Ch. 2 – 802.11 and NICs Part 3 – 802.11 PHY Cisco Fundamentals of Wireless LANs version 1.1 Rick Graziani Cabrillo College Spring 2005 Note: Includes information

Ch. 2 – 802.11 and NICsPart 3 – 802.11 PHY

Cisco Fundamentals of Wireless LANs version 1.1

Rick Graziani

Cabrillo College

Spring 2005

Note: Includes information which is in Cisco online curriculum Module 2 and Module 3

Rick Graziani [email protected] 2

Topics

• Overview of Waves

• EM Spectrum

• 802.11 PHY Physical Layer Technologies– PLCP– PMD

• 802.11 Technologies– FHSS – 802.11– DSSS- 802.11– HR/DSSS – 802.11b– OFDM – 802.11a– ERP – 802.11g

• Comparing 802.11a, 802.11b, 802.11g

Overview of Waves


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.


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.

www.ewart.org.uk


Waves

• 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


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.

• When he talks 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.

www.ewart.org.uk


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.

interactive activity 3.1.1


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).


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).

Go to interactive activity 3.1.2 Amplitude and Frequency


Relationship between time and frequency

• The inverse relationship between time (t), the period in seconds, and frequency (f), in Hz, is indicated by the following formulas:

t = 1/f (time = 1 / frequency)

f = 1/t (frequency = 1 / time)

Examples:

1 second

• t = 1/f 1 second = 1 / 1 Hz (1 cycle per second)

• f = 1/t 1 Hz = 1 / 1 second

½ second

• t = 1/f ½ second = 1 / 2 Hz (2 cycles per second)

• f = 1/t 2 Hz = 1 / ½ second

1/10,000,000th of a second

• t = 1/f 1/10,000,000th of a second = 1 / 10,000,000 Hz (cycles/sec) = 1 / 10 MHz

• f = 1/t 10 MHz = 1 / 1/10,000,000th of sec


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.

180° Phase Shift

Go to interactive activity 3.1.2 Amplitude, Frequency, and Phase


Analog to digital conversion

1. Analog wave amplitudes are sampled at specific instances in time.

2. Each sample is assigned a discrete value.

3. Each discrete value is converted to a stream of bits.

Go to interactive activity 3.1.3


Bandwidth

• There are two common ways of looking at bandwidth: – Analog bandwidth– Digital bandwidth

• Analog bandwidth – Analog bandwidth can refer to the range of frequencies . – Analog bandwidth is described in units of frequency, 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.


Bandwidth

• 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.

EM (Electromagnetic) Spectrum


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.


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


Basics of EM waves

• wavelength (λ), frequency (f), speed of light (c)• A wave of 1 cycle per second, has a wavelength of 300,000,000

meters or 300,000 kilometers or 180,000 miles.• Speed of a bit doesn’t go beyond the speed of light, Dr. Einstein says

we all go “poof” (my words, not his)• Speed is a function of increasing the number of waves, bits, in the

same amount of space, I.e. bits per second

300,000 kilometers

or 180,000 miles

150,000 km 150,000 km


Basics of EM waves

• Other interesting calculations

Size of a Wave


Size of a Wave

• It’s important to visualize the physical size of a wireless signal because the physical size determines: – How that signal interacts with its environment– How well it is propagated from antenna to antenna– The physical size of the antenna (the smaller the signal

size, the smaller the antenna)


Speed of light = 186,000 miles/sec or 300,000,000 meters/sec (approx.)

1 second

186,000 miles

Mile: 0 Mile: 186,000

Speed of Light

Start here End here

1 mile

• 5,280 feet per mile; so 186,000 miles = 982,080,000 feet

• 63,360 inches per mile; so 186,000 miles = 11,784,960,000 inches


Wavelength http://eosweb.larc.nasa.gov/EDDOCS/wavelength.html

• Speed of the wave = Frequency x Wavelength

• Wavelength = Speed of the wave or speed of light / Frequency • Speed of light =

– 186,000 miles/sec or – 982,080,000 feet/sec or – 11,784,960,000 inches/sec

• Wavelength = Speed of the wave or speed of light/ Frequency

• 10.93 feet = 982,080,000 feet per sec / 90,000,000 cycles per sec

All About Wavelength

http://eosweb.larc.nasa.gov/EDDOCS/wavelength.html


Speed of light = 186,000 miles/sec

Length of rope 186,000 miles long

1 second

Mile: 0, beginning of rope

0 seconds

After 1/2 second

After 1 second

Mile: 186,000, end of rope

• Length of rope 186,000 miles long traveling at the speed of light, 186,000 miles/second

• In 1 second we would see the entire length of rope go by.

