CWNA Guide to Wireless LANs, Second Edition 3-13

Chapter 3

How Wireless Works

At a Glance

Instructor’s Manual Table of Contents

·  Overview

·  Objectives

·  Teaching Tips

·  Quick Quizzes

·  Class Discussion Topics

·  Additional Projects

·  Additional Resources

·  Key Terms

Lecture Notes

Overview

In this chapter, the students will explore the fundamentals of how wireless technology works. Although wireless transmissions can take place using infrared or radio waves as discussed in Chapter 2, radio wave transmission is the preferred method used today and is the focus of this chapter. The students begin by looking at the principles behind sending voice and data through radio wave transmissions. Next, they learn how radio frequency waves actually behave and how this behavior can be measured. Finally, the students will find out about one of the most important elements of a radio wave system, namely the antenna.

Chapter Objectives

·  Explain the principals of radio wave transmissions

·  Describe RF loss and gain, and how it can be measured

·  List some of the characteristics of RF antenna transmissions

·  Describe the different types of antennas

Teaching Tips

Radio Wave Transmission Principals

  1. Explain that understanding the principles of radio wave transmission is important not only for troubleshooting wireless LANs but also to create a context for understanding wireless terminology.

What Are Radio Waves?

  1. Define the term electromagnetic wave. Stress that in wireless communication, data travels on radio waves, which are a form of electromagnetic wave. Mention the similarity between electromagnetic waves and the waves emitted from a hose, illustrating with Figure 3-1.
  1. Discuss light and heat waves briefly, mentioning their physical limitations.
  1. Introduce the concept of radio waves. Explain how radio waves are generated and how they propagate. Discuss the physical differences between radio waves and light or heat wave, using Table 3-1 to facilitate the discussion.

Teaching

Tip / All forms of electromagnetic energy, from gamma rays to radio waves, travel through space in waves.

Teaching

Tip / Guglielmo Marconi first used radio waves to transmit a Morse code signal across the Atlantic Ocean in 1901. This type of transmission was originally called “wireless.” An international conference in Berlin in 1906 officially changed the name “wireless” to “radio,” from the Latin word radius meaning a ray or beam.

Teaching

Tip / The electromagnetic spectrum is divided in octaves, the natural way to represent frequencies. An octave represents eight diatonic degrees or a gradual frequencyincreasing of a 10-factor. The spectrum is also divided into bands. Each band represents around 3 octaves.

Analog vs. Digital Transmissions

  1. Discuss the form of analog and digital transmissions, illustrating with Figure 3-2 and 3-4 respectively. Stress that analog signals are continuous, while digital signals are discrete.
  1. Explain the concepts of modulation and demodulation. Explain how a modem is used in the process of the transmission of digital signals over an analog medium.

Teaching

Tip / Make sure that the students realize that computers operate using digital signals (binary code is discrete, thus it is digital). When analog data, such as a video image or an audio sound, needs to be stored on the computer, it must be first converted into a digital format.
  1. Explain that wireless transmissions are digital transmissions.


Frequency

  1. Explain the concept of frequency as it relates to waves. Illustrate long waves and short waves with Figures 3-5 and 3-6 respectively.
  1. Explain that the term hertz is used as the unit of measurement for frequency. Mention the terms kilohertz, megahertz, and gigahertz.
  1. Explain the concept of a sine wave, illustrating with Figure 3-7. Discuss the characteristics of a sine wave.

Teaching

Tip / In electrical terms the cycle produces what is known as an alternating current (AC) because it flows between positive (+) and negative (-). AC is the type of current that runs to the electrical outlets in a house. Direct current (DC) is found in batteries. With DC the current flows only from one terminal (+) to the other (-) and does not alternate.
  1. Using Table 3-2 as a guide, discuss the various terminology used for electricity.

Teaching

Tip / The formula for voltage (V=I*R) was first proposed in 1827 by German physicist George Ohm and is called Ohm’s Law.
  1. Explain that the frequency of the radio wave can be changed by modifying the voltage. Discuss the concept of a carrier signal. Illustrate with Figure 3-8.

Teaching

Tip / It is important that students think about radio waves, and waves in general, in three dimensions.

Modulation

  1. Explain that the carrier signal sent by radio transmissions is simply a continuous electrical signal, which itself carries no information.
  1. Discuss the three types of modulation that allow a carrier signal to carry information. Explain that modulation can be done on either analog or digital transmissions.

