Ch. 3 Wireless Radio Technology

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

Radio Wave Transmission Principles

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

terminology

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.

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.

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

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

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

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. (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.

Working with Wireless Power

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…

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.”

Point A Point B¼ 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

3 times the distance 1/9 the power of Point A

5 times the distance 1/25 the power of Point A

10 times the distance 1/100 the power of A

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.

Wireless Power Ratios

• 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.

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

dB = 10 log10 (Pfinal/Pref)

Mw = 10 (dBm/10)

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) - FYIx = 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

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 2Now 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 / 2Now 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.

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