Fundamentals of Wireless LANs 1.2

Module 3:

Wireless Radio Technology

Module Overview

Module Overview

• In this module, the student will learn about wireless technology and radio waves.

• This module will explore the technology and the mathematics of radio, so that the reader can understand how invisible radio waves work to make so many things possible, including WLANs.

Waves – Sine Waves

• A waveform is a representation of how alternating current (AC) varies with time.– The most familiar AC waveform is the sine wave,

which derives its name from the fact that the current or voltage varies with the mathematical sine function of the elapsed time

• Frequency measured in cycles per second or Hertz (Hz). • A million cycles per second is represented by megahertz

(MHz)• A billion cycles per second represented by gigahertz (GHz)

Sine Wave

There is an inverse relationship between time and frequency: t = 1/ff = 1/t

Sine Wave Properties

• Amplitude – The distance from zero to the maximum value of each alternation is called the amplitude.

• Period – The time it takes for a sine wave to complete one cycle is defined as the period of the waveform. – The distance traveled by the sine wave during this period is

referred to as its wavelength.

• Wavelength – Indicated by the Greek lambda symbol λ. It is the distance between one value to the same value on the next cycle.

• Frequency – The number of repetitions or cycles per unit time is the frequency, typically expressed in cycles per second, or Hz.

Watts

• One definition of energy is the ability to do work. • There are many forms of energy, including:

– electrical energy– chemical energy– thermal energy– gravitational potential energy

• The metric unit for measuring energy is the Joule. • Energy can be thought of as an amount. • 1 Watt = I Joule of energy / one second

– If one Joule of energy is transferred in one second, this is one watt (W) of power.

Watts

• The U.S. Federal Communications Commission allows a maximum of 4 watts of power to be emitted in point-to-multipoint WLAN transmissions in the unlicensed 2.4-GHz band.

• Typical WLAN NICS transmit at 100 mW. • Typical Access Points can transmit between 30 to 100 mW

(plus the gain from the Antenna).

Watts

• Power levels on a single WLAN segment are rarely higher than 100 mW, enough to communicate for up to three-fourths of a kilometer or one-half of a mile under optimum conditions.

• Access points generally have the ability to radiate from 30 to100 mW, depending on the manufacturer.

• Outdoor building-to-building applications (bridges) are the only ones that use power levels over 100 mW.

Decibels

• The decibel (dB) is a unit that is used to measure electrical power.– The dB is measured on a base 10 logarithmic

scale – The base increases ten-fold for every ten dB

measured

• The formula for calculating dB is:dB = 10 log10 (Pfinal/Pref)

Calculating dB

• dB = The amount of decibels. – This usually represents a loss in power such as when the wave

travels or interacts with matter, but it can also represent a gain as when traveling through an amplifier.

• Pfinal = The final power. – This is the delivered power after some process has occurred.

• Pref = The reference power. – This is the original power.

• There are also some general rules for approximating the dB and power relationship: – An increase of 3 dB = Double the power – A decrease of 3 dB = Half the power – An increase of 10 dB = Ten times the power – A decrease of 10 dB = One-tenth the power

Decibel Reference

The power gain or loss in a signal is determined by comparing it to this fixed reference point, the milliwatt.

dB milliWatt (dBm)

• dB milliWatt (dBm) – This is the unit of measurement for signal strength or power level.

• If a person receives a signal at one milliwatt, this is a loss of zero dBm. However, if a person receives a signal that is 0.001 milliwatt, then a loss of 30 dBm occurs. – This loss is represented as -30 dBm.

• To reduce interference with others, the 802.11b WLAN power levels are limited to the following: – 36 dBm EIRP by the FCC – 20 dBm EIRP by ETSI

EIRP = Effective Isotropic Radiated Power

dB dipole (dBd)

• dB dipole (dBd) – This refers to the gain an antenna has, as compared to a dipole antenna at the same frequency.

