CCNA Wireless Official Exam Certification Guide part 8 ppt

Note: While at the CCNA wireless level, you should not need to calculate the Fresnel zone.. One tool that is used in wireless networks to give RSSI values is called Network Stumbler.. Th

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 http://en.wikipedia.org/wiki/Fresnel_zone#Determining_ Fresnel_zone_clearance

Also, you can find an online Fresnel zone clearance calculator at http://www.terabeam 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

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 http://www.netstumbler.com/

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

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)

Chapter 3: WLAN RF Principles 45

Table 3-2 Key Topics for Chapter 3

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

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

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?

Table 4-1 “Do I Know This Already?” Section-to-Question Mapping

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?

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?

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

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

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

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?

b. WGB

c. Repeater

d. Hub

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