf.org/lombry/qsl-propa.htm
3. Analog Signal Transmission:
http://www.omega.com/literature/transactions/volume2/analogsignal.html
4. Digital Signal Transmission:
http://www.omega.com/literature/transactions/volume2/digitalsignal.html
5. Digital modulation, ASK, FSK and PSK:
http://www.cs.ucl.ac.uk/staff/S.Bhatti/D51-notes/node12.html
6. Antennas:
http://www.aticourses.com/antennas_tutorial.htm
7. EIRP Limitations for 802.11 WLANs:
http://www.wi-fiplanet.com/tutorials/article.php/1428941
8. Accurate Gain/Phase Measurement at Radio Frequencies up to 2.5 GHz:
http://www.analog.com/library/analogDialogue/archives/35-05/AD8302/
9. Signal Propagation in the 900 MHz to 5 GHz Wireless Bands (pdf):
http://www.highfrequencyelectronics.com/Archives/Sep03/HFE0903_Tutorial.pdf
10. Reflection, Refraction, and Diffraction:
http://lectureonline.cl.msu.edu/~mmp/kap13/cd372.htm
11. Antenna Polarization Considerations in Wireless Communications Systems (pdf):
http://www.eie.fceia.unr.edu.ar/ftp/Antenas%20y%20Propagacion/AntennaPolarization-14B32.pdf
12. RF Propagation Basics (pdf):
http://www.sputnik.com/docs/rf_propagation_basics.pdf
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13. Antenna Tutorial (pdf):
http://www.aerocomm.com/Docs/Antenna_Tutorial.pdf
Key Terms
 10’s and 3’s Rules of RF Math: Basic rules of thumb for measuring RF power and
loss. Refer to Table 3-3.
 Absorption: The assimilation of RF signals into a material.
 Amperes (amps): The measure of the flow of electrical current.
 Amplification: The action that causes a gain.
 Amplitude: The height of the carrier waves.
 Amplitude modulation (AM): An analog modulation that changes the height of the
carrier waves.
 Amplitude shift keying (ASK): A binary modulation technique similar to amplitude
modulation that changes the height of the carrier.
 Analog signal: A continuous signal.
 Antenna: A copper wire or similar device that has one end up in the air and the other
end connected to the ground or a grounded device that transmits or receives radio
signals.
 Antenna diversity: The use of multiple antennas, inputs, and receivers to overcome
multipath distortion.
 Antenna gain: An increase in amplification by the antenna shape.
 Attenuation: The negative difference in amplitude between signals.
 Beamwidth: A measure of focus of a RF signal.
 Carrier signal: The signal on which a radio transmission is sent.
 Current (I): The flow of electrical energy.
 Cycle: The changing event that creates different radio frequencies.
 dBm: The reference point that relates the logarithmic relative decibel (dB) scale to the
linear milliwatt (mW).
 Decibels (dB): The measurement of RF power gain and loss on a relative scale.
 Diffraction: When RF signals bend because of striking a rough surface.
 Digital signal: A signal in discrete or separate units.
 Dipole antenna: An antenna that radiates its signal in all directions equally.
 Electromagnetic waves: Waves of energy through which light, heat, and radio signals
move through space.
 Equivalent (or Effective) Isotropically Radiated Power (EIRP): The power radiated
out by the antenna of a wireless system.
 Free space path loss: The weakening of signal strength as it disperses.
 Frequency: The rate at which an electrical current alternates creating different radio
wave transmissions.
 Frequency modulation (FM): An analog modulation that changes the number of
waves used to represent one cycle.
 Frequency shift keying (FSK): A binary modulation technique that changes the
frequency of the carrier signal.
 Fresnel zone: An area in which the RF signal strength can be calculated.
 Gain: The positive difference in amplitude between two signals.
 Gigahertz (GHz): One billion hertz.
 Hertz (Hz): The unit of measure for a radio frequency cycle.
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 Highly-directional antenna: An antenna that sends a narrowly focused signal beam.
 Impedance: The total amount of resistance to the flow of electrical current.
 Intentional radiator: A device (minus the antenna) that is specifically designed to
create and generate radio frequency signals.
 Kilohertz (KHz): One thousand hertz.
 Loss: The negative difference in amplitude between signals.
 Megahertz (MHz): One million hertz.
 Milliwatt (mW): One thousandth of a watt of power.
 Modem: A device that converts analog signals to a digital format or digital to analog
format for transmission.
 Multipath distortion: Refracted or diffracted signals that reach the receiving antenna
late.
 Ohms: The measure of the restriction of the flow of current.
 Omni-directional antenna: An antenna that radiates its signal equally in all directions.
 Oscillator signal: A waveform that illustrates cycles or changes in frequency.
 Phase modulation (PM): An analog modulation that changes the starting point of the
cycle.
 Phase shift keying (PSK): A binary modulation technique that starts and stops the
signal to represent a binary digit.
 Polarization: The orientation of radio waves as they leave the antenna; usually
horizontal or vertical.
 Radio wave: An electromagnetic wave through which radio transmissions are sent and
received.
 Radiotelephony wave: Same as radio wave.
 Receive Signal Strength Indicator (RSSI): The value transmitted by chipsets in
802.11 devices that can be used to determine signal strength.
 Reflection: When RF signals bounce back after striking a material.
 Refraction: When RF signals bend due to a change in atmospheric condition.
 Resistance (R): Measure of the restriction of the flow of electrical current.
 RF line of sight (RF LOS): A straight line transmission of an RF signal.
 Scattering: The reflection of an RF signal by small objects, such as raindrops.
 Semi-directional antenna: An antenna that focuses RF signals in one direction.
 Sine wave: A wave that illustrates cycles or changes in frequency.
 Transmitter Power Output (TPO): A measure of the power being delivered to the
transmitting antenna.
 Visual line of sight: A line between two points so that vision is unobstructed between
the points.
 Voltage (V): Electrical pressure on a wire.
 Voltage Standing Wave Ratio (VSWR): Impedance caused by a reflected signal.
 Volts: The measure of electrical pressure on a wire.
 Watts: The measure of electrical energy.
 Wavelength: The distance between successive amplitude peaks.
 Wave propagation: The pattern of RF radio wave dispersal.