CCNA Wireless Official Exam Certification Guide part 7 pdf

32 CCNA Wireless Official Exam Certification Guide
1. Which of the following best describes a frequency that is seen 1 million times per
second?
a. 1 Hz
b. 1000000 Mb
c. 1 joule
d. 1 MHz
2. What does amplitude measure?
a. Distance from high crest to high crest horizontally in a waveform
b. Distance between two access points
c. Distance from low crest to midspan in a waveform
d. Height of wave from lowest crest to highest crest
3. EIRP is calculated using which of the following formulas?
a. EIRP = transmitter power – cable loss + antenna gain
b. EIRP = interference – cable loss + antenna gain
c. EIRP = cable gain – cable loss + antenna gain
d. EIRP = transmitter loss + cable loss + antenna gain
4. Metal desks, glass, light fixtures, and computer screens can contribute to which influ-
ence on wireless transmissions?
a. Scattering
b. Refraction
c. Reflection
d. Absorption
5. Carpet, human bodies, and walls can contribute to which influence on wireless
transmission?
a. Scattering
b. Refraction
c. Reflection
d. Absorption
6. In the Free Path Loss model, objects that are farther away from a transmitter receive
the same amount of signal as those that are closer to the transmitter. True or False?

a. True
b. False
04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 32
Chapter 3: WLAN RF Principles 33
7. If a signal is being spread about by microparticles, it is experiencing which influence
on wireless transmissions?
a. Scattering
b. Spreading
c. Scarring
d. Splitting
e. Refracting
8. Multipath causes which of the following issues? (Choose all that apply.)
a. Redundant connectivity
b. The signal becoming out of phase, which can potentially cancel the signal
c. The signal being received by multiple devices in the path, causing security
concerns
d. Portions of the signal being reflected and arriving out of order
9. Scattering is caused by humidity. True or False?
a. True
b. False
10. For line of sight (LOS) transmissions, what can determine where signals can become
out of phase?
a. Free Path Zone
b. EIRP
c. Fresnel Zone
d. Phase Zone
11. Link budget is used to do which of the following? (Choose two.)
a. Account for all the receivers on a link
b. Account for all the gains and losses
c. Determine how much money you can spend on a wireless deployment

d. Factor in EIRP and attenuation for a transmission
04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 33
34 CCNA Wireless Official Exam Certification Guide
Foundation Topics
Characteristics of Wireless Networks
Many influences can act on a wireless transmission. For that reason, it is important to un-
derstand what is actually involved in a wireless transmissions so you know exactly what is
being affected. This section reviews what a wavelength is, how frequency it is used in
wireless transmission, and what the purpose of amplitude is. In addition, it covers how Ef-
fective Isotropic Radiated Power (EIRP) is calculated and what it defines.
Review of Wavelength
A wavelength is the distance between successive crests of a wave. This is how wavelength
is measured. Most people have seen examples of sound waves. By measuring the distance
between the crest of each wave, you can determine the wavelength. This is a distinctive
feature of radio waves that are sent from a transmitter. Thinking back to what was dis-
cussed in Chapter 1, the waveform takes on a form called a sine wave.
The waveform starts as an AC signal that is generated by a transmitter inside an access
point (AP) and is then sent to the antenna, where it is radiated as a sine wave. During this
process, current changes the electromagnetic field around the antenna, so it transmits
electric and magnetic signals.
The wavelength is a certain size, measured from one point in the AC cycle to the next
point in the AC cycle. This in turn is called a waveform. Following are some quick facts
about waveforms that you may relate to:
■ AM radio waveforms are 400 to 500 meters long.
■ Wireless waveforms in wireless LANS are only a few centimeters.
■ Waveforms sent by satellites are approximately 1 mm long.
Review of Frequency
Because the term frequency is thrown around quite a bit in wireless networking, you need
to have a clear understanding of it. Frequency, as discussed in Chapter 1, determines how
often the signal is seen. It is the rate at which something occurs or is repeated over a par-

