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 http://id.mind.net/~zona/mstm/ 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 http://www.phy.hk/wiki/ 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 . can spend on a wireless deployment d. Factor in EIRP and attenuation for a transmission 04_15 872 02115_ch03.qxd 9/29/08 2:42 PM Page 33 34 CCNA Wireless Official Exam Certification Guide Foundation. obstacles, the four APs that you originally thought 04_15 872 02115_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. with wireless, only the pile of glass is replaced with microparticles of dust or water. Key Topi c Key Topi c 04_15 872 02115_ch03.qxd 9/29/08 2:42 PM Page 39 40 CCNA Wireless Official Exam Certification