• A dipole antenna is the smallest, least gain practical antenna that can be made.

dB isotropic (dBi)

• dB isotropic (dBi) – This refers to the gain a given antenna has, as compared to a theoretical isotropic, or point source, antenna.– An isotropic antenna cannot exist in the real world,

but it is useful for calculating theoretical coverage and fade areas.

• A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna. – For example, a simple dipole antenna has a gain of

2.14 dBi or 0 dBd.

Effective Isotropic Radiated Power

• Effective Isotropic Radiated Power (EIRP) – is defined as the effective power found in the main lobe of a transmitter antenna.

• EIRP is equal to the sum of the antenna gain, in dBi, plus the power level, in dBm, into that antenna.

http://en.wikipedia.org/wiki/EIRP

Gain

• Gain – This refers to the amount of increase in energy that an antenna adds to an RF signal.

• There are different methods for measuring gain, depending on the chosen reference point.

• Cisco Aironet wireless is standardized on dBi to specify gain measurements.

• Some antennas are rated in dBd. – To convert any number from dBd to dBi, simply add

2.14 to the dBd number.

Electromagnetic Waves – EM Waves

• The EM spectrum is simply a name that scientists have given to the set of all types of radiation.– Radiation is energy that travels in waves and spreads out over

distance.

• All EM waves travel at the speed of light in a vacuum and have a characteristic wavelength (λ) and frequency (f) which can be determined by using the following equation:

c = λ x f, where c = the speed of light (3 x 108 m/s) • EM waves exhibit the following properties:

– reflection or bouncing – refraction or bending – diffraction or spreading around obstacles – scattering or being redirected by particles

EM Radiation

EM waves can be classified by their frequency in Hz or their wavelength in meters.

Visible Light

Eight EM Sections1. Power waves – These are the slowest of all EM radiation and therefore also have the

lowest energy and the longest wavelength. 2. Radio waves – This is the same kind of energy that radio stations emit into the air for

a radio to capture and play. However, other things such as stars and gases in space also emit radio waves. Many communication functions use radio waves.

3. Microwaves – Microwaves will cook popcorn in just a few minutes. In space, astronomers use microwaves to learn about the structure of nearby galaxies.

4. Infrared (IR) light – Infrared is often thought of as being the same thing as heat, because it makes our skin feel warm. In space, IR light maps the dust between stars.

5. Visible light – This is the range that is visible to the human eye. Visible radiation is emitted by everything from fireflies to light bulbs to stars. It is also emitted by fast-moving particles hitting other particles.

6. Ultra-violet (UV) light – It is well known that the sun is a source of ultraviolet (UV) radiation. It is the UV rays that cause our skin to burn. Stars and other hot objects in space emit UV radiation.

7. X-rays – A doctor uses X-rays to look at bones and a dentist uses them to look at teeth. Hot gases in the universe also emit X-rays.

8. Gamma rays – Natural and man-made radioactive materials can emit gamma rays. Big particle accelerators that scientists use to help them understand what matter is made of can sometimes generate gamma rays. However, the biggest gamma-ray generator of all is the universe, which makes gamma radiation in many ways.

The EM spectrum has eight major sections, which are presented in order of increasing frequency and energy, and decreasing wavelength:

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ISM Bands of Spectrum

In the US, it is the FCC that regulates spectrum use. In Europe, the European Telecommunications Standards Institute (ETSI) regulates the spectrum usage.

Noise

• A very important concept in communications systems, including WLANs, is noise.

• In the context of telecommunications, noise can be defined as undesirable voltages from both natural and technological sources. – Since noise is just another signal that produces waves, the noise

will be added to other signals – including wireless data!• Sources of noise in a WLAN include the electronics in the

WLAN system, plus radio frequency interference (RFI), and electromagnetic interference (EMI) found in the WLAN environment.

• Gaussian, or white noise affects all frequencies equally. • Narrowband interference would only interfere with some

radio stations or channels of a WLAN.

Modulation Techniques

• A carrier frequency is an electronic wave that is combined with the information signal and carries it across the communications channel. – For WLANs, the carrier frequency is 2.4 GHz or 5 GHz.

