CWNA Guide to Wireless LANs, Second Edition Chapter Three How Wireless Works

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 CWNA Guide to Wireless LANs, Second Edition

Radio Wave Transmission Principles Understanding principles of radio wave transmission is important for: Troubleshooting wireless LANs Creating a context for understanding wireless terminology CWNA Guide to Wireless LANs, Second Edition

What Are Radio Waves? Electromagnetic wave: Travels freely through space in all directions at speed of light Radio wave: When electric current passes through a wire it creates a magnetic field around the wire As magnetic field radiates, creates an electromagnetic radio wave Spreads out through space in all directions Can travel long distances Can penetrate non-metallic objects CWNA Guide to Wireless LANs, Second Edition

What Are Radio Waves? (continued) Table 3-1: Comparison of wave characteristics CWNA Guide to Wireless LANs, Second Edition

Analog vs. Digital Transmissions Figure 3-2: Analog signal Figure 3-4: Digital signal CWNA Guide to Wireless LANs, Second Edition

Analog vs. Digital Transmissions (continued) Analog signals are continuous Digital signals are discrete Modem (MOdulator/DEModulator): Used when digital signals must be transmitted over analog medium On originating end, converts distinct digital signals into continuous analog signal for transmission On receiving end, reverse process performed WLANs use digital transmissions CWNA Guide to Wireless LANs, Second Edition

Frequency Figure 3-5: Long waves Figure 3-6: Short Waves CWNA Guide to Wireless LANs, Second Edition

Frequency (continued) Frequency: Rate at which an event occurs Cycle: Changing event that creates different radio frequencies When wave completes trip and returns back to starting point it has finished one cycle Hertz (Hz): Cycles per second Kilohertz (KHz) = thousand hertz Megahertz (MHz) = million hertz Gigahertz (GHz) = billion hertz CWNA Guide to Wireless LANs, Second Edition

Frequency (continued) Figure 3-7: Sine wave CWNA Guide to Wireless LANs, Second Edition

Frequency (continued) Table 3-2: Electrical terminology CWNA Guide to Wireless LANs, Second Edition

Frequency (continued) Frequency of radio wave can be changed by modifying voltage Radio transmissions send a carrier signal Increasing voltage will change frequency of carrier signal CWNA Guide to Wireless LANs, Second Edition

Frequency (continued) Figure 3-8: Lower and higher frequencies CWNA Guide to Wireless LANs, Second Edition

Modulation Carrier signal is a continuous electrical signal Carries no information Three types of modulations enable carrier signals to carry information Height of signal Frequency of signal Relative starting point Modulation can be done on analog or digital transmissions CWNA Guide to Wireless LANs, Second Edition

Analog Modulation Amplitude: Height of carrier wave Amplitude modulation (AM): Changes amplitude so that highest peaks of carrier wave represent 1 bit while lower waves represent 0 bit Frequency modulation (FM): Changes number of waves representing one cycle Number of waves to represent 1 bit more than number of waves to represent 0 bit Phase modulation (PM): Changes starting point of cycle When bits change from 1 to 0 bit or vice versa CWNA Guide to Wireless LANs, Second Edition

Analog Modulation (continued) Figure 3-9: Amplitude CWNA Guide to Wireless LANs, Second Edition

Analog Modulation (continued) Figure 3-10: Amplitude modulation (AM) CWNA Guide to Wireless LANs, Second Edition

Analog Modulation (continued) Figure 3-11: Frequency modulation (FM) CWNA Guide to Wireless LANs, Second Edition

Analog Modulation (continued) Figure 3-12: Phase modulation (PM) CWNA Guide to Wireless LANs, Second Edition

Digital Modulation Advantages over analog modulation: Better use of bandwidth Requires less power Better handling of interference from other signals Error-correcting techniques more compatible with other digital systems Unlike analog modulation, changes occur in discrete steps using binary signals Uses same three basic types of modulation as analog CWNA Guide to Wireless LANs, Second Edition

Digital Modulation (continued) Figure 3-13: Amplitude shift keying (ASK) CWNA Guide to Wireless LANs, Second Edition

Digital Modulation (continued) Figure 3-14: Frequency shift keying (FSK) CWNA Guide to Wireless LANs, Second Edition

Digital Modulation (continued) Figure 3-15: Phase shift keying (PSK) CWNA Guide to Wireless LANs, Second Edition

Radio Frequency Behavior: Gain Gain: Positive difference in amplitude between two signals Achieved by amplification of signal Technically, gain is measure of amplification Can occur intentionally from external power source that amplifies signal Can occur unintentionally when RF signal bounces off an object and combines with original signal to amplify it CWNA Guide to Wireless LANs, Second Edition

Radio Frequency Behavior: Gain (continued) Figure 3-16: Gain CWNA Guide to Wireless LANs, Second Edition

Radio Frequency Behavior: Loss Loss: Negative difference in amplitude between signals Attenuation Can be intentional or unintentional Intentional loss may be necessary to decrease signal strength to comply with standards or to prevent interference Unintentional loss can be cause by many factors CWNA Guide to Wireless LANs, Second Edition

Radio Frequency Behavior: Loss (continued) Figure 3-18: Absorption CWNA Guide to Wireless LANs, Second Edition

Radio Frequency Behavior: Loss (continued) Figure 3-19: Reflection CWNA Guide to Wireless LANs, Second Edition

Radio Frequency Behavior: Loss (continued) Figure 3-20: Scattering CWNA Guide to Wireless LANs, Second Edition

Radio Frequency Behavior: Loss (continued) Figure 3-21: Refraction CWNA Guide to Wireless LANs, Second Edition

Radio Frequency Behavior: Loss (continued) Figure 3-22: Diffraction CWNA Guide to Wireless LANs, Second Edition

