11 Chapter 1 - Introduction to Wireless LANs Key Terms Before taking the exam, you should be familiar with the following terms: access layer core layer distribution layer FCC IEEE IEEE 802.11 IEEE 802.11a IEEE 802.11b IEEE 802.11g last mile SOHO WISP CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. Chapter 1 - Introduction to Wireless LANs 12 Review Questions 1. Which one of the following does a wireless LAN provide that a wired network does not? A. Mobility B. Centralized security C. Reliability D. VPN security 2. Which one of the following would not be an appropriate use of a wireless LAN? A. Connecting two buildings together that are on opposite sides of the street B. Connecting two computers together in a small office so they can share a printer C. Connecting a remote home to a WISP for Internet access D. Connecting two rack-mounted computers together 3. Why is a wireless LAN a good choice for extending a network? Choose all that apply. A. Reduces the cost of cables required for installation B. Can be installed faster than a wired network C. The hardware is considerably less expensive D. Eliminates a significant portion of the labor charges for installation 4. Wireless ISPs provide which one of the following services? A. Small office/home office services B. Connectivity for large enterprises C. Last mile data delivery D. Building-to-building connectivity 5. Wireless LANs are primarily deployed in which one of the following roles? A. Backbone B. Access C. Application D. Core CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. 13 Chapter 1 - Introduction to Wireless LANs 6. Why would a mobile office be a good choice for using a wireless LAN? Choose all that apply. A. It would take less time to setup than wiring a network B. The equipment could be removed easily if the office moves C. It would not require any administration D. It is a more centralized approach 7. Which one of the following is the IEEE family of standards for wireless LANs? A. 802.3 B. 803.5 C. 802.11 D. 802.1x 8. As a consultant, you have taken a job creating a wireless LAN for an office complex that will connect 5 buildings in close vicinity together. Given only this information, which one of the following wireless LAN implementations would be most appropriate for this scenario? A. Last-mile data service from a WISP B. Point-to-point bridge links between all buildings C. Point-to-multipoint bridge link from a central building to all remote buildings D. One central antenna at the main building only 9. Which of the following are challenges that WISPs face that telephone companies and cable companies do not? Choose all that apply. A. Customers located more than 18,000 feet from a central office B. High costs of installing telephone lines or copper cabling C. Trees as line of sight obstructions D. Rooftop access for antenna installation 10. In what organization did the use of spread spectrum wireless data transfer originate? A. WECA B. WLANA C. FCC D. U.S. Military CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. Chapter 1 - Introduction to Wireless LANs 14 11. Which one of the following is the most recently approved IEEE standard for wireless LANs? A. 802.11a B. 802.11b C. 802.11c D. 802.11g 12. Which one of the following IEEE standards for wireless LANs is not compatible with the standard currently known as Wi-Fi™? A. 802.11 B. 802.11g C. 802.11a D. 802.11b 13. Which one of the following IEEE 802.11 standards for wireless LANs utilizes the 5 GHz UNII bands for its radio signal transmissions? A. 802.11b B. Bluetooth C. 802.11 D. 802.11g E. 802.11a 14. A WISP would take advantage of which one of the following applications for wireless LANs? A. Last Mile data delivery B. Building-to-building bridging C. Classroom connectivity D. Home network connectivity 15. Who makes the laws that govern the usage of wireless LANs in the United States? A. IEEE B. WECA C. FCC D. FAA CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. 15 Chapter 1 - Introduction to Wireless LANs Answers to Review Questions 1. A. The most alluring feature of a wireless network is the freedom to move about while remaining connected to the network. Wired networks cannot offer this feature. 2. D. Generally speaking, computers that are rack-mounted together are servers, and servers should be connected to a high-speed, wired backbone. Wireless networks are meant for mobile access rather than server room connectivity. 3. A, B, D. Cabling a facility is a time-consuming and expensive task. Wireless networks can quickly and inexpensively be installed and configured. 4. C. Wireless Internet Service Providers (WISPs) provide last mile data delivery service to homes and businesses. In this fashion, they compete directly against wired ISPs such as telephone and cable companies. 