CWNA guide to wireless LANs 2nd ch03

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 Edit 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 Edit 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 Edit What Are Radio Waves? (continued) Table 3-1: Comparison of wave characteristics CWNA Guide to Wireless LANs, Second Edit Analog vs Digital Transmissions Figure 3-2: Analog signal Figure 3-4: Digital signal CWNA Guide to Wireless LANs, Second Edit 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 Edit Frequency Figure 3-5: Long waves Figure 3-6: Short Waves CWNA Guide to Wireless LANs, Second Edit 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 Edit Frequency (continued) Figure 3-7: Sine wave CWNA Guide to Wireless LANs, Second Edit 10 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 Edit 38 Characteristics of RF Antenna Transmissions (continued) • Wave propagation: Pattern of wave dispersal Figure 3-26: Sky wave propagation CWNA Guide to Wireless LANs, Second Edit 39 Characteristics of RF Antenna Transmissions (continued) Figure 3-27: RF LOS propagation CWNA Guide to Wireless LANs, Second Edit 40 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 not encounter obstructions • Antenna diversity: Uses multiple antennas, inputs, and receivers to overcome multipath distortion CWNA Guide to Wireless LANs, Second Edit 41 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 Edit 42 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 Edit 43 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 Edit 44 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 Edit 45 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 Edit 46 Antenna Types and Their Installations (continued) Figure 3-29: Omni-directional antenna CWNA Guide to Wireless LANs, Second Edit 47 Antenna Types and Their Installations (continued) Figure 3-30: Semi-directional antenna CWNA Guide to Wireless LANs, Second Edit 48 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 Edit 49 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 Edit 50 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 Edit 51 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 Edit 52 ... to Wireless LANs, Second Edit Frequency (continued) Figure 3-7: Sine wave CWNA Guide to Wireless LANs, Second Edit 10 Frequency (continued) Table 3-2: Electrical terminology CWNA Guide to Wireless. .. combines with original signal to amplify it CWNA Guide to Wireless LANs, Second Edit 24 Radio Frequency Behavior: Gain (continued) Figure 3-16: Gain CWNA Guide to Wireless LANs, Second Edit 25 Radio... change frequency of carrier signal CWNA Guide to Wireless LANs, Second Edit 12 Frequency (continued) Figure 3-8: Lower and higher frequencies CWNA Guide to Wireless LANs, Second Edit 13 Modulation