Radio Frequency Behavior: Loss (continued) Figure 3-23: VSWR CWNA Guide to Wireless LANs, Second Edition

RF Measurement: RF Math RF power measured by two units on two scales: Linear scale: Using milliwatts (mW) Reference point is zero Does not reveal gain or loss in relation to whole Relative scale: Reference point is the measurement itself Often use logarithms Measured in decibels (dB) 10’s and 3’s Rules of RF Math: Basic rule of thumb in dealing with RF power gain and loss CWNA Guide to Wireless LANs, Second Edition

RF Measurement: RF Math (continued) Table 3-3: The 10’s and 3’s Rules of RF Math CWNA Guide to Wireless LANs, Second Edition

RF Measurement: RF Math (continued) dBm: Reference point that relates decibel scale to milliwatt scale Equivalent Isotropically Radiated Power (EIRP): Power radiated out of antenna of a wireless system Includes intended power output and antenna gain Uses isotropic decibels (dBi) for units Reference point is theoretical antenna with 100 percent efficiency CWNA Guide to Wireless LANs, Second Edition

RF Measurement: WLAN Measurements In U.S., FCC defines power limitations for WLANs Limit distance that WLAN can transmit Transmitter Power Output (TPO): Measure of power being delivered to transmitting antenna Receive Signal Strength Indicator (RSSI): Used to determine dBm, mW, signal strength percentage Table 3-4: IEEE 802.11b and 802.11g EIRP CWNA Guide to Wireless LANs, Second Edition

Antenna Concepts Radio waves transmitted/received using antennas Figure 3-24: Antennas are required for sending and receiving radio signals CWNA Guide to Wireless LANs, Second Edition

Characteristics of RF Antenna Transmissions Polarization: Orientation of radio waves as they leave the antenna Figure 3-25: Vertical polarization CWNA Guide to Wireless LANs, Second Edition

Characteristics of RF Antenna Transmissions (continued) Wave propagation: Pattern of wave dispersal Figure 3-26: Sky wave propagation CWNA Guide to Wireless LANs, Second Edition

Characteristics of RF Antenna Transmissions (continued) Figure 3-27: RF LOS propagation CWNA Guide to Wireless LANs, Second Edition

Characteristics of RF Antenna Transmissions (continued) Because RF LOS propagation requires alignment of sending and receiving antennas, ground-level objects can obstruct signals Can cause refraction or diffraction Multipath distortion: Refracted or diffracted signals reach receiving antenna later than signals that do not encounter obstructions Antenna diversity: Uses multiple antennas, inputs, and receivers to overcome multipath distortion CWNA Guide to Wireless LANs, Second Edition

Characteristics of RF Antenna Transmissions (continued) Determining extent of “late” multipath signals can be done by calculating Fresnel zone Figure 3-28: Fresnel zone CWNA Guide to Wireless LANs, Second Edition

Characteristics of RF Antenna Transmissions (continued) As RF signal propagates, it spreads out Free space path loss: Greatest source of power loss in a wireless system Antenna gain: Only way for an increase in amplification by antenna Alter physical shape of antenna Beamwidth: Measure of focusing of radiation emitted by antenna Measured in horizontal and vertical degrees CWNA Guide to Wireless LANs, Second Edition

Characteristics of RF Antenna Transmissions (continued) Table 3-5: Free space path loss for IEEE 802.11b and 802.11g WLANs CWNA Guide to Wireless LANs, Second Edition

Antenna Types and Their Installations Two fundamental characteristics of antennas: As frequency gets higher, wavelength gets smaller Size of antenna smaller As gain increases, coverage area narrows High-gain antennas offer larger coverage areas than low-gain antennas at same input power level Omni-directional antenna: Radiates signal in all directions equally Most common type of antenna CWNA Guide to Wireless LANs, Second Edition

Antenna Types and Their Installations (continued) Semi-directional antenna: Focuses energy in one direction Primarily used for short and medium range remote wireless bridge networks Highly-directional antennas: Send narrowly focused signal beam Generally concave dish-shaped devices Used for long distance, point-to-point wireless links CWNA Guide to Wireless LANs, Second Edition

Antenna Types and Their Installations (continued) Figure 3-29: Omni-directional antenna CWNA Guide to Wireless LANs, Second Edition

Antenna Types and Their Installations (continued) Figure 3-30: Semi-directional antenna CWNA Guide to Wireless LANs, Second Edition

WLAN Antenna Locations and Installation Because WLAN systems use omni-directional antennas to provide broadest area of coverage, APs should be located near middle of coverage area Antenna should be positioned as high as possible If high-gain omni-directional antenna used, must determine that users located below antenna area still have reception CWNA Guide to Wireless LANs, Second Edition

Summary A type of electromagnetic wave that travels through space is called a radiotelephony wave or radio wave An analog signal is a continuous signal with no breaks in it A digital signal consists of data that is discrete or separate, as opposed to continuous The carrier signal sent by radio transmissions is simply a continuous electrical signal and the signal itself carries no information CWNA Guide to Wireless LANs, Second Edition

Summary (continued) Three types of modulations or changes to the signal can be made to enable it to carry information: signal height, signal frequency, or the relative starting point Gain is defined as a positive difference in amplitude between two signals Loss, or attenuation, is a negative difference in amplitude between signals RF power can be measured by two different units on two different scales CWNA Guide to Wireless LANs, Second Edition

Summary (continued) An antenna is a copper wire or similar device that has one end in the air and the other end connected to the ground or a grounded device There are a variety of characteristics of RF antenna transmissions that play a role in properly designing and setting up a WLAN CWNA Guide to Wireless LANs, Second Edition