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### Ch. 3 Wireless Radio Technology

### 802.11 Physical Layer Technologies

### Ch. 3 Wireless Radio Technology

Cisco Fundamentals of Wireless LANs version 1.1

Rick Graziani

Cabrillo College

Spring 2005

Note

- 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.

Rick Graziani graziani@cabrillo.edu

Acknowledgements

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

Rick Graziani graziani@cabrillo.edu

Overview of Waves

- Wave is a “disturbance or variation” that travels through a medium.
- The medium through which the wave travels may experience some local oscillations as the wave passes, but the particles in the medium do not travel with the wave.
- Just like none of the individual people in the stadium are carried around when they do the wave, they all remain at their seats.

Rick Graziani graziani@cabrillo.edu

Waves

- Waves are one way in which energy can move from one place to another.
- The waves that you see at the beach are the result of the kinetic energy of water particles passing through the water.
- Other types of energy (such as light, heat, and radio waves) can travel in this way as well.
- The distance between 2 peaks (or 2 troughs) is called a wavelength
- The deepest part of a trough or the highest part of a peak is called the amplitude
- The frequency is the number of wavelengths that pass by in 1 second

www.ewart.org.uk

Rick Graziani graziani@cabrillo.edu

Longitudinal Waves

- Longitudinal sound waves in the air behave in much the same way.
- As the sound wave passes through, the particles in the air oscillate back and forth from their equilibrium positions but it is the disturbance that travels, not the individual particles in the medium.
- Rick talks in a loud voice and he causes the air near his mouth to compress.
- A compression wave then passes through the air to the ears of the people around him.
- A longitudinal sound wave has to travel through something - it cannot pass through a vacuum because there aren't any particles to compress together.
- It has a wavelength; a frequency and an amplitude.

Curriculum 3.1.1

www.ewart.org.uk

Rick Graziani graziani@cabrillo.edu

Transverse Waves

- Transverse waves on a string are another example.
- The string is displaced up and down, as the wave travels from left to right, but the string itself does not experience any net motion.
- A light wave is a transverse wave.
- If you look at the waves on the sea they seem to move in one direction .... towards you.
- However, the particles that make up the wave only move up and down.
- Look at the animation, on the right, although the wave seems to be moving from left to right the blue particle is only moving up and down.

Rick Graziani graziani@cabrillo.edu

Sine waves

- The sine wave is unique in that it represents energy entirely concentrated at a single frequency.
- An ideal wireless signal has a sine waveform
- With a frequency usually 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).

Rick Graziani graziani@cabrillo.edu

Sine waves

- Amplitude – The distance from zero to the maximum value of each alternation is called the amplitude.
- The amplitude of the positive alternation and the amplitude of the negative alternation are the same.
- 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 Hertz (Hz).

Curriculum 3.1.2

Rick Graziani graziani@cabrillo.edu

Sine waves

- One full period or cycle of a sine wave is said to cover 360 degrees (360°).
- It is possible for one sine wave to lead or lag another sine wave by any number of degrees, except zero or 360.
- When two sine waves differ by exactly zero° or 360°, the two waves are said to be in phase.
- Two sine waves that differ in phase by any other value are out of phase, with respect to each other.

Go to interactive activity 3.1.2 Amplitude, Frequency, and Phase

180° Phase Shift

Rick Graziani graziani@cabrillo.edu

Analog to digital conversion

- Analog wave amplitudes are sampled (measuring the analog wave) at specific instances in time.
- More samples means more bits.
- Sampling rate more than twice the frequency is not efficient.
- Each sample is assigned a discrete value.
- Each discrete value is converted to a stream of bits.

Go to interactive activity 3.1.3

Rick Graziani graziani@cabrillo.edu

Fourier synthesis

Go to interactive activity 3.3.3

Whatis.com

- Jean Baptiste Fourieris responsible for one of the great mathematical discoveries.
- When two EM waves occupy the same space, their effects combine to form a new wave of a different shape.
- It works by combining a sine wave signal and sine-wave or cosine-wave harmonics (signals at multiples of the lowest, or fundamental, frequency) in certain proportions.
- A square wave, or a square pulse, can be built by using the right combination of sine waves.