Teaching

Tip / Make sure the students understand that, although WLAN transmissions are digital, understanding analog modulation helps in the understanding of digital modulation.

Analog Modulation

  1. Discuss the concepts of amplitude and amplitude modulation, illustrating with Figure 3-9 and 3-10 respectively. Mention that the height of the carrier can be changed so that a higher wave represents a 1 bit while a lower wave represents a 0 bit.

Teaching

Tip / Amplitude modulation is most frequently used by broadcast radio stations. However, AM is often susceptible to interference from outside sources such as lightning from a thunderstorm, and is not generally used for data transmissions.
  1. Discuss the concept of frequency modulation, illustrating with Figure 3-11. Explain that, when using frequency modulation, the number of waves needed to represent a 1 bit is more than the number of waves needed to represent a 0 bit.

Teaching

Tip / Like amplitude modulation, frequency modulation is often used by broadcast radio stations. However, unlike AM, FM is not as susceptible to interference from outside sources.
  1. Discuss the concept of phase modulation. Illustrate with Figure 3-12. Mention that the change in the starting point of a cycle takes place only when the bits being transmitted change from a 1 bit to a 0 bit or vice versa.

Teaching

Tip / Although radio broadcasts use either amplitude modulation (AM) or frequency modulation (FM), television broadcasts actually use AM, FM, and phase modulation (PM). Television video uses AM, the sound uses FM, and the color information uses PM.

Digital Modulation

  1. Using the list on page 84 of the text as a guide, discuss the advantages of digital modulation over analog modulation. Mention that almost all wireless transmission uses digital modulation.
  1. Explain that, with digital modulation, the changes to a carrier signal are in discrete steps using binary signals. Explain that digital modulation uses the same three types of modulation that analog modulation uses.
  1. Discuss the amplitude shift keying technique for digital modulation. Illustrate with Figure 3-13.

Teaching

Tip / Digital (binary) modulation is still shown as a standard sine wave.
  1. Discuss the frequency shift keying technique for digital modulation. Illustrate with Figure 3-14.
  1. Discuss the phase shift keying technique for digital modulation. Illustrate with Figure 3-15.

Radio Frequency Behavior and Measurement

  1. Explain that one of the key elements of managing a wireless LAN is the understanding of how radio frequency (RF) signals behave.

RF Behavior

  1. Explain that the behavior of an RF signal can be categorized by whether something adds power to the signal or takes power away from the signal.

Gain

  1. Define the term gain. Explain that gain is achieved by an amplification of the signal. Illustrate with Figure 3-16.
  1. Mention that gain can occur intentionally or unintentionally, and describe generally how this occurs.

Loss

  1. Define the terms loss and attenuation. Illustrate with Figure 3-17.
  1. Explain the reasons why intentional loss may be implemented.
  1. Explain that loss is usually unintentional, and that there are many factors that can result in signal loss. Using the list on pages 86 through 89 of the text as a guide, discuss in detail the phenomena that can result in signal loss. Illustrate with Figures 3-18 through 3-23.

Teaching

Tip / A useful tool for illustrating some of the types of loss is an animation. Some animations of these phenomena can be found online. For instance, animations illustrating reflection, refraction, and diffraction can be found at:
http://lectureonline.cl.msu.edu/~mmp/kap13/cd372.htm

RF Measurement

  1. Explain that, because RF signals can be affected by the environment, it is sometimes necessary to be able to calculate the gain or loss of the signal. Mention that, for WLAN network managers, approximations can often be sufficient, but these measurements are still important.

RF Math

  1. Explain that RF power can be measured by two different units on two different scales.

2.  Discuss the milliwatts scale, mentioning that it is a linear scale. Explain that the reference point on this scale is zero, or the absence of power. Stress that this scale does not account for the relationship of loss or gain to the whole.

  1. Discuss the concept of a relative scale. Stress that the relative scale is not as precise as a linear scale, but presents a better picture of the loss or gain relative to the whole.
  1. Discuss the concept of a logarithm, and explain that relative scales often are logarithmic.

