CCNA Wireless Official Exam Certification Guide part 8 ppt

42 CCNA Wireless Official Exam Certification Guide
Determining Signal Strength Influences
Although it might seem hopeless to deploy a network that is susceptible to various forms
of interference, you can fight back. To fight back, you need to know what tools are avail-
able out there for you to determine signal strength, noise levels, and potential sources of
interference in the path. This section discusses these tools.
The Fresnel Zone
To give you a little background, Augustin-Jean Fresnel was a French physicist and civil en-
gineer who lived from 1788 to 1827. He correctly assumed that light moved in a wavelike
motion transverse to the direction of propagation. His assumption, or claim, was correct.
Because of his work, a method for determining where reflections will be in phase and out
of phase between sender and receiver is based on his name. This method determines what
is called the Fresnel zone.
Here is how Fresnel did it. First he divided the path into zones. The first zone should be at
least 60 percent clear of obstructions. To visualize this, you can think of the shape of a
football, which is wider in the middle. However, with the Fresnel zone calculation, you use
an equation to determine what the size of the ball is at the middle. This helps to determine
the width that a wave will be so you can make sure that no obstacles are in the path.
Note: While at the CCNA wireless level, you should not need to calculate the Fresnel zone.
You can find the actual formula at />Fresnel_zone_clearance.
Also, you can find an online Fresnel zone clearance calculator at abeam.
com/support/calculations/fresnel-zone.php.
Note: Indoor signals are too short to be affected.
Figure 3-10 illustrates the height an antenna would need to be at different distances to
overcome this. For example, for a 2.4-GHz system, at 7 miles you need to have the anten-
nas mounted at 45 to 50 feet.
Although this is just an example, the numbers are pretty close, and at least you can get
more of a visual of what you are up against in the real world. Again, do not spend too
much time on this in preparation for the CCNA wireless exam, because it is not a concept
you will be tested on.
Received Signal Strength Indicator

The Received Signal Strength Indicator (RSSI) measurement uses vendor-specified values.
Because of this, you cannot rely on it to compare different vendors. In the end, all this
gives you is a grading of how much signal was received.
Keep in mind that the measurement is vendor specific, so the scale that is used might vary.
For example, one vendor might use a scale of 0 to 100, whereas another might use a scale
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Chapter 3: WLAN RF Principles 43
2.4 GHz Systems
5 GHz Systems

Wireless Link-Distance
in Miles
1
5
10
15
20
25
1
5
10
15
20
25
9
20
28
35
40
45

Approximate Value “F”
(60% Fresnal Zone at 2.4 GHz)
14
31
43
53
61
68
3
5
13
28
50
78
Approximate Value “C”
Earth Curvature
3
5
13
28
50
78
12
25
41
63
90
123
Value “H” Antenna Mounting
Height with No Obstructions

17
36
56
81
111
146
Total Distance
Fresnel @ 60%
Earth Curvature
Antenna
Height
Figure 3-10 Sample Bridge Calculator
of 0 to 60. The scale is usually represented in dBm, so the two scales would not match up.
It is also up to the vendor to determine what dBm is represented by 0 and what dBm is
represented by 100.
One tool that is used in wireless networks to give RSSI values is called Network Stumbler.
Note: You can find the free Network Stumbler software at />RSSI is acquired during the preamble stage of receiving an 802.11 frame. RSSI has been re-
placed with Receive Channel Power Indicator (RCPI), which is a functional measurement
covering the entire received frame with defined absolute levels of accuracy and resolution.
To gain these measurements, you can use a CB21AG card and the Aironet Desktop Utility
(ADU), which are covered in Chapter 16, “Wireless Clients.” The CB21AG card is the most
widely adopted card used by Airmagnet and OmniPeek.
Signal-to-Noise Ratio
Signal-to-noise ratio (SNR) is the term used to describe how much stronger the signal is
compared to the surrounding noise that corrupts the signal. To understand this, suppose
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44 CCNA Wireless Official Exam Certification Guide
you walk into a crowded park with many screaming kids and speak in a normal voice
while on the phone. The odds are that the noise is going to be so loud that the person on
the other end will not be able to distinguish your words from all the noise around you

that is also being transmitted over the phone. This is how the wireless network operates. If
the outside influences are causing too much noise, the receivers cannot understand the
transmissions.
When the software that runs your wireless card reports this measurement, it is best to
have a higher number, but this is also built on the RSSI value, so it is vendor determined.
Note: You can explore SNR levels in the Network Stumbler application previously men-
tioned. Remember that the values are valid only for the Network Stumbler application.
Other applications might report different SNR values.
Link Budget
Link budget is a value that accounts for all the gains and losses between sender and re-
ceiver, including attenuation, antenna gain, and other miscellaneous losses that might oc-
cur. This can be useful in determining how much power is needed to transmit a signal that
the receiving end can understand.
The following is a simple equation to factor link budget:
Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) – Losses (dB)
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Chapter 3: WLAN RF Principles 45
Table 3-2 Key Topics for Chapter 3
Key Topic Item Description Page Number
Figure 3-1 The Free Path Loss model 36
Figure 3-3 The absorption issue 38
Figure 3-4 The reflection issue 39
Figure 3-5 The multipath issue 39
Figure 3-9 Line of sight 45
Exam Preparation Tasks
Review All Key Concepts
Review the most important topics from this chapter, noted with the Key Topics icon in the
outer margin of the page. Table 3-2 lists a reference of these key topics and the page num-
ber where you can find each one.
Definition of Key Terms