Rick Graziani graziani@cabrillo.edu

Bandwidth

- Analog bandwidth
- Analog bandwidth can refer to the range of frequencies, or cycles per second, which is measured in Hz.
- There is a direct correlation between the analog bandwidth of any medium and the data rate in bits per second that the medium can support.
- Digital bandwidth
- Digital bandwidth is a measure of how much information can flow from one place to another, in a given amount of time.
- Digital bandwidth is measured in bits per second.
- When dealing with data communications, the term bandwidth most often signifies digital bandwidth.

Rick Graziani graziani@cabrillo.edu

Basics of EM waves

- EM waves exhibit the following properties:
- reflection or bouncing
- refraction or bending
- diffraction or spreading around obstacles
- scattering or being redirected by particles
- This will be discussed in greater detail later in this module.
- Also, the frequency and the wavelength of an EM wave are inversely proportionally to one another.

Rick Graziani graziani@cabrillo.edu

Basics of EM waves

- There are a number of properties that apply to all EM waves, including:
- Direction
- Frequency
- Wavelength
- Power
- Polarization
- Phase.

Rick Graziani graziani@cabrillo.edu

Basics of EM waves

- EM (Electromagnetic)spectrum a set of all types of radiation when discussed as a group.
- Radiation is energy that travels in waves and spreads out over distance.
- The visible light that comes from a lamp in a house and radio waves that come from a radio station are two types of electromagnetic waves.
- Other examples are microwaves, infrared light, ultraviolet light, X-rays, and gamma rays.

Rick Graziani graziani@cabrillo.edu

Basics of EM waves

- 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)
- Wavelength x Frequency = Speed of light
- Speed of light = 180,000 miles/sec or

300,000 kilometers/sec or

300,000,000 meters/sec

Interactive activity 3.3.1

Rick Graziani graziani@cabrillo.edu

EM Spectrum Chart

Interactive activity 3.3.2

- One of the most important diagrams in both science and engineering is the chart of the EM spectrum .
- The typical EM spectrum diagram summarizes the ranges of frequencies, or bands that are important to understanding many things in nature and technology.
- EM waves can be classified according to their frequency in Hz or their wavelength in meters.
- The most important range for this course is the RF (Radio Frequency) spectrum.

Rick Graziani graziani@cabrillo.edu

EM Spectrum Chart

- The RF spectrum includes several frequency bands including:
- Microwave
- Ultra High Frequencies (UHF)
- Very High Frequencies (VHF)
- This is also where WLANs operate.
- The RF spectrum ranges from 9 kHz to 300 GHz.
- Consists of two major sections of the EM spectrum: (RF Spectrum)
- Radio Waves
- Microwaves.
- The RF frequencies, which cover a significant portion of the EM radiation spectrum, are used heavily for communications.
- Most of the RF ranges are licensed, though a few key ranges are unlicensed.

Rick Graziani graziani@cabrillo.edu

Rick Graziani graziani@cabrillo.edu

Rick Graziani graziani@cabrillo.edu

Licensed Frequencies

- Frequency bands have a limited number of useable different frequencies, or communications channels.
- The electromagnetic spectrum is a finite resource.
- One way to allocate this limited, shared resource is to have international and national institutions that set standards and laws as to how the spectrum can be used.
- In the US, it is the FCC that regulates spectrum use.
- In Europe, the European Telecommunications Standards Institute (ETSI) regulates the spectrum usage.
- Frequency bands that require a license to operate within are called the licensed spectrum.
- Examples include amplitude modulation (AM) and frequency modulation (FM) radio, ham or short wave radio, cell phones, broadcast television, aviation bands, and many others.
- In order to operate a device in a licensed band, the user must first apply for and be granted the appropriate license.

Rick Graziani graziani@cabrillo.edu

ISM (Industrial, Scientific, and Medical) & U-NII (Unlicensed National Information Infrastructure)

- Some areas of the spectrum have been left unlicensed.
- This is favorable for certain applications, such as WLANs.
- An important area of the unlicensed spectrum is known as the industrial, scientific, and medical (ISM) bands and the U-NII (Unlicensed National Information Infrastructure)
- ISM – 802.11b, 802.11g
- U-NII – 802.11a
- These bands are unlicensed in most countries of the world.
- The following are some examples of the regulated items that are related to WLANs:
- The FCC has defined eleven 802.11b DSSS channels and their corresponding center frequencies. ETSI has defined 13.
- The FCC requires that all antennas that are sold by a spread spectrum vendor be certified with the radio with which it is sold.
- Unlicensed bands are generally license-free, provided that devices are low power.
- After all, you don’t need to license your microwave oven or portable phone.

