Cisco WLAN - Chapter. 03 : wireless radio technology

Ch. 3 Wireless Radio
Technology
Cisco Fundamentals of Wireless LANs version 1.2

Rick Graziani graziani@cabrillo.edu 2
Note
• Much of the information in this Module has been
presented previously in the Module 2 PowerPoints and will
not be included in this presentation.
• Some of this information should be a review from CCNA 1:
– Sine waves, modulation, etc.
– Please review your CCNA materials if needed.
• This module contains several mathematical formulas.
– Examples will be included, but we will not discuss them
in any detail, nor will you be responsible for them on any
exam.

Acknowledgements
• Thanks Jack Unger and his
book Deploying License-Free
Wireless Wide-Area Networks
• Published by Cisco Press
• ISBN: 1587050692
• Published: Feb 26, 2003

Wireless Propagation
• There are several important simplifications which can be made.
• In a vacuum, 2.4 GHz microwaves travel at the speed of light.
• Once started, these microwaves will continue in the direction they were emitted
forever, unless they interact with some form of matter.
• In the atmosphere, the microwaves are traveling in air, not in a vacuum.
• This does not significantly change their speed.
• Similar to light, when RF travels through transparent matter, some of the waves
are altered.
• 2.4 & 5 GHz microwaves also change, as they travel through matter.
• Amount of alteration depends heavily on the frequency of the waves and the
matter.
• Wireless propagation is the
total of everything that happens
to a wireless signal as the signal
travels from Point A to Point B.
• The study of how EM waves
travel and interact with matter
can become extremely complex.

Wireless Propagation
Mental picture
• Wave is not a spot or a line, but a moving wave.
• Like dropping a rock into a pond.
• Wireless waves spread out from the antenna.
• Wireless waves pass through air, space, people, objects,…

Attenuation
• Attenuation is the loss in amplitude that occurs whenever a signal
travels through wire, free space, or an obstruction.
• At times, after colliding with an object the signal strength remaining is
too small to make a reliable wireless link.
Same wavelength (frequency),
less amplitude.

Attenuation and Obstructions
• Longer the wavelength (lower frequency) of the wireless signal, the
less the signal is attenuated.
Same wavelength
(frequency), less
amplitude.
• Shorter the wavelength (higher frequency) of the wireless signal, the
more the signal it is attenuated.

Attenuation and Obstructions
• The wavelength for the AM (810 kHz) channel is 1,214 feet
• The larger the wavelength of the signal relative to the size of the
obstruction, the less the signal is attenuated.
• The shorter the wavelength of the signal relative to the size of the
obstruction, the more the signal is attenuated.

Free-Space Waves
• Free-space wave is a signal that propagates from Point A
to Point B without encountering or coming near an
obstruction.
• The only amplitude reduction is due to “free space loss”
(coming).
• This is the ideal wireless scenario.

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

• 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

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

Weather - Precipitation
Precipitation: Rain, snow, hail, fog, and sleet.
• Rain, Snow and Hail
– Wavelength of 2.4 GHz 802.11b/g signal is 4.8 inches
– Wavelength of 5.7 GHz 802.11a signal is 2 inches
– Much larger than rain drops and snow, thus do not significantly
attenuate these signals.
• At frequencies 10 GHz and above, partially melted snow and hail do
start to cause significant attenuation.

Weather - Precipitation
• Rain can have other effects:
– Get inside tiny holes in antenna systems, degrading the
performance.
– Cause surfaces (roads, buildings, leaves) to become more
reflective, increasing multipath fading.
• Tip: Use unobstructed paths between antennas, and do not try to blast
through trees, or will have problems.

Weather - Ice
• Ice buildup on antenna systems can:
– Reduce system performance
– Physically damage the antenna system
Collapsed tower

Weather - Wind
• The affect of wind:
– Antenna on the the mast or tower can turn, decreasing the aim of
the antenna.
– The mast or tower can sway or twist, changing the aim.
– The antenna, mast or tower could fall potentially injuring someone
or something.

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.

