cwts certified wireless technology specialist official study guide exam pw0 070 phần 5 potx

Answers to Review Questions 157 Answers to Review Questions 1. C. IEEE wireless LAN devices use half-duplex communication. Half duplex is defined as two-way communication only one way at a time. Wired LANs can use full-duplex commu- nication, which is two-way communication transmitting in both directions simultaneously. An example of diplex is to combine signals from two different frequencies into a single transmitter/receiver. 2. A. DSSS devices operate in the 2.4 to 2.5 GHz ISM band. OFDM devices operate in the 5GHz UNII bands. 3. B. 802.11b operates in the 2.4 GHz ISM band. A total of three access points can be co-located before interference becomes an issue. 4. D. The IEEE 802.11n draft 2.0 amendment devices use MIMO, multiple radio chains, and antennas to operate. 802.11a/b/g devices use one radio and may use multiple antennas for diversity. 5. C. Spread spectrum technology sends data over many subcarrier frequencies. Narrowband technology is not used in IEEE-based WLANs but is used in other technology such as radio and TV. Wireless broadband provides high-speed wireless data communications and wire- less internet over a wide area network. Wideband uses a wide range of frequencies, and spectral mask refers to the signal levels of the radio frequency. 6. B, C, D. 802.11b channels need to be separated by at least five channels or 25 MHz to be considered non-overlapping. Channels 3 and 9 are separated by six channels, channels 6 and 11 are separated by five channels, and channels 2 and 8 are separated by six channels. All of these scenarios are non-overlapping channels. 7. A. CSMA/CA uses collision avoidance. CSMA/CD uses collision detection. CSMA/CR and CSMA/DSSS do not exist. 8. A. DSSS uses Barker code at 1 Mbps. CCK is for 5.5 Mbps. DBPSK and DQPSK are modu- lation technologies, not spreading codes. 9. A. FM radio stations use narrowband communication, which is high power and narrow frequency. WLANs use spread spectrum technology, which is low power and wide frequency. 10. C. HR/DSSS channels are 22 MHz wide. FHSS uses 1 MHz subcarrier frequencies. OFDM, ERP-OFDM, and HT-OFDM use 20 MHz–wide channels, and HT-OFDM can also use 40 MHz–wide channels. 11. D. Bluetooth operates in the 2.4 GHz band and can cause interference with WLAN devices that operate in the 2.4 GHz band, including FHSS, DSSS, and OFDM. 12. D. OFDM can be used in 802.11a or 802.11g and supports a maximum data rate of 54 Mbps. 802.11b supports a maximum data rate of 11 Mbps. OFDM is also used with 802.11n Draft devices, but the maximum data rate is 300 Mbps. 38893c05.indd 157 5/19/09 6:05:03 AM 158 Chapter 5 N Access Methods, Architectures, and Spread Spectrum Technology 13. C. HT-OFDM can support data rates as high as 300 Mbps, OFDM supports a maximum of 54 Mbps, and DSSS supports a maximum of 11 Mbps. Ethernet is not a wireless LAN technology. 14. C, D. IEEE 802.11a wireless LANs operate in the 5 GHz UNII bands. 802.11b/g wireless LANs operate in the 2.4 GHz ISM band. 15. D. IEEE 802.11b and 802.11g amendments are interoperable. 802.11a networks operate in the 5 GHz UNII bands and therefore are incompatible with 802.11b/g. 16. D. 802.11b operates in the 2.4 GHz ISM band and will allow for 14 channels. The channels that can be used will depend on where the wireless LAN is located. 17. A. FHSS uses 1 MHz subcarrier frequencies to transfer data. 20 MHz–wide, 22 MHz– wide, and 40 MHz–wide channels are used with other technologies. 18. B. The IEEE 802.11b amendment specifies data rate of 5.5 and 11 Mbps. OFDM allows for data rates up to 54 Mbps and is used in IEEE 802.11a and IEEE 802.11g amendments. 19. C. FHSS constantly changes frequencies while transmitting data in a WLAN. DSSS, OFDM, and MIMO use set channels and frequencies to transmit data. 20. D. Current MIMO technology allows for up to 300 Mbps. One way this is accomplished is by using multipath as a benefit rather than a hindrance. 