• Using carrier frequencies in WLANs has added complexity because the carrier frequency is changed by frequency hopping or direct sequence chipping, to make the signal more immune to interference and noise.

Spread Spectrum (SS)

• Spread-spectrum technology makes data transmission possible in the ISM bands

• SS diffuses radio signals over a wide range of frequencies– The FCC requires that devices using the ISM bands

use SS transmissions for data• By spreading data transmission over a wide

range of frequencies, the transmission will look like noise to other non 802.11 devices– This also allows spread-spectrum devices to be more

resilient to noise

Spread-Spectrum Technologies

• 802.11 uses three types of spread-spectrum technologies:– Frequency Hopping (FHSS) systems jump from

one frequency to another – legacy – Direct Sequence (DSSS) spread the signal over

a wide range of frequencies – 802.11b/g– Orthogonal Frequency Division Multiplexing

(OFDM) – 802.11a/g

Frequency Hopping

• Frequency hopping (FH) systems are the least costly to produce but allow for the lowest data rates

• FH rapidly changes from one frequency to another during data transmission using a predetermined pattern– This pattern is pseudorandom which means it is practically,

never the same• The receiver radio is synchronized to the

hopping sequence of the transmitting radio to enable the receiver to be on the right frequency at the right time. – The amount of time a sender stays at a particular frequency is

known as the dwell time

FHSS

• FHSS is a spread spectrum technique that uses frequency agility to spread data over more than 83 MHz of spectrum.

• Frequency agility is the ability of a radio to change transmission frequency quickly, within the useable RF frequency band.

FHSS (cont.)

• Frequency hopping avoids interference between two stations using the same band by using different hopping sequences– If any two stations do interfere with each other,

the interference is for such a short time that it appears as transient noise

Direct Sequence Spread Spectrum

• In the US, each channel operates from one of 11 defined center frequencies and extends 11 MHz in each direction

• For example, Channel 1 operates from 2.401 GHz to 2.423 GHz, which is 2.412 GHz plus or minus 11 MHz. Channel 2 uses 2.417 plus or minus 11 MHz, and so on. – There is significant overlap between adjacent channels. Center

frequencies are only 5 MHz apart, yet each channel uses 22 MHz of analog bandwidth.

– In fact, channels should be co-located only if the channel numbers are at least five apart. Channels 1 and 6 do not overlap, Channels 2 and 7 do not overlap, and so on.

• In Europe, ETSI has defined a total of 14 channels, which allows for four different sets of three non-overlapping channels.

Direct Sequence Spread-Spectrum (DSSS)

• Whereas FHSS uses each frequency for a short period of time in a repeating pattern, DSSS uses a wide frequency range of 22 MHz all of the time.

• Non-overlapping channels have 25 MHz of frequency between them which gives them a 3MHz buffer

• Each data bit becomes a chipping sequence, or a string of chips that are transmitted in parallel, across the frequency range.– This is also referred to as the chipping code

Chipping Code Example

1 = 00110011011 0 = 11001100100 0 = 11001100100 1 = 00110011011

802.11b Channels—FCC

2.4 GHz Channel Sets

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Regulatory DomainRegulatory DomainChannel IdentifierChannel Identifier

Channels- 2.4 GHz DSSS

• 11 “chips per bit” means each bit sent redundantly• 11 Mbps data rate• 3 access points can occupy same area

11 Channels – each channel 22 MHz wide

1 set of 3 non-overlapping channels

14 Channels – each channel 22 MHz wide

4 sets of 3 non-overlapping channels, only one set used at a time

Non-overlapping Channels - again

802.11b Throughput• 802.11b uses three different types of modulation,

depending upon the data rate used: – Binary phase shift keyed (BPSK) — BPSK uses one phase to

represent a binary 1 and another to represent a binary 0, for a total of one bit of binary data. – BPSK is utilized to transmit data at 1 Mbps. 

– Quadrature phase shift keying (QPSK) — With QPSK, the carrier undergoes four changes in phase and can thus represent two binary bits of data. – QPSK is utilized to transmit data at 2 Mbps.