5. B. The access layer of the industry standard design model is where users attach to the network. Wireless network devices are most generally installed in this capacity. There are times when wireless networks may be used in a distribution role, such as building-to-building bridging, but a very large percentage of wireless networks are used strictly for access. 6. A, B. In the setup and teardown of a mobile office, cabling is the most significant task. In a small office, many of the common problems of a wireless network are not experienced so time-consuming tasks such as site surveys are not required. Centralized connection points (called access points) are minimal so wiring is minimal. 7. C. The 802.11 family of standards specifically address wireless LANs. There are many flavors of standards addressing many types of wireless technologies and various topics related to wireless technologies. For example, 802.11, 802.11b, 802.11g, and 802.11a are all specifications of wireless LANs systems whereas 802.11f addresses inter-access point protocol and 802.11i addresses wireless LAN security. The 802.1x standard is for port-based network access control. 8. C. Since using a single antenna would likely have severe problems with coverage and many point-to-point bridge links (forming a partial or full mesh) would be highly expensive, the only logical alternative is to use point-to-multipoint bridge connectivity between buildings. This is an economically sound and highly effective solution. 9. C, D. Wireless Internet Service Providers (WISPs) face problems with line of sight limitations of 2.4 GHz and 5 GHz wireless LAN systems. Antennas must be installed on rooftops or higher if possible in most cases. Trees and hills both pose problems to WISPs for the same reason. 10. D. During WWII, actress Hedy Lamarr and composer George Antheil co-invented the frequency hopping communications technique. The U.S. military began using frequency hopping spread spectrum communications in 1957 well before the broad commercial use that spread spectrum systems enjoy today. CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. Chapter 1 - Introduction to Wireless LANs 16 11. A. The first wireless LAN standard was the 802.11 standard using the 2.4 GHz ISM band, approved in 1997. Following 802.11 was 802.11b raising the top speed to 11 Mbps and limiting use to DSSS technology only. Following 802.11b was 802.11a, which uses the 5 GHz UNII bands. The 802.11g standard is in draft form, and has not yet been completed. 12. C. Wi-Fi is the hardware compatibility standard created and maintained by WECA for 802.11b devices. IEEE 802.11g devices use the 2.4 GHz ISM band are backwards compatible with 802.11b. 802.11a devices use a different set of frequencies and a different modulation type from 802.11b, and are thus incompatible. 13. E. The IEEE 802.11, 802.11b, 802.11g, Bluetooth, and HomeRF all use the 2.4 GHz ISM bands, whereas the 802.11a standard uses the 5 GHz UNII bands. 14. A. WISPs are direct competitors for telephone companies and cable companies in providing last-mile connectivity to businesses and residences in the broadband Internet services market. 15. C. The Federal Communications Commission (FCC) makes the laws regarding frequency band usage (licensed and unlicensed) in the United States. The IEEE makes standards regarding wireless LANs, which use RF frequencies. WECA makes hardware compatibility standards called Wi-Fi and Wi-Fi5, and the Federal Aviation Commission (FAA) controls airspace and aviation vehicles. CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. Radio Frequency (RF) Fundamentals CWNA Exam Objectives Covered:  Define and apply the basic concepts of RF behavior:  Gain  Loss  Reflection  Refraction  Diffraction  Scattering  VSWR  Amplification & attenuation  Identify and understand application of basic RF antenna concepts:  Visual LOS  RF LOS  The Fresnel Zone  Intentional Radiator  EIRP  Wave propagation  Understand and apply the basic components of RF mathematics:  Watt  Milliwatt  Decibel (dB)  dBm  dBi CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. CHAPTER 5 CHAPTER 2 In This Chapter RF Behavior Principles of Antennas RF Mathematics Chapter 2 – RF Fundamentals 18 In order to understand the wireless aspects of a wireless LAN, an administrator must have a solid foundation in the fundamentals of radio frequency (RF) theory. In this chapter we will discuss the properties of RF radiation and how its behavior in certain situations can affect the performance of a wireless LAN. Antennas will be introduced to create a good understanding of their uses and properties. We will discuss the mathematical relationships that exist in RF circuits and why they are important, as well as how to perform the necessary RF math calculations. To a wireless LAN administrator, an understanding of RF concepts is essential to