Define the following key terms from this chapter, and check your answers in the Glossary:
wavelength, frequency, amplitude, EIRP, Free Path Loss, absorption, reflection, multipath,
phase (in-phase/out-of-phase), scattering, refraction, line of sight, SNR, link budget
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This chapter covers the following subjects:
General Wireless Topologies: Discusses wire-
less LAN topologies from a high-level perspective.
Original 802.11 Topologies: Discusses wireless
network topologies defined by the IEEE.
Vendor-Specific Topology Extensions: Explains
how vendors extend network topologies.
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CHAPTER 4
WLAN Technologies and Topologies
When you work in a wireless network, you can encounter a number of technologies and
deployment options. Sometimes your situation calls for a peer-to-peer connection, and
other times you will want to connect to users who are in another room or on another
floor, yet on the same network. In this chapter you will learn what these networks are and
when they are appropriate. You will also look at which types of equipment are appropriate
for certain situations and environments.
Use the “Do I Know This Already?” quiz to gauge whether you should read the entire
chapter or if you should simply jump to the “Exam Preparation Tasks” section and review.
If in doubt, read through the whole chapter!
“Do I Know This Already?” Quiz
The “Do I Know This Already?” quiz helps you determine your level of knowledge of this
chapter’s topics before you begin. Table 4-1 details the major topics discussed in this chap-
ter and their corresponding quiz questions.
1. Which of the following topologies can be used with clients closer than 20 feet?
a. WLAN
b. WWAN

c. WPAN
d. WMAN
Table 4-1 “Do I Know This Already?” Section-to-Question Mapping
Foundation Topics Section Questions
General Wireless Topologies 1–7
Original 802.11 Topologies 8–20
Vendor-Specific Topology Extensions 21–26
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2. True or false: A WLAN uses 802.16b.
a. True
b. False
3. What topology is most often seen in a LAN and is designed to connect multiple de-
vices to the network?
a. WMAN
b. WPAN
c. WLAN
d. WWAN
4. In what frequency ranges does a wireless LAN operate? (Choose two.)
a. 2.2 GHz
b. 2.4 GHz
c. 2.4 MHz
d. 5 GHz
e. 5 MHz
5. What type of speed can you expect from a WMAN?
a. Broadband
b. WAN
c. Ethernet
d. Dialup modem
6. What is the name of the common WMAN technology?
a. WiMAN

b. WiMAX
c. Wi-Fi
d. WiNET
7. True or false: Deploying a WWAN is relatively inexpensive, so it’s common for enter-
prise customers to deploy their own.
a. True
b. False
8. Which of the following are 802.11 topologies for LANs? (Choose all that apply.)
a. Adsense
b. Ad hoc
c. Infrastructure
d. Internal
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Chapter 4: WLAN Technologies and Topologies 49
9. What does BSS stand for?
a. Basic Service Signal
b. Basic Service Separation
c. Basic Service Set
d. Basic Signal Server
10. If an AP is not used in a wireless network, this is called which of the following?
a. Independent Basic Service Set
b. Solitary Service Set
c. Single-Mode Set (SMS)
d. Basic Individual Service Set
11. For two devices to communicate without an access point, you must define which of
the following?
a. A group name
b. A password
c. A network number

d. A key
12. True or false: When operating in infrastructure mode, an AP is operating in full-du-
plex mode.
a. True
b. False
13. What device does an access point act as to connect wireless clients to a wired network?
a. Hub
b. Bridge
c. Router
d. Repeater
14. What is another name for wireless clients?
a. Stations
b. End nodes
c. Clients
d. Mobile APs
15. An access point is what kind of device?
a. Support device
b. Network device
c. Perimeter device
d. Infrastructure device
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16. What is the name for the area of coverage offered by a single access point?
a. VSA
b. MSA
c. TSA
d. BSA
17. When more than one AP connects to a common distribution, what is the network
called?
a. Extended Service Area
b. Basic Service Area

c. Local Service Area
d. WMAN
18. Clients connect to which of the following to access the LAN via a wireless AP?
a. SSID
b. SCUD
c. BSID
d. BSA
19. When one area exists, what is the name of the service set advertised by an AP?
a. BBSM
b. BSUP
c. BSSID
d. SSIG
20. Using MBSSIDs indicates which of the following?
a. More than one AP is advertising SSIDs.
b. More than one SSID is being advertised by one AP.
c. The AP sees more than one SSID.
d. There are multiple MACs on one SSID.
21. What can you use to connect an isolated wired network to a LAN?
a. WLAN
b. WGB
c. Repeater
d. Hub
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Chapter 4: WLAN Technologies and Topologies 51
22. Cisco offers which types of wireless bridges? (Choose two.)
a. aWGB
b. bWGB
c. uWGB
d. cWGB

23. For topologies where cable lengths prohibit placing an AP in certain locations, what
solution can be used?
a. Install a new switch that’s closer.
b. Install a hub instead.
c. Install a repeater.
d. Install a wireless client.
24. How much overlap is needed with an AP when a wireless repeater is used?
a. 10 to 15 percent
b. 100 percent
c. 50 percent
d. 40 to 80 percent
25. True or false: Outdoor mesh networks support only point-to-point topologies.
a. True
b. False
26. Mesh deployments are appropriate when __________ is a major concern.
a. Connectivity
b. Security
c. Cost
d. Speed
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