0 second

Speed of Light

1 second



Length of rope 186,000 miles longMile: 0, beginning of rope


• So, if 1 Hz is 1 cycle per second, traveling at the speed of light….

• The length of the wave would be 186,000 miles long (300,000,000 meters).

1 second0 second

Speed of Light – 1 Hz

186,000 miles




• 2 Hz is 2 cycles per second, traveling at the speed of light….

• The length of each wave would be 186,000/2 or 93,000 miles long (150,000,000 meters).

1 second0 second

Speed of Light – 2 Hz

93,000 miles



• 11,784,960,000 inches in a mile

• 1 Hz wave = 11,784,960,000 inches (11 billion inches)

• 2 Hz wave = 11,784,960,000 / 2 = 6 billion inches (give or take)

• What would a wave the size of 11 GHz wave be?

Speed of Light – Let’s do inches

11,784,960,000 inches

6,000,000,000 inches



• Size of the rope divided by the number of pieces = size of each piece

• About 1 inch! (11,784,960,000 in. per sec / 11,000,000,000 pieces or cycles or Hz)

• Same as slicing up the 186,000 mile rope into 11 billion equal pieces.

• Each piece is 1 inch, 11 billion pieces equal 11 billion inches, the size of our rope traveling at 186,000 miles per second.

Speed of Light – Lets do inches

1 second0 second …1 2 11 billion



1 inch

Length of rope 11.8 billion inches long



• 11 inches! (Actually, 11.8 inches because we rounded off values.)

(approx.: 11,784,960,000 inches per sec / 1,000,000,000 cycles per sec)

• Same as slicing up the 186,000 mile rope into 1 billion equal pieces.

• Each piece is 11 inches, 1 billion pieces equal 11 billion inches, the size of our rope traveling at 186,000 miles per second.

Speed of Light – Lets do inches

1 second0 second …1 2 1 billion



11.8 inches



RADM Grace Hopper

• Grace Hopper, “Mother of Cobol”

• The size of a nanosecond, 11.8 inches

• The distance the speed of light travels in a billionth of a second.


• Same as slicing up the 186,000 mile rope into 2.4 billion equal pieces.

• Each piece is 4.8 inches or 12 cm (.12 meters)


• 2.4 billion pieces equal 11 billion inches, the size of our rope traveling at 186,000 miles per second.

Size of a 2.4 GHz WLAN wave

1 second0 second …1 2 2.4 billion



4.8 inches



• Same as slicing up the 186,000 mile rope into 5.8 billion equal pieces.

• Each piece is 2 inches or 5 cm (.05 meters)


• 5.8 billion pieces equal 11 billion inches, the size of our rope traveling at 186,000 miles per second.

Size of a 5.8 GHz WLAN wave

1 second0 second …1 2 5.8 billion



2 inches


Basics of EM Waves


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.


Basics of EM waves

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


EM Spectrum Chart

• 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.


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.


EM Spectrum Chart

Nasa.gov


Nasa.gov

Rick Graziani [email protected] 41www.britishlibrary.net


Licensed Frequencies

• Frequency bands have a limited number of useable different frequencies, or communications channels.

• Many parts of the EM spectrum are not useable for communications and many parts of the spectrum are already used extensively for this purpose.

• 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.


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.


Fourier synthesis (More than we need…)

• When two EM waves occupy the same space, their effects combine to form a new wave of a different shape.

• For example, air pressure changes caused by two sound waves added together.

• Jean Baptiste Fourier is responsible for one of the great mathematical discoveries.

• He proved that a special sum of sine waves, of harmonically related frequencies, could be added together to create any wave pattern.

• Harmonically related frequencies are simply frequencies that are multiples of some basic frequency.

• Use the interactive activity to create multiple sine waves and a complex wave that is formed from the additive effects of the individual waves.

• Finally, a square wave, or a square pulse, can be built by using the right combination of sine waves.

• The importance of this will be clarified when modulation is discussed.


Fourier synthesis

Whatis.com• Fourier synthesis is a method of electronically constructing a signal

with a specific, desired periodic waveform. • 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.

Go to interactive activity 3.3.3


http://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.

http://www.sfu.ca/sonic-studio/handbook/Sound/Law_Of_Superposition.aiff

C:\My Information2\Cisco CNA\Rick's Curriculum\Cisco Semester Wireless\Presentations - Spring 2005\Law_Of_Superposition.aif

802.11 Physical Layer Technologies

PLCP

PMD

Note: The information presented here is just to introduce these terms and concepts. Many of the “how’s” and “why’s” are beyond the scope of this

material. Don’t get lost in the detail!