Rick Graziani graziani@cabrillo.edu

Signals

See Figure 2 in 3.4.1

- One of the most important facts of the information age is that data can be represented electrically by voltage patterns on wires and in electronic devices.
- The data in electronic devices, which is represented by voltage patterns, can be converted to radio waves and radio waves can be converted to voltage patterns.
- Since voltages are much easier to measure than directly measuring the radio waves, an understanding of voltage patterns can be very helpful in the study of WLANs, which are made up of electronic devices.

Rick Graziani graziani@cabrillo.edu

Time Domain Analysis

- The study of how signals vary with time is called time domain analysis.
- To understand frequency-domain analysis as it relates to WLANs, it is helpful to first examine a more familiar radio system, namely commercial broadcast FM radio.
- The different stations each have a different center or carrier frequency so that they do not transmit on the same frequencies.
- The strength of the signal at the FM radio receiver may be weak or strong.
- These same factors exist in a WLAN. For example, to gain the most benefit from multiple APs in the same location, it is important that they do not overlap in frequency.

Rick Graziani graziani@cabrillo.edu

Digital Signals in Time

Interactive activity 3.4.1

- Imagine several sine waves all added together at one time. The resulting wave is more complex than a pure sine wave. There are several tones and the graph of these tones will show several individual lines, each corresponding to the frequency of one tone.
- The pattern of voltage changes versus time is called a square wave. Figure above illustrates a very simple example in which there are only two voltage levels, which will be interpreted as either a one or a zero.
- Remember the Fourier synthesis and that a square wave can be built by using the right combination of sine waves.

Rick Graziani graziani@cabrillo.edu

Modulation Techniques – Carrier frequencyInteractive Activity 3.5.2

- A carrier frequency is an electronic wave that is combined with the information signal and carries it across the communications channel.
- An FM radio station typically has call letters associated with it, such as KPBS or 101.1 MHz. For WLANs, the carrier frequency is 2.4 GHz or 5 GHz.
- Using carrier frequencies in WLANs has an added complexity, in that the carrier frequency is changed by frequency hopping or direct sequence chipping, to make the signal more immune to interference and noise.
- The process of recovering the information from the carrier waves is called demodulation..

Rick Graziani graziani@cabrillo.edu

- There are three aspects of the basic carrier wave that can be modulated: Amplitude, Frequency Phase or angle.
- The three corresponding techniques are as follows:
- Amplitude modulation (AM)
- Frequency modulation (FM)
- Phase modulation (PM)
- Most communication systems use some form or combination of these three basic modulation techniques.
- Amplitude shift keying (ASK) - Turning the amplitude all the way off
- Frequency shift keying (FSK) - Hopping to an extreme frequency
- Phase shift keying (PSK) - Shifting the phase 180 degrees

Rick Graziani graziani@cabrillo.edu

http://www.sfu.ca/sonic-studio/handbook/Fourier_Synthesis.htmlhttp://www.sfu.ca/sonic-studio/handbook/Fourier_Synthesis.html

Sound Example:Addition of the first 14 sine wave harmonics resulting in the successive approximation of a sawtooth wave.

Rick Graziani graziani@cabrillo.edu

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.

Rick Graziani graziani@cabrillo.edu

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,…

Rick Graziani graziani@cabrillo.edu

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.

Rick Graziani graziani@cabrillo.edu

Attenuation and Obstructions

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

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

Same wavelength (frequency), less amplitude.

Rick Graziani graziani@cabrillo.edu

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.

Rick Graziani graziani@cabrillo.edu

Reflected Waves

Interactive Activity 3.7.3

- 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.

Rick Graziani graziani@cabrillo.edu

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.

Rick Graziani graziani@cabrillo.edu

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).

Rick Graziani graziani@cabrillo.edu

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.

