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 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 04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 42 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 04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 43 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) 04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 44 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 04_1587202115_ch03.qxd 9/29/08 2:42 PM Page 45 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. 05_1587202115_ch04.qxd 9/29/08 2:44 PM Page 46 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 05_1587202115_ch04.qxd 9/29/08 2:44 PM Page 47 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 48 CCNA Wireless Official Exam Certification Guide 05_1587202115_ch04.qxd 9/29/08 2:44 PM Page 48 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 05_1587202115_ch04.qxd 9/29/08 2:44 PM Page 49 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 50 CCNA Wireless Official Exam Certification Guide 05_1587202115_ch04.qxd 9/29/08 2:44 PM Page 50 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 05_1587202115_ch04.qxd 9/29/08 2:44 PM Page 51 . False 8. Which of the following are 80 2.11 topologies for LANs? (Choose all that apply.) a. Adsense b. Ad hoc c. Infrastructure d. Internal 48 CCNA Wireless Official Exam Certification Guide 05_1 587 202115_ch04.qxd. Repeater d. Hub 50 CCNA Wireless Official Exam Certification Guide 05_1 587 202115_ch04.qxd 9/29/ 08 2:44 PM Page 50 Chapter 4: WLAN Technologies and Topologies 51 22. Cisco offers which types of wireless. corrupts the signal. To understand this, suppose 04_1 587 202115_ch03.qxd 9/29/ 08 2:42 PM Page 43 44 CCNA Wireless Official Exam Certification Guide you walk into a crowded park with many screaming