• We have looked at the data link layer, now we will look at the physical layer.

• As you can see there are multiple physical layer technologies involved with both similarities and differences between them.

• The job of the PHYs is to provide the wireless transmission mechanisms for the MAC.

• By keeping the PHY transmission mechanisms independent of the MAC it allows for advances in both of these areas.



• The physical layer is divided into two sublayers:– PLCP (Physical Layer Convergence Procedure)– PMD (Physical Medium Dependent)

• All of this is needed to help ensure that the data goes from the receiver to the transmitter over this “hostile” wireless environment with noise, and all kinds of “mean, nasty ugly things”. (Arlo Guthrie)



PLCP (Physical Layer Convergence Procedure)• All PLCPs provide the interface to transfer data octets between the

MAC and the PMD.• “Primitives” (fields) that tell the PMD when to begin and end

communications.• The PCLP is the “handshaking layer” that enables the MAC protocol

data units (MPDUs), fancy name for MAC frame, to be transmitted between the MAC over the PMD.


PLCP (Physical Layer Convergence Procedure)

• PPDU (PLCP Protocol Data Unit) adds “encapsulation”• The PDSU (PLCP Data Service Unit) is the data the PCLP is responsible for

delivering. – Depending upon the protocol the encapsulated MAC frame is sometimes

called the PSDU (PLCP Service Data Unit) or MPDU (MAC Protocol Data Unit). All these acronyms! You got to be kidding!

• More on this after the PMD concepts

General 802.11 FrameIP PacketL

LC

PDSU



PMD (Physical Medium Dependent)

• The PMD is responsible for transmitting the actual bits it receives from the PLCP into the air, over the wireless, and sometimes hostile, medium.

• The PHY concepts and building blocks are: Scrambling Coding Interleaving Symbol mapping and modulation

• Let’s look at these to see what wireless technologies do in order to help transmit bits over a hostile wireless medium and increase the chance that the information can be read by the receiver.



• Scrambling– A method for sending and receiving data to make it look more

random than it is.– Receivers do not tend to like long strings of 0’s or 1’s.– The data is scrambled by the transmitter and descrambled by the

receiver.

Original Data Bits Scrambler Scrambled Data Bits

Original Data Bits Descrambler Scrambled Data Bits

Transmission Medium


PMD - Coding

• Coding– After the data is scrambled it is coded.– Coding is a mechanism that enables high transmission over a noisy

channel (like wireless).– Coding does this by replacing sequences with longer sequences.– An example of a coding:

• Scrambled data: 0 1 1 0 1• Coded data: 000000 111111 111111 000000 111111• Transmission: 000000 111111 111111 000000 111111

– The idea is that multiple bits are sent so if some bits can are corrupted (interference), the receiver can still determine the original bits.

– This is effective because noise tends to happen in relative pulses and not across the entire frequency band.

X X X X

Frequency

Spread Signal of coded bits

Noise


802.11 Chipping Sequence – Barker Sequence

• 802.11 encodes data by taking 1 Mbps data stream into an 11 MHz chip stream.

• The spreading sequence or chipping sequence or Barker sequence.• Converts a data bit into chips, 11 bits.

– 0 into 00000000000– 1 into 11111111111

• The expanded data bit is then exclusive ORed (XORed) with a spreading sequence (Barker) resulting in the chipped sequence which is transmitted over the wireless medium.

Scrambled Data Bit

1

Expanded Data Bit

11111111111

Barker Sequence

10110111000XOR 01001000111

Transmitted Chipped Sequence


802.11 Chipping Sequence – Barker Sequence

Scrambled Data Bit

0

Expanded Data Bit

00000000000

Barker Sequence

10110111000XOR 10110111000


Scrambled Data Bit

1

Expanded Data Bit

11111111111

Barker Sequence

10110111000XOR 01001000111


Original Data Bit

1

Either one

XOR

0 XOR 0 -> 0 1 XOR 1 -> 0 0 XOR 1 -> 1


PMD Concepts and Building Blocks

Original Data Bits Scrambler Block Coder

Original Data Bits Descrambler Block Decoder

Modulated over Transmission Medium

Block Interleaver

Block Interleaver

• Sometimes bit errors are not independent events but occur in batches, or bursts.

• Because of this, interleavers are used to spread out adjacent bits and block of error that might occur.

• The idea it to spread out the adjacent bits.