Working with Wireless Power
More on all these in a moment…
• Power can be:
– Increased (gain)
– Decreased (loss)
• Power can be:
– Relative (ex: twice as much power or ½ as much power)
– Absolute (ex: 1 watt or 4 watts)
• Both relative and absolute power are always referenced to initial power
level:
– Relative power level
– Absolute power level
• Wireless power levels become very small, very quickly after leaving the
transmitting antenna.
• Wireless power levels are done in dB.
• Wireless power levels do not decrease linearly with distance, but
decrease inversely as the square of the distance increases…

Inverse square law
• “Signal strength does not fade in a linear manner, but inversely as the
square of the distance.
• This means that if you are a particular distance from an access point
and you move measure the signal level, and then move twice a far
away, the signal level will decrease by a factor of four.”
WildPackets White Paper on my web site.
Point A Point B
¼ the power of Point A
Twice the distance

Inverse square law
• Double the distance of the wireless link, we receive only ¼ of the
original power.
• Triple the distance of the wireless link, we receive only 1/9 the original
power.
• Move 5 times the distance, signal decreases by 1/25.
Point A
2 times the distance
¼ the power of Point A
10 20 30 40 50 100
1/9 the power of Point A
1/25 the power of Point A
1/100 the power of A

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.
• In WLANs, power levels as low as one milliwatt (mW), or one one-
thousandth (1/1000th) of a watt, can be used for a small area.
• 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.

Ratios
• Ratio is a comparison between two quantities.
• Ratios use a colon (:) to divide the two quantities.
2 Pennies 1 Penny
2 Pennies : 1 Penny
2 : 1 Ratio
100 Pennies 1 Penny
100 Pennies : 1 Penny
100 : 1 Ratio

Wireless Power Ratios
• Every dB (decibel) value is a ratio.
• These are three wireless power ratios; each uses 1 Watt (1 W) of
power as their reference point.
• The decibel (dB) is a unit that is used to measure electrical power.
• A dB is one-tenth (1/10th) of a Bel, which is a unit of sound named
after Alexander Graham Bell.
• The dB is measured on a base 10 logarithmic scale.
• The base increases ten-fold for every ten dB measured.
2 Watts
1 w
1 w
1 w 1 w
1 w
1 w 1 w
1 w
1 w
1 w
1 w
1 w
1 w
1 w
1 w
1 w
1 w
1 Watt 4 Watts 1 Watt 8 Watts 1 Watt
2:1 Ratio =
+ 3 dBW
4:1 Ratio =
+ 6 dBW
8:1 Ratio =
+ 9 dBW

Decibels
• The decibel scale allows people to work more easily with large
numbers.
• A similar scale called the Richter Scale.
– The Richter scale is logarithmic, that is an increase of 1 magnitude
unit represents a factor of ten times in amplitude.
– The seismic waves of a magnitude 6 earthquake are 10 times
greater in amplitude than those of a magnitude 5 earthquake.
– Each whole number increase in magnitude represents a tenfold
increase in measured amplitude; as an estimate of energy.
10x
10x

Decibels - FYI
• Calculating dB
The formula for calculating dB is as follows:
dB = 10 log10 (Pfinal/Pref)
– dB = The amount of decibels.
• This usually represents:
– a loss in power such as when the wave travels
or interacts with matter,
– 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.

Logarithms – Just another way of
expressing powers (10n) - FYI
x = ay
loga x = y
• Example: 100 = 102
• This is equivalent to saying that the base-10 logarithm of 100 is 2; that
is:
100 = 102 same as log10 100 = 2
• Example 2: 1000 = 103 is the same as: log10 1000 = 3
• Notes:
– With base-10 logarithms, the subscript 10 is often omitted;
log 100 = 2 same as log 1000 = 3
– When the base-10 logarithm of a quantity increases by 1, the quantity itself
increases by a factor of 10, ie. 2 to 3 increases the quantity 100 to 1000.
– A 10-to-1 change in the size of a quantity, resulting in a logarithmic
increase or decrease of 1, is called an order of magnitude.
– Thus, 1000 is one order of magnitude larger than 100.

