Ch. 3 Wireless Radio Technology

1
Ch. 3 Wireless Radio Technology
2
Acknowledgements

• Thanks Rick Graziani, Networking Professor with
  Cabrillo College, for allowing me to use your
  presentation material
• Thanks Jack Unger and his book Deploying
  License-Free Wireless Wide-Area Networks
• Published by Cisco Press
• ISBN 1587050692
• Published Feb 26, 2003
• Thanks Mark Ciampa and his book CWNA Guide to
  Wireless LANs
• Published by Course Technology
• ISBN 0-619-21579-8

3
Radio Wave Transmission Principles

• Understanding principles of radio wave
  transmission is important for
• Troubleshooting wireless LANs
• Creating a context for understanding wireless
  terminology

4
Attenuation
Same wavelength (frequency), less amplitude.

• 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.

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Attenuation and Obstructions

• Shorter the wavelength (higher frequency) of the
  wireless signal, the more the signal it is
  attenuated.

Same wavelength (frequency), less amplitude.

• Longer the wavelength (lower frequency) of the
  wireless signal, the less the signal is
  attenuated.

6
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.

7
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.

8
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.

9
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.

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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.

11
Multipath Reflection
Delay spread is a parameter used to signify
multipath. The delay of reflected signal is
measured in nanoseconds (ns). The amount of delay
spread varies for indoor home, office, and
manufacturing environments. Multipath and
Diversity Article from Cisco
12
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.

13
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.

14
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.

15
Weather - Ice
Collapsed tower

• Ice buildup on antenna systems can
• Reduce system performance
• Physically damage the antenna system

16
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.

17
Refraction

• Refraction (or bending) of signals is due to
  temperature, pressure, and water vapor content in
  the atmosphere. (Also could be the a result of
  the difference in air density)
• Amount of refractivity depends on the height
  above ground.
• Refractivity is usually largest at low elevations.

18
Working with Wireless Power
19
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 (0dbm)
• Absolute power level (1mw)
• Wireless power levels become very small, very
  quickly after leaving the transmitting antenna.
• Wireless power levels are done in dBm.
• Wireless power levels do not decrease linearly
  with distance, but decrease inversely as the
  square of the distance increases

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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.

Twice the distance
Point A
Point B
¼ the power of Point A
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Inverse square law
10
20
30
40
50
100
Point A
10 times the distance 1/100 the power of A
3 times the distance 1/9 the power of Point A
2 times the distance ¼ the power of Point A
5 times the distance 1/25 the power of Point A

• 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.

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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).

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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.

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Wireless Power Ratios
1 w
1 w
dB 10 log10 (Pfinal/Pref)
1 w
1 w
1 w
1 w
Mw 10 (dBm/10)
1 w
1 w
1 w
1 w
1 w
1 w
1 w
1 w
1 w
1 w
1 w
2 Watts
1 Watt
4 Watts
1 Watt
8 Watts
1 Watt
21 Ratio 3 dBW
41 Ratio 6 dBW
81 Ratio 9 dBW

• Every dB (decibel) value is a ratio.
• The dB is measured on a base 10 logarithmic
  scale.
• The base increases ten-fold for every ten dB
  measured.

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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.

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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.

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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

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Decibel references
WLANs work in milliwatts or 1/1,000th of a Watt

• 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.

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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

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Ref.

• 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

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Interactive Activity Calculating decibels
End
Start
Change
10 dBm

• 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.

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RF Receivers
-90 dBm
End
Start
Change

• 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.

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• 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

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

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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.

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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

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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.

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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.

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Last note

• As signal strength decreases, so will the
  transmission rate.
• An 802.11b clients 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