Transcript Fundamentals of Wireless LANs 1.2

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/f
f = 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-tomultipoint 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:
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–
–
–
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:
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–
–
–
reflection or bouncing
refraction or bending
diffraction or spreading around obstacles
scattering or being redirected by particles
EM Radiation
Visible Light
EM waves can be classified by their frequency in Hz or their wavelength in meters.
Increasing frequency and energy / decreasing wavelength
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 fastmoving 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:
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 nonoverlapping 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
Channel
Identifier
Center
Frequency
Regulatory Domain
Americas
Europe, Middle East and Asia
Japan
Israel
1
2412 MHz
X
X
X
2
2417 MHz
X
X
X
3
2422 MHz
X
X
X
X
4
2427 MHz
X
X
X
X
5
2432 MHz
X
X
X
X
6
2437 MHz
X
X
X
X
7
2442 MHz
X
X
X
X
8
2447 MHz
X
X
X
X
9
2452 MHz
X
X
X
X
10
2457 MHz
X
X
X
11
2462 MHz
X
X
X
12
2467 MHz
X
X
13
2472 MHz
X
X
14
2484 MHz
X
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
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•
•
11 “chips per bit” means each bit sent redundantly
11 Mbps data rate
3 access points can occupy same area
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
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%,
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
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.
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
Diffracted
Signal
• 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.
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.
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.
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.