the implementation, expansion, maintenance, and troubleshooting of the wireless network. Radio Frequency Radio frequencies are high frequency alternating current (AC) signals that are passed along a copper conductor and then radiated into the air via an antenna. An antenna converts/transforms a wired signal to a wireless signal and vice versa. When the high frequency AC signal is radiated into the air, it forms radio waves. These radio waves propagate (move) away from the source (the antenna) in a straight line in all directions at once. If you can imagine dropping a rock into a still pond (Figure 2.1) and watching the concentric ripples flow away from the point where the rock hit the water, then you have an idea of how RF behaves as it is propagated from an antenna. Understanding the behavior of these propagated RF waves is an important part of understanding why and how wireless LANs function. Without this base of knowledge, an administrator would be unable to locate proper installation locations of equipment and would not understand how to troubleshoot a problematic wireless LAN. FIGURE 2.1 Rock into a pond CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. 19 Chapter 2 – RF Fundamentals RF Behaviors RF is sometimes referred to as "smoke and mirrors" because RF seems to act erratically and inconsistently under given circumstances. Things as small as a connector not being tight enough or a slight impedance mismatch on the line can cause erratic behavior and undesirable results. The following sections describe these types of behaviors and what can happen to radio waves as they are transmitted. Gain Gain, illustrated in Figure 2.2, is the term used to describe an increase in an RF signal's amplitude. Gain is usually an active process; meaning that an external power source, such as an RF amplifier, is used to amplify the signal or a high-gain antenna is used to focus the beamwidth of a signal to increase its signal amplitude. FIGURE 2.2 Power Gain Gain as seen by an Oscilloscope Peak Amplitude after Gain Peak Amplitude before Gain Gain of DSSS as seen by a spectrum analyzer However, passive processes can also cause gain. For example, reflected RF signals can combine with the main signal to increase the main signal's strength. Increasing the RF signal's strength may have a positive or a negative result. Typically, more power is better, but there are cases, such as when a transmitter is radiating power very close to the legal power output limit, where added power would be a serious problem. Loss Loss describes a decrease in signal strength (Figure 2.3). Many things can cause RF signal loss, both while the signal is still in the cable as a high frequency AC electrical signal and when the signal is propagated as radio waves through the air by the antenna. Resistance of cables and connectors causes loss due to the converting of the AC signal to heat. Impedance mismatches in the cables and connectors can cause power to be reflected back toward the source, which can cause signal degradation. Objects directly in the propagated wave's transmission path can absorb, reflect, or destroy RF signals. Loss can be intentionally injected into a circuit with an RF attenuator. RF attenuators are CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. Chapter 2 – RF Fundamentals 20 accurate resistors that convert high frequency AC to heat in order to reduce signal amplitude at that point in the circuit. FIGURE 2.3 Power Loss Loss as seen by an Oscilloscope Peak Amplitude before Loss Peak Amplitude after Loss Loss of DSSS as seen by a spectrum analyzer There are many things that can affect an RF signal between the transmitter and receiver. In order for gains or losses to be relevant to the implementation of wireless LANs, they must be quantifiable. The section in this chapter about RF mathematics will discuss quantifiable loss and gain and how to calculate and compensate for them. Being able to measure and compensate for loss in an RF connection or circuit is important because radios have a receive sensitivity threshold. A sensitivity threshold is defined as the point at which a radio can clearly distinguish a signal from background noise. Since a receiver’s sensitivity is finite, the transmitting station must transmit a signal with enough amplitude to be recognizable at the receiver. If losses occur between the transmitter and receiver, the problem must be corrected either by removing the objects causing loss or by increasing the transmission power. Reflection Reflection, as illustrated in Figure 2.4, occurs when a propagating electromagnetic wave impinges upon an object that has very large dimensions when compared to the wavelength of the propagating wave. Reflections occur from the surface of the earth, buildings, walls, and many other obstacles. If the surface is smooth, the reflected signal may remain intact, though there is some loss due to absorption and scattering of the signal. CWNA Study Guide © Copyright 2002 Planet3 Wireless, Inc. [...]