38893c05.indd 158 5/19/09 6:05:03 AM Chapter 6 WLAN Antennas and Accessories THE FOLLOWING CWTS EXAM OBJECTIVES ARE COVERED IN THIS CHAPTER: Identify RF signal characteristics and the applications of  basic RF antenna concepts Passive gain  Beamwidths  Simple diversity  Polarization  Identify the purpose, features, and functions of and the  appropriate installation or configuration steps for the fol- lowing types of antennas Omnidirectional/dipole  Semidirectional  Highly directional  Identify the use of the following WLAN accessories and  explain how to select and install them for optimal perfor- mance and regulatory domain compliance RF cables  RF connectors  Lightning arrestors and grounding rods  Describe the proper locations and methods for installing  RF antennas Pole/mast mount  Ceiling mount  Wall mount  38893c06.indd 159 5/18/09 4:07:28 PM Antennas are an essential part of a successful wireless LAN deployment. From the transmitter perspective, an antenna will take the energy from the transmission system, transform it into radio waves, and propagate it through the free air. From the receiver perspective, an antenna performs the opposite task—it receives the radio waves, transforms them back to AC signals, and finally sends the information to a computer or other device. Many factors are involved in determining the proper antenna to be used in an applica- tion or deployment of a wireless LAN. These factors include: Indoor or outdoor installation  Distance between transmitter and receiver  Frequency to be used  Horizontal or vertical polarization  Aesthetics  Cost  Manufacturer  Intended use  Mounting brackets  Electrical characteristics  Height  Location  Local ordinances  Basic RF Antenna Concepts It is important to understand some of the basic theory, characteristics, and terminology associated with antennas prior to learning how they operate. Becoming familiar with this will help in making decisions when it comes to sales and support of antennas and wireless LAN systems. Some of the terminology for characteristics of antennas is listed here: RF lobes—Shape of the RF patterns  Beamwidth—Horizontal and vertical measurement angles  38893c06.indd 160 5/18/09 4:07:29 PM Basic RF Antenna Concepts 161 Antenna charts—Azimuth and elevation  Gain—Changing the RF coverage pattern  Polarization—Horizontal or vertical  RF Lobes The term lobe has many meanings depending on the context in which it is used. Typically it is used to define the projecting part of an object. In anatomical terms, an example would be part of the human ear known as the ear lobe. In botanical terms, a lobe is the divided part of a leaf. As a radio frequency technology term, lobe refers to the shape of the RF energy emitted from an antenna element. RF lobes are determined by the physical design of the antenna. Antenna design also determines how the lobes project from an antenna element. The effect of antenna design and the shape of the RF lobes are two reasons why choos- ing the correct antenna is a critical part of a wireless LAN design. Antennas may project many lobes of RF signal, some of which are not intended to be usable areas of coverage. The type of antenna utilized—omnidirectional, semidirectional, or highly directional parabolic dish—will determine the usable lobes. These antennas as well as the RF radiation patterns they project will be discussed in more detail later in this chapter. Figure 6.1 shows an example of RF lobes emitted from an antenna element. FIGURE 6.1 RF lobes’ shape and coverage area are affected by type of antenna. Highly directional parabolic dish antenna Main signalSide lobes Beamwidth The design of an antenna will determine how RF propagates and the specific patterns in which it propagates from an antenna element. As mentioned earlier, the patterns of energy emitted from an antenna are known as lobes. For antennas, the beamwidth is the angle of measurement of the main RF lobe measured at the half-power point or –3 dB point. Beam- width is measured both horizontally and vertically, in degrees. 38893c06.indd 161 5/18/09 4:07:29 PM 162 Chapter 6  WLAN Antennas and Accessories Azimuth and elevation charts available from the antenna manufacturer will show the beamwidth angles. The azimuth refers to the horizontal RF coverage pattern, and the elevation is the vertical RF coverage pattern. The azimuth is the view from above or the “bird’s-eye view” of the RF pattern; in some cases it will be 360°. Think of the elevation as a side view. If you were to look at a mountain from the side view, it would have a certain height or elevation measured in feet or meters. For example, Pikes Peak, a mountain in the front range of the Rocky Moun- tains, has an elevation of 14,115 feet (4,302 meters). Figure 6.2 shows a representation of horizontal and vertical beamwidths. FIGURE 6.2 Horizontal (azimuth) and vertical (elevation) beamwidths measured at the half power point Vertical beamwidth Horizontal beamwidth Reading Azimuth and Elevation Charts Understanding how to read an azimuth and elevation chart is good to know from a techni- cal sales, design, or integration perspective. Knowing these patterns will help when making hardware recommendations for customers based upon needed coverage and device use. These charts show the angles of RF propagation from both the azimuth (horizontal or looking down) and the elevation (vertical or side view). These charts give a general idea of the shape of the RF propagation lobe based on antenna design. 