CWNA guide to wireless LANs 2nd ch03

Digital Transmissions continued • Analog signals are continuous • Digital signals are discrete • Modem MOdulator/DEModulator: Used when digital signals must be transmitted over analog m

CWNA Guide to Wireless LANs, Second Edition

Chapter Three

How Wireless Works

• Explain the principals of radio wave transmissions

• Describe RF loss and gain, and how it can be

Radio Wave Transmission Principles

• Understanding principles of radio wave

transmission is important for:

– Troubleshooting wireless LANs

– Creating a context for understanding wireless

terminology

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

What Are Radio Waves? (continued)

Table 3-1: Comparison of wave characteristics

Analog vs Digital Transmissions

Figure 3-4: Digital signal

Figure 3-2: Analog signal

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

Figure 3-5: Long waves

Figure 3-6: Short Waves

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

Frequency (continued)

Figure 3-7: Sine wave

Frequency (continued)

Table 3-2: Electrical terminology

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

Frequency (continued)

Figure 3-8: Lower and higher frequencies

– Relative starting point

• Modulation can be done on analog or digital

transmissions

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

Analog Modulation (continued)

Figure 3-9: Amplitude

Analog Modulation (continued)

Figure 3-10: Amplitude modulation (AM)

Analog Modulation (continued)

Figure 3-11: Frequency modulation (FM)

Analog Modulation (continued)

Figure 3-12: Phase modulation (PM)

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

Digital Modulation (continued)

Figure 3-13: Amplitude shift keying (ASK)

Digital Modulation (continued)

Figure 3-14: Frequency shift keying (FSK)

Digital Modulation (continued)

Figure 3-15: Phase shift keying (PSK)

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

Radio Frequency Behavior: Gain

(continued)

Figure 3-16: Gain

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

Radio Frequency Behavior: Loss

(continued)

Figure 3-18: Absorption

Radio Frequency Behavior: Loss

(continued)

Figure 3-19: Reflection

Radio Frequency Behavior: Loss

(continued)

Figure 3-20: Scattering

Radio Frequency Behavior: Loss

(continued)

Figure 3-21: Refraction

Radio Frequency Behavior: Loss

(continued)

Figure 3-22: Diffraction

Radio Frequency Behavior: Loss

(continued)

Figure 3-23: VSWR

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

RF Measurement: RF Math

(continued)

Table 3-3: The 10’s and 3’s Rules of RF Math

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

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

Antenna Concepts

• Radio waves transmitted/received using antennas

Figure 3-24: Antennas are required for sending and receiving radio signals

Characteristics of RF Antenna

Transmissions

• Polarization: Orientation of radio waves as they

leave the antenna

Figure 3-25: Vertical polarization

Characteristics of RF Antenna Transmissions (continued)

• Wave propagation: Pattern of wave dispersal

Figure 3-26: Sky wave propagation

Characteristics of RF Antenna Transmissions (continued)

Figure 3-27: RF LOS propagation

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

Characteristics of RF Antenna Transmissions (continued)

• Determining extent of “late” multipath signals can

be done by calculating Fresnel zone

Figure 3-28: Fresnel zone

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

Characteristics of RF Antenna Transmissions (continued)

Table 3-5: Free space path loss for IEEE 802.11b and 802.11g WLANs

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

Antenna Types and Their Installations

• Highly-directional antennas: Send narrowly

focused signal beam

– Generally concave dish-shaped devices

– Used for long distance, point-to-point wireless links

Antenna Types and Their Installations

(continued)

Figure 3-29: Omni-directional antenna

Antenna Types and Their Installations

(continued)

Figure 3-30: Semi-directional antenna

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

• The carrier signal sent by radio transmissions is

simply a continuous electrical signal and the signal itself carries no information

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

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