Fundamentals of Wireless LANs 1.2

1
Fundamentals of Wireless LANs 1.2

• Module 3
• Wireless Radio Technology

2
Module Overview
3
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.

4
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)

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Sine Wave
There is an inverse relationship between time and
frequency t 1/f f 1/t
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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
  ?. 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.

7
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.

8
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).

9
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.

10
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)

11
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

12
Decibel Reference
The power gain or loss in a signal is determined
by comparing it to this fixed reference point,
the milliwatt.
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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
14
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.

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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.

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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
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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.

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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 (?)
  and frequency (f) which can be determined by
  using the following equation
• c ? 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

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

1. 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.

Increasing frequency and energy / decreasing
wavelength
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.
22
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.

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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.

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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

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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

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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

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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.

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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

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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.

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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

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Chipping Code Example
1 00110011011 0 11001100100 0
11001100100 1 00110011011
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802.11b ChannelsFCC
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2.4 GHz Channel Sets
Regulatory Domain
Center Frequency
Channel Identifier
Americas
Europe, Middle East and Asia
Japan
Israel
X X X X X X X X X X X X X
X X X X X X X X X X X X X X
1 2 3 4 5 6 7 8 9 10 11 12 13 14
2412 MHz 2417 MHz 2422 MHz 2427 MHz 2432 MHz 2437
MHz 2442 MHz 2447 MHz 2452 MHz 2457 MHz 2462
MHz 2467 MHz 2472 MHz 2484 MHz
X X X X X X X X X X X
X X X X X X X
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Channels- 2.4 GHz DSSS

• 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

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

35
Non-overlapping Channels - again
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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).

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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

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DSSS Modulation and Data Rates
The D in the beginning stands for Differential
http//en.wikipedia.org/wiki/Phase-shift_keying
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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
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OFDM Subcarriers
41
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.

42
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.

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Refraction
Sub-Refraction
Refraction (straight line)
Normal Refraction
Earth

• 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,

44
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.

45
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.

46
Reflected Waves

• When a wireless signal encounters an obstruction,
  normally two things happen
• Attenuation The shorter the wavelength of the
  signal relative to the size of the obstruction,
  the more the signal is attenuated.
• 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.

47
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.

48
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).

49
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.

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Reflections
51
Microwave Reflections
Multipath Reflection

• 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.

52
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.

53
Diffraction
Diffracted Signal

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

54
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.

55
Multipath Reflection

• 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.

56
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

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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.

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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.