38893c06.indd 162 5/18/09 4:07:30 PM Basic RF Antenna Concepts 163 Antenna manufacturers test antenna designs in a laboratory. Using the correct instru- ments, an engineer is able to create the azimuth and elevation charts. These charts show only approximate coverage area based on the readings taken during laboratory testing and do not take into consideration any environmental conditions such as obstacles or interference. The following image shows an example of an azimuth and elevation chart. 90° 120° 150° 180° 210° 240° 270° 300° 330° 0°1020 3 30° 60° 90° 120° 150° 180° 210° 240° 270° 300° 330° 0°1020 3 30° 60° Vertical Horizontal Im age provIded by www.L-com.com. Understanding how to read one of these charts is not very complicated. Notice the chart is a circular pattern with readings from 0° to 360°, and there are many rings within these charts. The outermost ring shows the strongest signal from the testing process of this antenna. The inner rings show measurements and dB ratings less than the strongest measured signal from the outside ring. A good-quality chart will show the most accurate readings from the testing process. A sales or technical support professional can use these charts to get an idea of how the radiation pattern would look based on a specific antenna type and model. Antenna Gain The gain of an antenna provides a change in coverage that is a result of the antenna focusing the area of RF propagation. This gain is produced from the physical design of the antenna ele- ment. In Chapter 4, “Radio Frequency (RF) Fundamentals for Wireless LAN Technology,” we looked at various characteristics of radio frequency. One of these characteristics is amplitude, which was defined as the height (voltage level) of a sine wave. The amplitude is created by varying voltage over a period of time and is measured at the peaks of the signal from top to bottom. Amplification of an RF signal will result in gain. An antenna is a device that 38893c06.indd 163 5/18/09 4:07:31 PM 164 Chapter 6  WLAN Antennas and Accessories can change the coverage area, therefore propagating an RF signal further. Antenna gain is measured in decibels isotropic (dBi), which is a change in power as a result of increasing the isotropic energy. Isotropic energy is defined as energy emitted equally in all directions. The sun is a good example of isotropic energy, emitting energy in a spherical fashion equally in all directions. Figure 6.3 shows an example of energy being emitted from an isotropic radiator. FIGURE 6.3 A perfect isotropic radiator emits energy equally in all directions. Passive Gain It’s actually quite intriguing how an antenna can provide passive gain, a change in coverage without the use of an external power source. Because of how antennas are designed, they focus isotropic energy into a specific radiation pattern. Focusing this energy increases cov- erage in a particular direction. A common example used to describe passive gain is a mag- nifying glass. If a person is standing outside on a beautiful sunny day, the sun’s energy is not very intense because it is being diffused across the entire earth’s hemisphere. Therefore, there is not enough concentrated energy to cause any harm or damage in a short period of time. However, if this person was to take a magnifying glass and point one side of the mag- nifying glass toward the sun and the other side toward a piece of paper, more than likely the paper would start to heat very quickly. This is because the convex shape of the magnify- ing glass focuses or concentrates the sun’s energy into one specific area, therefore increasing the heat to that area. Antennas are designed to function in the same way by focusing the energy they receive from a signal source into a specific RF radiation pattern. Depending on the design of the antenna element, as the gain of an antenna increases, both the horizontal and vertical radiation patterns will also increase. Figure 6.4 shows a drawing of a wireless LAN system with 100 mW of power at the antenna. Because of passive gain, the antenna emits 200 mW of power. 38893c06.indd 164 5/18/09 4:07:31 PM Basic RF Antenna Concepts 165 FIGURE 6.4 Access point supplying 100 mW of power and an antenna with a gain of 3 dBi for an output at the antenna of 200 mW Access point Antenna gain of 3 dBi Power at antenna 100 mW Antenna power out 200 mW Exercise 6.1 is a simple way to demonstrate passive gain. EXERCISE 6.1 Demonstrate Passive Gain You can demonstrate passive gain by using a standard 8.5” × 11.0” piece of notebook paper or cardstock. 