– Complementary Code Keying (CCK) — CCK uses a complex set of functions known as complementary codes to send more data by representing 4 or 8 binary bits.– CCK is can transmit data at 5.5 Mbps (4 bits) and 11 Mbps (8bits).

Complementary Code Keying (CCK)

• CCK is an alternative encoding method to PSK which can encode 4 to 8 bits into a code word

• The benefit of CCK is that it uses an 8-bit encoding scheme instead of an 11-bit encoding scheme to produce 1.375 times as much data transmission as PSK

• When CCK encodes 4 binary bits at a time it produces 5.5Mbps of throughput and when CCK encodes 8 bits at a time it produces 11Mbps of throughput

DSSS Modulation and Data Rates

The ‘D’ in the beginning stands for Differential

http://en.wikipedia.org/wiki/Phase-shift_keying

Orthogonal Frequency Division Multiplexing

• The 802.11a and 802.11g standards both use orthogonal frequency division multiplexing (OFDM), to achieve data rates of up to 54 Mbps.

• OFDM works by breaking one high-speed data carrier into several lower-speed subcarriers, which are then transmitted in parallel.

• Each high-speed carrier is 20 MHz wide and is broken up into 52 subchannels, each approximately 300 KHz wide – OFDM uses 48 of these subchannels for data, while

the remaining four are used for error correction.

http://www.wave-report.com/tutorials/OFDM.htm

http://en.wikipedia.org/wiki/COFDM

OFDM Subcarriers

OFDM (52 of 64 sub-carriers used)

802.11a Modulation• The 802.11a standard specifies that all 802.11a-compliant

products must support three basic data rates which include:Binary Phase Shift Keying (BPSK) – encodes 125 Kbps of data per channel, resulting in a 6,000-Kbps, or 6 Mbps Quadrature Phase Shift Keying (QPSK) – encodes to 250 Kbps per channel, yielding a 12 Mbps data rate. 16-level Quadrature Amplitude Modulation (16-QAM) – encodes 4 bits per hertz, achieving a data rate of 24 Mbps.

• In addition, the standard also lets the vendor extend the modulation scheme beyond 24 Mbps.64-level Quadrature Amplitude Modulation (64-QAM), which yields 8 bits per cycle or 10 bits per cycle, for a total of up to 1.125 Mbps per 300-KHz channel. With 48 channels, this results in a 54 Mbps data rate.

Refraction

• A surface is considered smooth if the size of irregularities is small relative to the wavelength. Otherwise, it is considered to be rough.

• Electromagnetic waves are diffracted around intervening objects.

• If the object is small relative to the wavelength, it has very little effect and the wave will pass around the object undisturbed.

• However, if the object is large a shadow will appear behind the object and a significant amount of energy is reflected back toward the source.

Refraction

• Refraction (or bending) of signals is due to temperature, pressure, and water vapor content in the atmosphere.

• Amount of refractivity depends on the height above ground.• Refractivity is usually largest at low elevations.• The refractivity gradient (k-factor) usually causes microwave signals to curve slightly

downward toward the earth, making the radio horizon father away than the visual horizon.

• This can increase the microwave path by about 15%,

Normal Refraction

Refraction (straight line)

Sub-Refraction

Earth

Refraction

• Radio waves also bend when entering different materials. • This can be very important when analyzing propagation in the

atmosphere. • It is not very significant in WLANs, but it is included here, as part of a

general background for the behavior of electromagnetic waves.

Reflection

• Reflection is the light bouncing back in the general direction from which it came.

• When waves travel from one medium to another, a certain percentage of the light is reflected. – This is called a Fresnel reflection.

Reflected Waves• When a wireless signal encounters an obstruction, normally two things

happen:

1. Attenuation – The shorter the wavelength of the signal relative to the size of the obstruction, the more the signal is attenuated.

2. Reflection – The shorter the wavelength of the signal relative to the size of the obstruction, the more likely it is that some of the signal will be reflected off the obstruction.