• It might get a couple of us, but it can’t get us all (hopefully).

• This along with the chipping sequence increases the chances that data still can be read by the receiver even with large blocks of data.

• We won’t go into the detail here.



• The PMD is responsible for transmitting the actual bits it receives from the PLCP into the air, over the wireless, and sometimes hostile, medium.

– Scrambling

– Coding

– Interleaving

– Symbol mapping and modulation

• These help transmit bits over a hostile wireless medium and increase the chance that the information can be read by the receiver.


FHSS – 802.11

DSSS- 802.11

HR/DSSS – 802.11b

OFDM – 802.11a

ERP – 802.11g



• 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



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

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



• 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.



• The transmitter will then use a small amount of time, referred to as the hop time, to move to the next frequency.

• When the list of frequencies has been completely traversed, the transmitter will start over and repeat the sequence.

• The receiver radio is synchronized to the hopping sequence of the transmitting radio to enable the receiver to be on the right frequency at the right time.

• The transmitter uses this hop sequence to select its transmission frequency.

• The carrier will remain at a given frequency for a specified period of time, which is referred to as the dwell time.



• FHSS radio hops between all of these channels in one of 78 orthogonal (non-colliding) patterns.

• Devices use all available channels equally in a 30 second period, about 0.4 seconds per channel.

• Note: Since FHSS is no longer used in 802.11 (a, b, g) we will not go into any more detail nor discuss the PLCP or modulation.



0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22Frequency MHz

FHSS Signal (1 MHz)

DSSS (Spread Spectrum) Signal (22 MHz)

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).



• 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



0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22Frequency MHz

FHSS Signal (1 MHz)

DSSS (Spread Spectrum) Signal (22 MHz)



• DSSS adds the following fields to the MAC frame to form the DSSS PPDU (PLCP Protocol Data Unit).

• We will look at these fields which will give us a better understanding of how the physical layer delivers bits over a wireless medium.

General 802.11 FrameIP PacketL

LC

PDSU



PLCP Preamble

• Sync– Provides synchronization for the receiving station.

• SFD (Start of Frame Delimiter) – Provides timing for the receiving station.

PCLP Header

• Signal – Specifies the modulation and data rate) for the frame

– DBPSK – 1 Mbps (PLCP Preamble and Header always sent at this rate)

– DQPSK – 2 Mbps

• Service – For future use

• Length – Number of microseconds required to transmit the MAC portion of the frame.

• CRC (Cyclic Redundancy Check) – CRC check for PCLP header fields.

PDSU


PLCP and MAC Error Statistics



Modulation

• DBPSK – 1 Mbps

– Differential Binary Phase Shift Keying

– One bit per phase change, two phases

– Each chip maps to a single symbol

– Uses one phase to represent a binary 1 and another to represent a binary 0, for a total of one bit of binary data.

• DQPSK – 2 Mbps

– Differential Quadrature Phase Shift Keying

– Two bits per phase change, four phases

– Maps two chips per symbol

– Uses four phases, each representing two bits.



• 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.

802.11b - High-Rate Direct-sequence spread-spectrum

(HR/DSSS)


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



• HR/DSSS uses 22 MHz channels in the 2.4 to 2.483 GHz range.



(Once again)

• HR/DSSS uses 22 MHz channels in the 2.4 to 2.483 GHz range.






• There are two PPDU frame types:

– Long – Same as DSSS PPDU

– Short – (above)

• The short PPDU minimizes overhead.

• The long PPD maintains backward compatibility with 802.11

• Both are basically the same PPDU as DSSS, except:

– Signal field includes addition data rates for 5.5 Mbps and 11 Mbps


Long

Short


ACU

• Enables short radio headers. You can enable the client adapter to use short radio headers only if the access point is also enabled to support short radio headers and is currently using them for all connected client adapters. If an access point connects to any client adapters that are using long headers, all client adapters in that cell must also use long headers, even if both your client adapter and the access point have enabled short radio headers.

• Short radio headers improve throughput. Long radio headers ensure compatibility with client adapters and access points that do not support short radio headers.

HELP Information


• Remember 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.• Regular phase shift encoding can only carry a few bits as detecting

smaller phase shifts requires more sophisticated and expensive electronics.

• IEEE 802.11 developed an alternative encoding method to Barker (802.11), the CCK (Complementary Code Keying).



• 802.11b uses CCK (Complementary Code Keying) instead of Barker.• CCK uses an 8-bit complex chip code.• Based on sophisticated mathematics.