ticular period or in a given sample or period. It is insufficient to say that frequency is how
often a signal is seen. If you are going to measure frequency, you need a period of time to
look at it. Frequency, which is usually measured in seconds, is the rate at which a vibration
occurs that constitutes a wave; this can be either in some form of material, as in sound
waves, or it can be in an electromagnetic field, as you would see in radio waves and light.
Because frequency refers to cycles, following are some quick facts to help you to under-
stand how it is measured:
■ 1 cycle = 1 Hz
■ Higher frequencies travel shorter distances
04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 34
Chapter 3: WLAN RF Principles 35
■ When a waveform is seen once in a second = 1 Hz
■ 10 times in a second = 10 Hz
■ 1 million times in a second = 1 MHz
■ 1 billion times in a second = 1 GHz
These are useful numbers that you can see throughout wireless networks.
Review of Amplitude
The vertical distance between crests in the wave is called amplitude. Different amplitude
can exist for the same wavelength and the same frequency. Amplitude is the quantity or
amount of energy that is put into a signal. Folks like the FCC and European Telecommuni-
cations Standards Institute (ETSI) regulate the amplitude.
Note: You can find a neat visualization of amplitude at />physics/waves/introduction/introductionWaves.html.
What Is Effective Isotropic Radiated Power?
When an access point sends energy to an antenna to be radiated, a cable might exist be-
tween the two. A certain degree of loss in energy is expected to occur in the cable. To
counteract this loss, an antenna adds gain, thus increasing the energy level. The amount of
gain you use depends on the antenna type. Note that both the FCC and ETSI regulate the
power that an antenna radiates. Ultimately, Effective Isotropic Radiated Power (EIRP) is
the power that results. EIRP is what you use to estimate the service area of a device.
To calculate EIRP, use the following formula:

EIRP = transmitter output power – cable loss + antenna gain
Influences on Wireless Transmissions
Now that you clearly understand wireless transmissions and what is involved, it is a good
time to discuss the influences on wireless signals. Some influences can stop a wireless sig-
nal from propagating altogether, whereas others might simply shorten the transmission
distance. Either way, you should be aware of these factors so you can plan and adjust your
deployment accordingly. In this section, you learn about the Free Path Loss model, ab-
sorption, reflection, scattering, multipath, refraction, and line of sight.
Understanding the Free Path Loss Model
To understand Free Path Loss, you can think of jumping smack into the middle of a pud-
dle. This would cause a sort of wave effect to spread in all directions away from you. The
closer to you that the wave is, the larger it is. Likewise, the farther away from you that
wave travels, the smaller it gets. After a certain distance, the wave widens so much that it
just disappears.
04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 35
36 CCNA Wireless Official Exam Certification Guide
Figure 3-1 The Free Path Loss Model
You might recall learning that an object that is in motion stays in motion until something
stops it. But nothing stops the wave. It just disappears. This is where you get the term free.
Take a look at Figure 3-1, and you can see that as the wave—or, in this case, the radiated
wireless signal—travels away from the source, it thins out. This is represented by the bold
dots becoming less and less bold.
You might also notice that the farther away the signal gets from the center, the sparser the
dots are. Figure 3-1 has a single transmitting device (you could relate that to an access
point) and many receiving devices. Not all the receiving stations get each one of the dots
or signals that the transmitter sent. A device closer to the transmitter usually gets a more
concentrated signal, and a receiver farther away might get only one dot.
Determining the range involves a determination of the energy loss and the distance. If you
place receivers outside of that range, they cannot receive wireless signals from the access
point and, in a nutshell, your network does not work.

Understanding Absorption
Earlier in this chapter, you learned that amplitude allowed a wave to travel farther. This can
be good, because you can cover a greater area, potentially requiring fewer access points
Key
Topi
c
04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 36
Chapter 3: WLAN RF Principles 37
Front Door
Side Door
Figure 3-2 Absorption Before Office Move-In
for your wireless deployment. By removing or reducing amplitude in a wave, you essen-
tially reduce the distance a wave can travel. A factor that influences wireless transmission
by reducing amplitude is called absorption.
An effect of absorption is heat. When something absorbs a wave, it creates heat in what-
ever absorbed the wave. This is seen in microwaves. They create waves that are absorbed
by your food. The result is hot food. A problem you can encounter is that if a wave is en-
tirely absorbed, it stops. While this effect reduces the distance the wave can travel, it does
not change the wavelength or the frequency of the wave. These two values do not change
as a wave is absorbed.
You might be asking what some possible sources of absorption are. Walls, bodies, and
carpet can absorb signals. Relate it to sound. If you had really loud neighbors who were
barbecuing outside your bedroom window, how could you deaden the sound? You could
hang a blanket on the window or board up the window. Things that absorb sound waves
also absorb data waves.
How can this affect your wireless deployment? Looking at Figure 3-2, you can see an of-
fice that has just been leased and ready to move in. After a quick site survey, you deter-
mine that four APs will provide plenty of coverage. This is because you cannot see
absorption. Nothing causes the issue.
Now look at Figure 3-3, which shows the same office after move-in. Notice that with the