Rick Graziani graziani@cabrillo.edu

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. Usually caused by poor signal quality levels or high RF signal strength

Multipath Reflection

Interactive Activity 3.7.5

Rick Graziani graziani@cabrillo.edu

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

Rick Graziani graziani@cabrillo.edu

Refraction

Sub-Refraction

- Refraction (or bending) of signals is due to temperature, pressure, and water vapor content in the atmosphere.
- When a ray of light traveling in one medium enters a second medium and is not perpendicular to the surface of this second medium, it bends
- 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%,

Interactive Activity 3.7.2

Refraction (straight line)

Normal Refraction

Earth

Rick Graziani graziani@cabrillo.edu

Working with Wireless Power

- 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 decibel (dB), 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.

Rick Graziani graziani@cabrillo.edu

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

Rick Graziani graziani@cabrillo.edu

Inverse square law

10

20

30

40

50

100

- 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

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

Rick Graziani graziani@cabrillo.edu

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.

Rick Graziani graziani@cabrillo.edu

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).

Rick Graziani graziani@cabrillo.edu

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.

Rick Graziani graziani@cabrillo.edu

Decibels

- The dB is measured on a base 10 logarithmic scale.
- The base increases ten-fold for every ten dB measured. 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

Rick Graziani graziani@cabrillo.edu

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.

Rick Graziani graziani@cabrillo.edu

Logarithms – Just another way of expressing powers (10n) - FYI

x = ay

logax = y

- Example: 100 = 102
- This is equivalent to saying that the base-10 logarithm of 100 is 2; that is:

100 = 102 same as log10100 = 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.

Rick Graziani graziani@cabrillo.edu

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

Rick Graziani graziani@cabrillo.edu

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

Rick Graziani graziani@cabrillo.edu

- 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

Rick Graziani graziani@cabrillo.edu

Interactive Activity – Calculating decibelsCurriculum 3.2.3

- 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.

End

Start

Change

+10 dBm

Rick Graziani graziani@cabrillo.edu

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.

End

+20 dBm

Start

Change

Rick Graziani graziani@cabrillo.edu

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.

+3dBm

End

Start

Change

Rick Graziani graziani@cabrillo.edu

Interactive Activity – Using decibels

Change

Start

End

+10 dBm

- 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.

Rick Graziani graziani@cabrillo.edu

Interactive Activity – Using decibels

Change

Start

+3 dBm

End

- 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.

Rick Graziani graziani@cabrillo.edu

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

End

Start

Change

Rick Graziani graziani@cabrillo.edu

Other decibel references besides mW

More on this when we discuss antennas.

- 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) - This refers to the gain a given antenna has, as compared to a theoretical isotropic, or point source, antenna. Unfortunately, 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 (EIRP) - EIRP is defined as the effective power found in the main lobe of a transmitter antenna. It is equal to the sum of the antenna gain, in dBi, plus the power level, in dBm, into that antenna.
- Gain - This refers to the amount of increase in energy that an antenna appears to add to an RF signal.

Rick Graziani graziani@cabrillo.edu

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%.

Rick Graziani graziani@cabrillo.edu

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%

Rick Graziani graziani@cabrillo.edu

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.

Rick Graziani graziani@cabrillo.edu

Noise

- In telecommunications, noise can be better defined as undesirable voltages from both natural and technological sources and can change the information.
- Sources of noise in a WLAN include the electronics in the WLAN system, plus radio frequency interference (RFI), and electromagnetic interference (EMI)
- Gaussian, or white noise - The spectrum analyzer graph of white noise is a straight line across all frequencies. Theoretically, Gaussian noise affects all different frequencies equally.
- Narrowband interference - The term band refers to a grouping of frequencies. A narrowband has a relatively smaller range of frequencies. FM radio is an example of narrowband interference.
- Both forms of noise are important in understanding WLANs.
- white noise would degrade the various channels equally.
- narrowband interference might disrupt only certain channels or spread spectrum components. It might even be possible to use a different channel to avoid the interference entirely.