1. Roll a piece of paper into a cone or funnel shape. 2. Speak at your normal volume and notice the sound of your voice as it propagates through the air. 3. Hold the cone-shaped paper in front of your mouth. 4. Speak at the same volume. 5. Notice that the sound of your voice is louder. This occurs because the sound is now focused into a specific area or radiation pattern, hence passive gain occurs. Active Gain Active gain will also provide an increase in signal strength. Active gain is accomplished by providing an external power source to a device in the wireless LAN system. An example of such a device is an amplifier. An amplifier is placed in series in the wireless LAN system and will increase the signal strength based on the gain of the amplifier. If an amplifier is used in a wireless LAN system, certain regulatory domains require that the amplifier must be certified as part of the system. It is best to carefully consider whether an amplifier is necessary before using such a device in an IEEE 802.11 wireless LAN sys- tem. Using an amplifier may nullify the system’s certification and potentially exceed the allowed RF limit. 38893c06.indd 165 5/18/09 4:07:32 PM 166 Chapter 6  WLAN Antennas and Accessories Antenna Polarization Antenna polarization describes how a wave is emitted from an antenna and the orientation of the electrical component or electric field of the waveform. To maximize signal, the trans- mitting and receiving antennas should be polarized in the same direction or as closely as possible. Antennas polarized the same way ensure the best possible signal. If the polarization of the transmitter and receiver are different, the power of the signal will decrease depending how different the polarization is. Figure 6.5 shows an example of horizontal and vertical polarized antennas. FIGURE 6.5 Horizontally and vertically polarized antennas Horizontally polarized antennas (vertical beam) Vertically polarized antennas (horizontal beam) With the large number of wireless LAN devices available, it is a challenging task to accomplish the same polarization for all devices on the network. Performing a wireless LAN site survey will show signal strength based upon several factors, including polariza- tion of access point antennas. This survey will help determine the received signal strength of the wireless LAN devices. Site surveys and antenna polarization will be discussed in more detail in Chapter 9, “Performing a WLAN Site Survey.” Antenna Polarization Example/Experiment It is fairly simple to demonstrate antenna polarization with a notebook computer or other wireless LAN device and either a wireless network adapter client utility or other third- party software that shows signal strength and/or signal to noise ratio. One such utility is InSSIDer, a free open source Wi-Fi network scanner for Windows Vista and Windows XP. The InSSIDer program is included on the CD that comes with this book. InSSIDer displays the received signal strength from the access points in the receiver area. 38893c06.indd 166 5/18/09 4:07:32 PM [...]... Frequency ranges 2400– 250 0 MHz Gain 30 dBi Horizontal beamwidth 5. 3° Vertical beamwidth 5. 3° Impedance 50 ohm Maximum power 100 W VSWR 150 mph (241 kph) Wind Loading Data Wind Speed (mph) Loading 100 5 lb 1 25 7 lb Azimuth and elevation charts are also available for patch/panel antennas Figure 6.11... Accessories n Ta b l e   6 4   ​ 15 dBi Yagi Antenna Specifications  (continued)  ​ Mechanical Specifications (continued) Radome material UV-inhibited polymer Connector 12² N-female Operating temperature –40° C to 85 C (–40° F to 1 85 F) Mounting 1-1/4” (32 mm) to 2” (51 mm) diameter masts Polarization Vertical and horizontal Flame rating UL 94HB RoHS-compliant Yes Wind survival > 150 mph (241 kph) Wind Loading . dBi Horizontal beamwidth 75 Vertical beamwidth 65 Impedance 50 ohm Maximum power 25 W VSWR <1 .5: 1 avg Mechanical Specifications Weight 0.4 lb. (.18 Kg) Dimensions 4 .5 x 4 .5 x 0.9˝ (114 x 114. Ethernet is not a wireless LAN technology. 14. C, D. IEEE 802.11a wireless LANs operate in the 5 GHz UNII bands. 802.11b/g wireless LANs operate in the 2.4 GHz ISM band. 15. D. IEEE 802.11b. of 54 Mbps. 802.11b supports a maximum data rate of 11 Mbps. OFDM is also used with 802.11n Draft devices, but the maximum data rate is 300 Mbps. 38893c 05. indd 157 5/ 19/09 6: 05: 03 AM 158 Chapter