furniture, cubicle walls, and other obstacles, the four APs that you originally thought
04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 37
38 CCNA Wireless Official Exam Certification Guide
Front Door
Side Door
Cubicle
Cubicle
Cubicle
Cubicle
Cubicle
Cubicle
Cubicle
Cubicle
Cubicle
Cubicle
Cubicle
Office Office
Office
Break Room
Figure 3-3 Absorption After Office Move-In
would be sufficient no longer provide the proper coverage because of the signal being ab-
sorbed. This is an illustration of absorption.
Understanding Reflection
Although absorption causes some problems, it is not the only obstacle that you are going
to encounter that will affect your wireless deployments. Another obstacle is reflection.
Reflection happens when a signal bounces off of something and travels in a different di-
rection. This can be illustrated by shining a flashlight on an angle at a mirror, which causes
it to reflect on an opposite wall. The same concept is true with wireless waveforms. You
can see this effect in Figure 3-4, where the reflection of the signal is reflected at the same
angle that it hits the mirror. You can also relate this to sources of interference in an office

environment. Although offices do not usually have mirrors lying around, they do have
other objects with similar reflective qualities, such as monitors and framed artwork with
glass facing.
Key
Topi
c
04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 38
Chapter 3: WLAN RF Principles 39
Incoming Wireless Signal
Reflected Wireless Signal
Reflective Surface
Figure 3-4 The Reflection Issue
Traffic Travels Across Multiple Paths;
some Traffic Arrives Later than Other Traffic.
Reflective Surface
Figure 3-5 The Multipath Issue
Reflection depends on the frequency. You will encounter some frequencies that are not af-
fected as much as others. This is because objects that reflect some frequencies might not
reflect others.
Understanding Multipath
Multipath is what happens when portions of signals are reflected and then arrive out of
order at the receiver, as illustrated in Figure 3-5.
One characteristic of multipath is that a receiver might get the same signal several times
over. This is dependent on the wavelength and the position of the receiver.
Another characteristic of multipath is that it can cause the signal to become out of
phase. When you receive out-of-phase signals, they can cancel each other out, resulting
in a null signal.
Understanding Scattering
The issue of wireless signals scattering happens when the signal is sent in many different
directions. This can be caused by some object that has reflective, yet jagged edges, such

as dust particles in the air and water. One way to illustrate the effects would be to consider
shining a light onto a pile of broken glass. The light that is reflected shoots off in many
different directions. The same is true with wireless, only the pile of glass is replaced with
microparticles of dust or water.
Key
Topi
c
Key
Topi
c
04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 39
40 CCNA Wireless Official Exam Certification Guide
Figure 3-6 Wireless Signal Scattering
Waveform
Waveform
Reflected
Waveform
Refracted
Glass with Water
Figure 3-7 The Refraction Issue
On a large scale, imagine that it is raining. Large raindrops have reflective capabilities.
When a waveform travels through those microparticles, it is reflected in many directions.
This is scattering. To visualize this, notice that Figure 3-6 involves a waveform traveling
between two sites on a college campus. During a heavy downpour of rain, the wireless
signal would be scattered in transit from one antenna to the next.
Scattering has more of an effect on shorter wavelengths, and the effect depends on fre-
quency. The result is that the signal weakens.
Understanding Refraction
Refraction is the change in direction of, or the bending of, a waveform as it passes
through something that is a different density. This behavior causes some of the signal to

be reflected away and part to be bent through the object. To better understand this con-
cept, Figure 3-7 demonstrates the effect of refraction. A waveform is being passed
through a glass of water. Notice that, because the glass is reflective, some of the light is re-
flected, yet some still passes through.
The waveform that is passed through the glass is now at a different angle.
04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 40
Chapter 3: WLAN RF Principles 41
Figure 3-8 Directional Antennas and Line of Sight
Curvature of the earth
Figure 3-9 Directional Antennas and LOS with Obstructions
Note: You can find a neat Java-based example of refraction at />englishhtm/RefractionByPrism.htm.
Because refraction usually has the most effect on outdoor signals, dryness refracts away
from the earth (as seen in dust particles), and humidity refracts toward the earth.
Understanding Line of Sight
As an object travels toward a receiver, it might have to deal with various obstructions that
are directly in the path. These obstructions in the path cause many of the issues just dis-
cussed—absorption, reflection, refraction, scattering. As wireless signals travel farther
distances, the signal widens near the midpoint and slims down nearer to the receiver.
Figure 3-8 illustrates where two directional antennas are sending a signal between the two
points. The fact that it appears to be a straight shot is called visual line of sight (LOS).
Although the path has no obvious obstacles, at greater distances the earth itself becomes
an obstacle. This means that the curvature of the earth, as well as mountains, trees, and
any other environmental obstacles, can actually interfere with the signal.
Even though you see the other endpoint as a direct line, you must remember that the sig-
nal does not. The signal in fact widens, as illustrated in Figure 3-9. What was not an obvi-
ous obstruction in Figure 3-8 is more clearly highlighted in Figure 3-9.
When you plan for LOS, you should factor in the closest obstacle.
Key
Topi
c

04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 41