... of +2 dB Example Given the RF circuit in Figure 2. 14, determine the power at all marked points in milliwatts CWNA Study Guide © Copyright 20 02 Planet3 Wireless, Inc 35 Chapter 2 – RF Fundamentals FIGURE 2. 14 Sample wireless LAN configuration Point A Point B cable connector Access point connector cable antenna connector Point C = = = = = = Access Point 100 mW 100 mW 100 mW 50 mW 25 mW 12. 5 mW 20 0 mW... Access point connector cable antenna connector Point C = = = = = = Access Point 100 mW 100 mW 100 mW 50 mW 25 mW 12. 5 mW 20 0 mW Point A Point B Point C -3 dB -3 dB -3 dB 2 2 2 2 2 2 2 2 2 Point D Point D + 12 dBi (x2 x2 x2 x2) x16 x16 x16 x16 Accurate Measurements Although these techniques are helpful and expedient in some situations, there are times when rounded or even numbers may not be available... factor of 2 one time yielding the following: 1 mW x 10 = 10 mW 10 mW x 10 = 100 mW 100 mW x 10 = 1,000 mW 1,000 mW x 10 = 10,000 mW 10,000 mW x 2 = 20 ,000 mW = 20 watts So, we now see that +43 dBm equals 20 watts of power Another example that takes into consideration measurement negative from the reference point would be -26 dBm CWNA Study Guide © Copyright 20 02 Planet3 Wireless, Inc Chapter 2 – RF Fundamentals... when planning a wireless LAN, and all of these factors are related mathematically The following section explains the units of measurement that are used to calculate power output when configuring wireless LAN devices CWNA Study Guide © Copyright 20 02 Planet3 Wireless, Inc Chapter 2 – RF Fundamentals 30 Units of Measure There are a few standard units of measure that a wireless network administrator should... October 1, 20 00 Equivalent Isotropically Radiated Power (EIRP) EIRP is the power actually radiated by the antenna element, as shown in Figure 2. 12 This concept is important because it is regulated by the FCC and because it is used in calculating whether or not a wireless link is viable EIRP takes into account the gain of the antenna CWNA Study Guide © Copyright 20 02 Planet3 Wireless, Inc 29 Chapter 2 – RF... in mW? A 20 0 mW B 400 mW C 800 mW D 1 W CWNA Study Guide © Copyright 20 02 Planet3 Wireless, Inc 41 Chapter 2 – RF Fundamentals 14 Given an access point with an output power of 20 dBm connected through a cable with a loss of 6 dB to an amplifier with a 10 dB gain, then through a cable with 3 dB of loss to an antenna with 6 dBi of gain, what is the EIRP in dBm? A 18 dBm B 23 dBm C 25 dBm D 27 dBm 15... factor of 10 yielding 500 mW of output power at the antenna element (referred to as EIRP) 12 B Converting 20 0 mW to dBm, we see that we start out with 23 dBm From this point, it's a simple addition/subtraction problem 23 dBm - 6 dB + 9 dBi = 26 dBm CWNA Study Guide © Copyright 20 02 Planet3 Wireless, Inc Chapter 2 – RF Fundamentals 44 13 C Starting with 100 mW of output power, and not being able to use... the job tasks of a network administrator CWNA Study Guide © Copyright 20 02 Planet3 Wireless, Inc 25 Chapter 2 – RF Fundamentals Solutions to VSWR To prevent the negative effects of VSWR, it is imperative that all cables, connectors, and devices have impedances that match as closely as possible to each other Never use 75Ohm cable with 50-Ohm devices, for example Most of today’s wireless LAN devices... power = 20 dBm Final power = 33 dBm Change in power, ∆P = 33 – 20 = +13 dB, the value is positive, indicating an increase in power If the power levels are given in milliwatts, the process can become more complicated: Initial power = 130 mW Final power = 5 .2 W Change in power, CWNA Study Guide © Copyright 20 02 Planet3 Wireless, Inc Chapter 2 – RF Fundamentals 36 P  ∆P = 10 log f  P   i   5.2W ... diffraction, and reflection in the same way as RF frequencies Figure 2. 9 illustrates LOS RF works very much the same way as visible light within wireless LAN frequencies with one major exception: RF LOS can also be affected by blockage of the Fresnel Zone CWNA Study Guide © Copyright 20 02 Planet3 Wireless, Inc Chapter 2 – RF Fundamentals 26 FIGURE 2. 9 Line of Sight Line of Site Imagine that you are looking . not a wireless link is viable. EIRP takes into account the gain of the antenna. CWNA Study Guide © Copyright 20 02 Planet3 Wireless, Inc. 29 Chapter 2 – RF Fundamentals FIGURE 2. 12 EIRP. today. CWNA Study Guide © Copyright 20 02 Planet3 Wireless, Inc. Chapter 1 - Introduction to Wireless LANs 16 11. A. The first wireless LAN standard was the 8 02. 11 standard using the 2. 4 GHz. beyond the scope of this text and the job tasks of a network administrator. CWNA Study Guide © Copyright 20 02 Planet3 Wireless, Inc. 25 Chapter 2 – RF Fundamentals Solutions to VSWR To prevent