Decibels
• There are also some general rules for approximating the
dB and power relationship:
– +3 dB = Double the power
– -3 dB = Half the power
– +10 dB = Ten times the power
– -10 dB = One-tenth the power

Decibel references
• dB has no particular defined reference
• Most common reference when working with WLANs is:
– dBm
– m = milliwatt or 1/1,000th of a watt
– 1,000 mW = 1 W (Watt)
• Milliwatt = .001 Watt or 1/1,000th of a watt
• Since the dBm has a defined reference, it can also be converted back
to watts, if desired.
• The power gain or loss in a signal is determined by comparing it
to this fixed reference point, the milliwatt.
WLANs work in
milliwatts or 1/1,000th of
a Watt

Decibel references
• Example:
– 1 mW = .001 Watts
– Using 1 mW as our reference we start at: 0 dB
– Using the dB formula:
• Doubling the milliwatts to 2 mW or .002 Watts we get +3 dBm
• +10 dBm is 10 times the original 1 mW value or 10 mW
• +20 dBm is 100 times the original 1 mW value or 100 mW

• dB milliWatt (dBm) - This is the unit of measurement for signal
strength or power level. (milliwatt = 1,000th of a watt or 1/1,000 watt)
• If the original signal was 1 mW and a device receives a signal at 1
mW, this is a loss of 0 dBm.
• However, if that same device receives a signal that is 0.001 milliwatt,
then a loss of 30 dBm occurs, or -30 dBm.
• -n dBm is not a negative number, but a value between 0 and 1.
• To reduce interference with others, the 802.11b WLAN power
levels are limited to the following:
– 36 dBm EIRP by the FCC (4 Watts)
– 20 dBm EIRP by ETSI
Ref.

Interactive Activity – Calculating decibels
• This activity allows the student to enter values for Power final and
Power reference, then calculates for decibels. Adding an antenna or
other type of amplification.
+10 dBm
Change
End
Start

Calculating decibels (FYI)
log10 100 = 2 same as 102 = 100
• 10 * log10 (10 / 1)
• 10 * log10 10 -> 10 to the ? = 10
• 10 * 1
• 10

+20 dBm
Change
End
Start

+3dBm
Change
End
Start

Interactive Activity – Using decibels
• This activity allows the student to enter a value for the decibels and a value for
the reference power resulting in the final power. Adding an antenna or other
type of amplification.
+10 dBm
Change
End
Start

Interactive Activity – Using decibels
• This activity allows the student to enter a value for the decibels and a value for
the reference power resulting in the final power. Adding an antenna or other
type of amplification.
+3 dBm
Change
End
Start

RF Receivers
• Radio receivers are very sensitive to and may be able to
pick up signals as small as 0.000000001 mW or –90 dBm,
or a 1 billionth of a milliwatt or 0.000000000001 W.
-90 dBm
Change
End
Start

• Doubled the distance 10ft to 20ft,
but have ¼ the signal.
• Signal strength decreased from –
47dB to –53dB.
• Decrease of 6dB
• 3dB + -3dB = ½ + ½ = ¼

Other decibel references besides mW
More on this
when we
discuss
antennas.

A simple decibel
conversion
• If a signal experiences a gain of 4,000 (gets 4,000 times bigger), what
is the gain in dB?
4,000 = 10 x 10 x 10 x 2 x 2
Now replace the multiplication-of factors by the addition-of factors of
dB:
4,000 = 10 dB + 10 dB + 10 dB + 3 dB + 3 dB = 36 dB
• If a signal experiences a gain of 4,000 (gets 4,000 times bigger), what
is the gain in dB? (Be creative!)
5,000 = 10 x 10 x 10 x 10 / 2
Now replace the multiplication-of factors by the addition-of factors of
dB and division by subtraction:
5,000 = 10 dB + 10 dB + 10 dB + 10 dB - 3 dB = 37 dB

ACU Status
• Current Signal Strength
– The Received Signal Strength Indicator (RSSI) for received packets.
The range is 0% to 100%.
• Current Signal Quality
– The quality of the received signal for all received packets. The range is
from 0% to 100%.