Microwave Reflections

• Microwave signals:– Frequencies between 1 GHz – 30 GHz (this can vary among experts).– Wavelength between 12 inches down to less than 1 inch.

• Microwave signals reflect off objects that are larger than their wavelength, such as buildings, cars, flat stretches of ground, and bodes of water.

• Each time the signal is reflected, the amplitude is reduced.

Reflection

• Reflection is the light bouncing back in the general direction from which it came.

• Consider a smooth metallic surface as an interface. • As waves hit this surface, much of their energy will be bounced or reflected. • Think of common experiences, such as looking at a mirror or watching

sunlight reflect off a metallic surface or water. • When waves travel from one medium to another, a certain percentage of the

light is reflected. • This is called a Fresnel reflection (Fresnel coming later).

Reflection

• Radio waves can bounce off of different layers of the atmosphere. • The reflecting properties of the area where the WLAN is to be installed are

extremely important and can determine whether a WLAN works or fails. • Furthermore, the connectors at both ends of the transmission line going

to the antenna should be properly designed and installed, so that no reflection of radio waves takes place.

Reflections

Microwave Reflections

• Advantage: Can use reflection to go around obstruction.• Disadvantage: Multipath reflection – occurs when reflections

cause more than one copy of the same transmission to arrive at the receiver at slightly different times.

Multipath Reflection

Diffraction

• The spreading out of a wave around an obstacle is called diffraction– This spreading is sometimes referred to as bending

around an obstacle.

• Radio waves undergo both small-scale and large-scale diffraction. – An example of small-scale diffraction is radio waves

in a WLAN spreading around indoors. – An example of large-scale diffraction is radio waves

spreading around a mountain peak, to an inaccessible area.

Diffraction

• Diffraction of a wireless signal occurs when the signal is partially blocked or obstructed by a large object in the signal’s path.

• A diffracted signal is usually attenuated so much it is too weak to provide a reliable microwave connection.• Do not plan to use a diffracted signal, and always try to obtain an unobstructed path between microwave

antennas.

Diffracted Signal

Multipath

• In many common WLAN installations, the radio waves emitted from a transmitter are traveling at different angles.

• They can reflect off of different surfaces and end up arriving at the receiver at slightly different times.

• Multipath interference can cause high RF signal strength, but poor signal quality levels. – If this interference is destructive enough, the

messages will not get through.

• Reflected signals 1 and 2 take slightly longer paths than direct signal, arriving slightly later.

• These reflected signals sometimes cause problems at the receiver by partially canceling the direct signal, effectively reducing the amplitude.

• The link throughput slows down because the receiver needs more time to either separate the real signal from the reflected echoes or to wait for missed frames to be retransmitted.

• Solution discussed later.

Multipath Reflection

Path-Loss• A crucial factor of any communications system is how

much power from the transmitter actually reaches the receiver.

• All of the previous different effects discussed earlier can be combined and described by what are known as path loss calculations. – Path loss calculations determine how much power is lost along

the communications path.• Free-space loss (FSL) is the signal attenuation that

would result if all absorbing, diffracting, obstructing, refracting, scattering, and reflecting influences were sufficiently removed so as to have no effect on propagation. - The formula is as follows: FSL (in dB) = 20 log10(f) + 20 log10(d) + 36.6

Path-Loss (cont.)

• Every time the distance from the transmitter to the receiver is doubled, the signal level is lowered (or increased) by 6 dB.

• Also, for each frequency, there is a series of wavelengths, where energy will escape out of the transmission line and enter the surrounding space. This is called the launch effect.

• The launch effect typically occurs at multiples of half-wavelengths of the signal.

Summary

• This module covered the mathematics and physics necessary for understanding how WLANs operate. Although it is not usually necessary to perform complex calculations to install a WLAN, an understanding of the underlying principles makes it easier to account for the many factors that can interfere with the proper operation of the WLAN.

• When performing a site survey for a new or existing WLAN, be sure to take into account factors such as refraction, reflection, and multipath distortion that were discussed in this module.