Rick Graziani graziani@cabrillo.edu

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

Rick Graziani graziani@cabrillo.edu

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/knowledgebaseAnswer/0,295199,sid63_gci976082,00.html

Rick Graziani graziani@cabrillo.edu

FHSS – 802.11

DSSS- 802.11

HR/DSSS – 802.11b

OFDM – 802.11a

ERP – 802.11g

802.11 Physical Layer Technologies

- The radio-based physical layers in 802.11 use three different spread-spectrum techniques:
- In 1997, the initial revision of 802.11 included:
- Frequency-hopping spread-spectrum (FHSS)
- Direct-sequence spread-spectrum (DSSS) – 802.11
- Infrared (IR)
- In 1999, two more physical layers were developed:
- Orthogonal Frequency Division Multiplexing (OFDM) – 802.11a
- High-Rate Direct-sequence spread-spectrum (HR/DSSS) – 802.11b
- In 2003, 802.11g was introduced which uses both HR/DSSS and OFDM:
- Extended Rate Physical (ERP) layer - 802.11g

Rick Graziani graziani@cabrillo.edu

802.11 Physical Layer Technologies

Frequency allocation in the EM spectrum

- Frequency-hopping spread-spectrum (FHSS)
- Direct-sequence spread-spectrum (DSSS) – 802.11
- Orthogonal Frequency Division Multiplexing (OFDM) – 802.11a
- High-Rate Direct-sequence spread-spectrum (HR/DSSS) – 802.11b
- Extended Rate Physical (ERP) layer - 802.11g

Original 802.11

Rick Graziani graziani@cabrillo.edu

802.11 - Frequency-hopping spread-spectrum (FHSS)

- Frequency-hopping spread-spectrum (FHSS) WLANs support 1 Mbps and 2 Mbps data rates.
- Widely deployed in the early days (1997) of WLANs.
- Electronics relatively inexpensive and had low power requirements.
- Uses unlicensed 2.4 GHz ISM (Industrial, Scientific, and Medical) band

Rick Graziani graziani@cabrillo.edu

802.11 - Frequency-hopping spread-spectrum (FHSS)

- Uses 79 non-overlapping channels. Across 2.402 to 2.480 GHz band
- Each channel is 1 MHz wide.
- Frequency hopping depends on rapidly changing the transmission frequency in a pseudo-random pattern, known as the hopping code.
- The initial advantage of using FHSS networks was the greater number of networks that could coexist with relatively high throughput and low collisions.
- With the advent of HR/DSSS this is no longer an advantage.

Interactive Activity 3.5.3

Rick Graziani graziani@cabrillo.edu

802.11 - Direct-sequence spread-spectrum (DSSS)

- Direct-sequence spread-spectrum (DSSS) defined in 1997 802.11 standard.
- Supports data rates of 1 Mbps and 2 Mbps
- In 1999 802.11 introduced 802.11b standard (HR/DSSS) to support 5.5 Mbps and 11 Mbps, which is backwards compatible with 802.11 (later).

Rick Graziani graziani@cabrillo.edu

802.11 - Direct-sequence spread-spectrum (DSSS)

- DSSS uses 22 MHz channels in the 2.4 to 2.483 GHz range.
- This allows for three non-overlapping channels (three channels that can coexist or overlap without causing interference), channels 1, 6 and 11 (coming).
- Uses 2.4 GHz ISM band

Rick Graziani graziani@cabrillo.edu

802.11 - Direct-sequence spread-spectrum (DSSS)

- 802.11 DSSS
- 802.11 DSSS uses a rate of 11 million chips per second or 1 million 11-bit Barker words per second.
- These 11 bit Barker words are transmitted over the 22 MHz spread spectrum at 1 million times per second.
- Each word is encoded as either 1-bit or 2-bits, corresponding with either 1.0 Mbps or 2.0 Mbps respectively.

Rick Graziani graziani@cabrillo.edu

802.11b - High-Rate Direct-sequence spread-spectrum (HR/DSSS)

- In 1999 802.11 introduced 802.11b standard (HR/DSSS)
- Data rates of 1 Mbps, 2 Mbps, 5.5 Mbps and 11 Mbps
- Backwards compatible with 802.11
- Uses 2.4 GHz ISM band

Rick Graziani graziani@cabrillo.edu

802.11b - High-Rate Direct-sequence spread-spectrum (HR/DSSS)

- HR/DSSS uses 22 MHz channels in the 2.4 to 2.483 GHz range.
- This allows for three non-overlapping channels (three channels that can coexist or overlap without causing interference), channels 1, 6 and 11 (coming).