Signal
• Signal Strength
– The signal strength for all received packets.
– The higher the value and the more green the bar graph is, the stronger the
signal.
– Differences in signal strength are indicated by the following colors: green
(strongest), yellow (middle of the range), and red (weakest).
– Range: 0 to 100% or -95 to -45 dBm
• Signal Quality
– The signal quality for all received packets. The higher the value and the
more green the bar graph is, the clearer the signal.
– Differences in signal quality are indicated by the following colors: green
(highest quality), yellow (average), and red (lowest quality).
– Range: 0 to 100%
• Overall Link Quality
– Overall link quality depends on the Current Signal Strength and Current
Signal Quality values.
– Excellent: Both values greater than 75%
– Good: Both values greater than 40% but one (or both) less than 75%
– Fair: Both values greater than 20% but one (or both) less than 40%
– Poor: One or both values less than 20%

Signal
• Signal Strength can also be seen in dBm
• Noise Level
– The level of background radio frequency energy in the 2.4-GHz band. The
lower the value and the more green the bar graph is, the less background
noise present.
– Range: -100 to -45 dBm
– Note This setting appears only if you selected signal strength to be
displayed in dBm.
• Signal to Noise Ratio
– The difference between the signal strength and the current noise level. The
higher the value, the better the client adapter's ability to communicate with
the access point.
– Range: 0 to 90 dB
– Note This setting appears only if you selected signal strength to be
displayed in dBm.

Signal
• You will notice that the maximum Signal Strength is –45 dBm and
lowest Noise Level is –105 dBm.
• Why these values?
• This is beyond the scope of this curriculum but has to do with how
Radio Performance is measured.
• The Cisco Press book, 802.11 Wireless LAN Fundamentals is a good
start for more information, but you will still need to do more research to
fully understand this.
• See the white paper from WildPackets: Converting Signal Strength
Percentage to dBm Values.

Real World Measurements
• Measurements from an antenna transmitting 100mW at 1 inch
• Remember a milliwatt is 1/1,000th of a Watt
• Experiment only, actual measure power would include antenna
loss/gain, and certain environmental factors.
1” 100 mW 20 dBm
2” 25 mW 13.9 dBm
4” 6.25 mW 7.9 dBm
8” 1.56 mW 1.9 dBm
16” 0.39 mW -4.08 dBm
32” .097 mW -10.1 dBm
64” .024 mW -16.1 dBm (5.3 ft)
128” .006 mW -22.2 dBm (10.6 ft)
256” .0015 mW -28.2 dBm (21.3 ft)

Last note…
• As signal strength decreases, so will the transmission rate.
• An 802.11b client’s speed may drop from 11 Mbps to 5.5
Mbps, to 2 Mbps, or even 1 Mbps.
• This can all be associated with a combination of factors
including:
– Distance
– Line of Sight
– Obstructions
– Reflection
– Multpath Reflection
– Refraction (partially blocked by obstruction)
– Diffraction (bending of signal)
– Noise and Interference

TechTarget.com
• “We have an office in a commercial building that is 3500-
4000 sq. ft. in one floor, with permanent walls separating
each office. Is a single access point for an 802.11a
implementation enough to cover this area? Is there a
formula for determining the bandwidth attenuation through
walls? “
• To design coverage for your office, nothing really
substitutes for a thorough site survey. However, here are
some estimates on RF signal loss due to obstructions,
courtesy of the Planet3 Wireless CWNA Study Guide:
• dry wall = 5-8 dB
• six inch thick solid-core wall = 15-20 dB.
• http://expertanswercenter.techtarget.com/eac/knowledgeba
seAnswer/0,295199,sid63_gci976082,00.html

Ch. 3 Wireless Radio
Technology
Cisco Fundamentals of Wireless LANs version 1.1
Rick Graziani
Cabrillo College
Spring 2005

Cisco WLAN - Chapter. 03 : wireless radio technology

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