– CCK uses a set of 64 8-bit code words– These code words have unique mathematical properties that allow

a receiver to distinguish them correctly from each other.– The 5.5 Mbps rate uses CCK to encode 4-bits per carrier.– The 11 Mbps rate uses CCK to encode 8-bits per carrier.

• Like DSSS 2 Mbps data rate, both the 5.5 Mbps and 11 Mbps rates uses DQPSK modulation technique.


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).



• 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.



• 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



• 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



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

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



• The fields are similar to other PPDU frame formats 802.11 and 802.11b.

• The Signal field specifies the data frame for the DATA part of the frame: 6, 9, 12, 18, 24, 36, 48, and 54 Mbps.



• OFDM works by breaking one high-speed data carrier into several lower-speed subcarriers, which are then transmitted in parallel.

• Each high-speed carrier is 20 MHz wide and is broken up into 52 subchannels, each approximately 300 KHz wide.

• OFDM uses 48 of these subchannels for data, while the remaining four are used for error correction.

• 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.




• 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.

• Orthogonality is best seen in the frequency domain, looking at a spectral analysis of a signal.

• 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.




• 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.




• 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



(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



• 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.• To keep a few faded channels from driving the bit error to high, OFDM

applies an error correction code COFDM across all the subchannels.• COFDM is beyond the scope of this curriculum.

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



• 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. (I will test this.)

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


802.11g / 802.11b Compatibility

802.11g compatibility with 802.11b, From the Broadband.com White Paper

• Protection Mechanisms: “Air Traffic Control”

– 802.11b radios do not hear the 802.11g OFDM signals.

– Protections mechanisms prevent 802.11b clients from transmitting, thinking the medium is free, when 802.11g devices are transmitting.

– 802.11g devices still communicate at the 802.11g data rates when protection is in use.

– 802.11g devices must transmit a short 802.11b rate message signal to 802.11b products to not transmit for a specified duration, because an 802.11g OFDM message is being transmitted.

– The 802.11b protection message causes additional overhead and reduced throughput for the 802.11g devices when at least one 802.11b device is present.

802.11g802.11b

Can’t hear 802.11g OFDM messages during CCA (Clear Channel Assessment), so will transmit and may cause collisions


802.11g / 802.11b Compatibility

802.11g compatibility with 802.11b, From the Broadband.com White Paper• Two 802.11 Protection Mechanism Standards: RTS/CTS and CTS-to-self

– RTS/CTS protection mechanism is the same 802.11 MAC operation earlier discussed between the 802.11g client and the AP, with all devices, including 802.11b, hearing the CTS from the AP.

– CTS-to-self protection mechanism sends a CTS message, using an 802.11b data rate, instead of the AP doing it, followed immediately my the 802.11g message.

• In either case, 802.11g throughput is still greater than the 802.11b throughput at the same distance.

• Maximum 802.11g throughput with mixed clients is 15 Mbps, or a data rate of about 33 Mbps.

802.11g802.11b 802.11g802.11b

RTS

CTS

CTS

RTS/CTS CTS-to-self



802.11g uses 5 PPDU formats

Long PPDU for 802.11 and 802.11b compatibility

Short PPDU for 802.11b compatibility

Data Rates 6, 9, 12, 18, 24, 36, 48 and 54 Mbps



“Long PPDU”

“Short PPDU”

802.11b compatibility: Backwards compatibility with 802.11

802.11b compatibility: Minimizes overhead

802.11g: Higher data rates



• The four lower data rates of 802.11g (1, 2, 5.5, 11 Mbps), like 802.11b uses CCK (Complementary Code Keying) - (802.11 uses Barker).– 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.

Comparing 802.11a, 802.11b, 802.11g



Broadband.com

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.


Relative Ranges

Broadband.com

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


Expected Throughputs

Broadband.com

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


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.


ACU and various client adapters

• Cisco ACU works with all adapters.



• Once the initial ACU application is downloaded and installed for one adapter, you need to download and install it for any other adapters as well.

• Subsequent installation will only install the drivers associated with that adapter.



• You can use the same profiles with the different adapters.


PLCP and MAC Error Statistics


http://www.cisco.com/en/US/products/hw/wireless/ps4555/products_data_sheet09186a00801ebc29.html

Ch. 2 – 802.11 and NICsPart 3 – 802.11 PHY

Cisco Fundamentals of Wireless LANs version 1.1

Rick Graziani

Cabrillo College

Documents

Ch. 2 – 802.11 and NICs Part 3 – 802.11 PHY Cisco Fundamentals of Wireless LANs version 1.1 Rick Graziani Cabrillo College Spring 2005 Note: Includes information