Rick Graziani graziani@cabrillo.edu

802.11b - High-Rate Direct-sequence spread-spectrum (HR/DSSS)

(Once again)

- HR/DSSS uses 22 MHz channels in the 2.4 to 2.483 GHz range.
- This allows for three non-overlapping channels (three channels that can coexist or overlap without causing interference), channels 1, 6 and 11 (coming).

Rick Graziani graziani@cabrillo.edu

802.11b - High-Rate Direct-sequence spread-spectrum (HR/DSSS)

Rick Graziani graziani@cabrillo.edu

802.11b modulation

- 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. This 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. This 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. One of the advantages of CCK over similar modulation techniques is that it suffers less from multipath distortion. Multipath distortion will be discussed later. CCK is utilized to transmit data at 5.5 Mbps and 11 Mbps.

Rick Graziani graziani@cabrillo.edu

802.11a – OFDM (Orthogonal Frequency Division Multiplexing)

- In 1999 802.11 introduced 802.11a standard same time as 802.11b
- Uses OFDM encoding.
- Data rates from 6 Mbps, to 54 Mbps
- Not compatible with 802.11b
- Uses 5 GHz band U-NII (Unlicensed National Information Infrastructure).

Rick Graziani graziani@cabrillo.edu

802.11a – OFDM (Orthogonal Frequency Division Multiplexing)

- Because 802.11a uses a higher frequency its devices require higher power, which means they use up more precious battery power on laptops and portable devices.

Rick Graziani graziani@cabrillo.edu

802.11a – OFDM (Orthogonal Frequency Division Multiplexing)

- 802.11a U-NII bands (Unlicensed National Information Infrastructure)
- 5.15 GHz to 5.25 GHz
- 5.25 GHz to 5.35 GHz
- 5.725 GHz to 5.825 GHz

Rick Graziani graziani@cabrillo.edu

802.11a – OFDM (Orthogonal Frequency Division Multiplexing)

- Uses four 20 MHz channels in each of the three U-NII bands
- Each 20 MHz 802.11a channel occupies four channels in the U-NII band (36 – 39, 40 – 43, etc.)
- Offers 8 lower and mid-band non-interfering channels
- As opposed to 3 with 802.11b/g
- Not all cards accept the upper band frequencies

4

8

Rick Graziani graziani@cabrillo.edu

802.11a – OFDM (Orthogonal Frequency Division Multiplexing)

- Offers 8 lower and mid-band non-interfering channels
- As opposed to 3 with 802.11b/g

www.networkcomputing.com/1201/1201ws1.html

Rick Graziani graziani@cabrillo.edu

802.11a – OFDM (Orthogonal Frequency Division Multiplexing)

- 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.
- OFDM uses the spectrum much more efficiently by spacing the channels much closer together.
- The spectrum is more efficient because all of the carriers are orthogonal to one another, thus preventing interference between closely spaced carriers.

www.networkcomputing.com/1201/1201ws1.html

Rick Graziani graziani@cabrillo.edu

802.11a – OFDM (Orthogonal Frequency Division Multiplexing)

- Orthogonal is a mathematical term derived from the Greek word orthos, meaning straight, right, or true.
- In mathematics, the word orthogonal is used to describe independent items.
- OFDM works because the frequencies of the subcarriers are selected in such a way that, for each subcarrier frequency, all other subcarriers will not contribute to the overall waveform.

www.networkcomputing.com/1201/1201ws1.html

Rick Graziani graziani@cabrillo.edu

802.11a – OFDM (Orthogonal Frequency Division Multiplexing)

- It is the different frequencies used (5 GHz and 2.4 GHz) and the different structure of the operating channels (OFDM and DSSS-HR/DSSS) that makes 802.11a incompatible with 802.11b devices.
- There are “dual band” access points that can operate in multimode modes (802.11a, b and g) – coming.

www.networkcomputing.com/1201/1201ws1.html

Rick Graziani graziani@cabrillo.edu

802.11a – OFDM (Orthogonal Frequency Division Multiplexing)

- OFDM (Orthogonal Frequency Division Multiplexing) is a mix of different modulation schemes to achieve data rates from 6 to 54 Mbps.
- Each subchannel in the OFDM implementation is about 300 KHz wide. 802.11a uses different types of modulation, depending upon the data rate used.
- The 802.11a standard specifies that all 802.11a-compliant products must support three modulation schemes.