5.  Explain that RF power gain and loss on a relative scale are measured in decibels.

6.  Using Table 3-3 as a guide, discuss the 10’s and 3’s Rules of RF Math.

Teaching

Tip / Make sure that the students understand that the Certified Wireless Network Administrator (CWNA) exam does not require you to perform logarithmic calculations but does require you to understand the concepts of the 10’s and 3’s Rules of RF Math.
  1. Explain that the reference point that relates the logarithmic relative decibel (dB) scale to the linear milliwatt scale is known as the dBm.
  1. Discuss the EIRP scale. Explain that EIRP uses the dBi unit of measurement. Mention that the reference point of dBi is a theoretical antenna with 100 percent efficiency.

WLAN Measurements

  1. Explain that in the United States, the Federal Communications Commission (FCC) defines power limitations for wireless LANS. Mention that the reason for these limitations is to limit the distance that they can transmit.
  1. Explain that the TPO is a measure of the power being delivered to the transmitting antenna. Mention that the TPO limitation set by the FCC for WLANs is 1,000 mW or 30 dBm.
  1. Using Table 3-4 as a guide, discuss the EIRP for 802.11b and 802.11g wireless networks.
  1. Explain the purpose of the RSSI. Mention that vendors use an algorithm or a look-up table to convert RSSI values to dBm, mW, or signal strength percentage.

Quick Quiz 1

  1. When an electric current passes through a wire, it creates a magnetic field in the space around the wire. As this magnetic field radiates or moves out, it creates an electromagnetic ______wave that spreads out through space in all directions.

Answer: radio

  1. A(n) ______signal is a continuous signal with no “breaks” in it.

Answer: analog

  1. The electrical pressure on a wire is measured in ______.

Answer: volts

  1. True or False: Modulation can only be performed on analog signals.

Answer: False

  1. ______modulation changes the starting point of a cycle.

Answer: Phase

  1. ______is a binary modulation technique similar to amplitude modulation.

Answer: Amplitude shift keying (ASK)

  1. ______is defined as the positive difference in amplitude between two signals.

Answer: Gain

  1. When an RF signal moves from one medium to another of a different density the signal actually bends instead of traveling in a straight line, in a process known as ______.

Answer: refraction

  1. ______is bending caused by an object in the path of the transmission.

Answer: Diffraction

  1. RF power gain and loss on a relative scale are measured in ______.

Answer: decibels (dB)

  1. The chipset in 802.11 devices provides a value called the ______which is used to determine different factors, such as when transmission is allowed, when a roaming device should be associated with a different access point, and what data rate (transmission speed) should be used for transmission

Answer: Receive Signal Strength Indicator (RSSI)

Antennas

  1. Provide an overview of antennas. Explain that without antennas, radio waves would be unable to travel long distances.

Antenna Concepts

  1. Explain that radio waves are transmitted and received by using antennas. Describe the general structure of an antenna.
  1. Discuss how radio waves are generated, transmitted, and received by antennas. Illustrate with Figure 3-24.
  1. Describe the purpose of an intentional radiator.

Characteristics of RF Antenna Transmissions

  1. Provide a brief overview of the characteristics of RF antenna transmissions.

Polarization

  1. Introduce the concept of polarization. Explain that waves follow the plane of their electrical fields, and the electric field is parallel to the radiating elements.
  1. Discuss vertical and horizontal polarization. Illustrate vertical polarization with Figure 3-25. Stress that devices with antennas that are not polarized in the same way are not able to communicate with each other effectively.

Teaching

Tip / Polarization is typically referred to as being horizontal or vertical, but the actual polarization can be at any angle. Circular polarization is also possible.

Wave Propagation

  1. Introduce the concept of wave propagation. Illustrate sky wave propagation and RF LOS propagation with Figures 3-26 and 3-27, respectively. Stress that RF LOS propagation is associated with WLANs.

Multipath Distortion

  1. Discuss the concept of multipath distortion. Explain that some signals sent via RF LOS propagation encounter obstructions that refract or diffract the signal, causing some of the signal to reach the receiving antenna later.

Teaching

Tip / The “late” multipath signals can actually reduce the power of the straight line signals.
  1. Explain that antenna diversity uses multiple antennas, inputs, and receivers to overcome multipath distortion.

Teaching

Tip / A wireless NIC can contain multiple embedded antennas.

Fresnel Zone

  1. Discuss the concept of the Fresnel Zone, using Figure 3-28 to illustrate. Mention which zones are strongest and which are weakest.

Teaching