48 subchannels for data

Rick Graziani graziani@cabrillo.edu

802.11a – OFDM (Orthogonal Frequency Division Multiplexing)

(How the modulation works is not important here.)

- BPSK (Binary Phase Shift Keying) – 1 bit per subchannel
- QPSK (Quadrature Phase Shift Keying) – 2 bits per subchannel
- 16 QAM (Quadrature Amplitude Moduation) – 4 bits using 16 symbols
- 64 QAM (Quadrature Amplitude Moduation) – 6 bits using 64 symbols

48 subchannels for data

Rick Graziani graziani@cabrillo.edu

802.11a – OFDM (Orthogonal Frequency Division Multiplexing)

- Coded orthogonal frequency division multiplexing (COFDM)delivers higher data rates and a high degree of multipath reflection recovery, thanks to its encoding scheme and error correction.
- The OFDM signal is subject to narrowband interference or deep fading.
- When this occurs the channel’s ability to carry data may go to zero because the received amplitude is so low.

Rick Graziani graziani@cabrillo.edu

802.11g – Extended Rate Physical (ERP) layer

- IEEE 802.11g standard was approved on June 2003.
- Introduces ERP, Extended Rate Physical layer support for data rate up to 54 Mbps.
- 2.4 GHz ISM band
- Borrows OFDM techniques from 802.11a
- Backwards compatible with 802.11b devices

Rick Graziani graziani@cabrillo.edu

802.11g – Extended Rate Physical (ERP) layer

- In an environment with only 802.11g devices, transmission will occur at the highest data rates that the signals allow.
- As soon as an 802.11b device is introduced to the BSS, 802.11b device(s) can only operate at 802.11 data rates.
- 802.11g devices will have lower data rates, however there are contradictions on what that is.
- Some documentation states that it will be at 802.11b rates. Other documentation states that it will be at 802.11g rates but with additional overhead causing overall throughput to decrease.

802.11g

802.11g

802.11g

802.11g

802.11g

802.11g

802.11b

802.11g

802.11g

802.11g

Rates up to 54 Mbps (802.11g)

Lower rates

Rick Graziani graziani@cabrillo.edu

802.11g – Extended Rate Physical (ERP) layer

- The four lower data rates of 802.11g (1, 2, 5.5, 11 Mbps), like 802.11b uses CCK (Complementary Code Keying) - (802.11usesBarker).
- CCK uses an 8-bit complex chip code.
- Based on sophisticated mathematics.
- CCK allows for the backward compatibility with 802.11b
- The higher data rates of 802.11g (6, 9, 12, 18, 24, 36, 48, and 54 Mbps) uses COFDM (like 802.11a).
- 802.11a is not compatible with 802.11g, different frequencies.

Rick Graziani graziani@cabrillo.edu

Data Rates at Varying Distances

5 GHz radio signals do not propagate as well as 2.4 GHz radio signals, so 802.11a devices are limited in range compared to 802.11b and 802.11g devices.

Broadband.com

Rick Graziani graziani@cabrillo.edu

Relative Ranges

Broadband.com

- 802.11a requires more APs for the same coverage area.

Rick Graziani graziani@cabrillo.edu

Expected Throughputs

Broadband.com

- Throughput includes overhead including MAC frame and MAC operations, PLCP header, etc..

Rick Graziani graziani@cabrillo.edu

WLAN User Requirements and Technology Characteristics

Broadband.com

- It is forecasted that 802.11g will quickly replace 802.11b.
- 802.11g Access Points automatically support 802.11b.
- Dual-band 802.11a/g and 802.11g Access Points become the two technologies to consider when migrating to 802.11g from 802.11b networks.
- Dual-band 802.11a/b Access Points become immediately obsolete.

Rick Graziani graziani@cabrillo.edu

Cisco Fundamentals of Wireless LANs version 1.1

Rick Graziani

Cabrillo College

Spring 2005

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