Ch. 2

1
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

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

3
Overview of Waves
4
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.

5
Waves
www.ewart.org.uk

• 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.

6
Waves
www.ewart.org.uk

• 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

7
Longitudinal Waves
www.ewart.org.uk

• 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.

8
Transverse Waves
interactive activity 3.1.1

• 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.

9
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).

10
Sine waves
Go to interactive activity 3.1.2 Amplitude and
Frequency

• 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
  ?.
• 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).

11
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

12
Sine waves
Go to interactive activity 3.1.2 Amplitude,
Frequency, and Phase
180 Phase Shift

• 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.

13
Analog to digital conversion
Go to interactive activity 3.1.3

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.

14
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.

15
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.

16
EM (Electromagnetic) Spectrum
17
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.

18
Basics of EM waves

• All EM waves travel at the speed of light in a
  vacuum and have a characteristic wavelength (?)
  and frequency (f), which can be determined by
  using the following equation
• c ? 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

19
Basics of EM waves
300,000 kilometers
or 180,000 miles
150,000 km
150,000 km

• wavelength (?), 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 doesnt 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

20
Basics of EM waves

• Other interesting calculations

21
Size of a Wave
22
Size of a Wave

• Its 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)

23
Speed of Light
Speed of light 186,000 miles/sec or 300,000,000
meters/sec (approx.)
Start here
End here
1 second
186,000 miles
Mile 0
Mile 186,000

• 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

24
Wavelength http//eosweb.larc.nasa.gov/EDDOCS/wave
length.html
All About Wavelength

• 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

25
Speed of Light
Speed of light 186,000 miles/sec
Mile 0, beginning of rope
Mile 186,000, end of rope
Length of rope 186,000 miles long
0 seconds
After 1/2 second
After 1 second
0 second
1 second
1 second

• 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.

26
Speed of Light 1 Hz
Speed of light 186,000 miles/sec
Mile 0, beginning of rope
Mile 186,000, end of rope
Length of rope 186,000 miles long
186,000 miles
1 second
0 second

• 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).

27
Speed of Light 2 Hz
Speed of light 186,000 miles/sec
Mile 0, beginning of rope
Mile 186,000, end of rope
Length of rope 186,000 miles long
93,000 miles
1 second
0 second

• 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).

28
Speed of Light Lets do inches
11,784,960,000 inches
6,000,000,000 inches

• 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?

29
Speed of Light Lets do inches
Mile 186,000, end of rope
Length of rope 186,000 miles long
Mile 0, beginning of rope
Length of rope 11.8 billion inches long
1
2
11 billion

1 inch
1 second
0 second

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

30
Speed of Light Lets do inches
Mile 186,000, end of rope
Length of rope 186,000 miles long
Mile 0, beginning of rope
Length of rope 11.8 billion inches long
1
2
1 billion

11.8 inches
1 second
0 second

• What would a wave the size of 1 GHz wave be?
• 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.

31
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.

32
Size of a 2.4 GHz WLAN wave
Mile 186,000, end of rope
Length of rope 186,000 miles long
Mile 0, beginning of rope
Length of rope 11.8 billion inches long
1
2
2.4 billion

4.8 inches
1 second
0 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)
• (approx. 11,784,960,000 inches per sec /
  2,450,000,000 cycles per sec)
• 2.4 billion pieces equal 11 billion inches, the
  size of our rope traveling at 186,000 miles per
  second.

33
Size of a 5.8 GHz WLAN wave
Mile 186,000, end of rope
Length of rope 186,000 miles long
Mile 0, beginning of rope
Length of rope 11.8 billion inches long
1
2
5.8 billion

2 inches
1 second
0 second

• 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)
• (approx. 11,784,960,000 inches per sec /
  5,800,000,000 cycles per sec)
• 5.8 billion pieces equal 11 billion inches, the
  size of our rope traveling at 186,000 miles per
  second.

34
Basics of EM Waves
35
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.

36
Basics of EM waves

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

37
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.

38
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.

39
EM Spectrum Chart
Nasa.gov
40
Nasa.gov
41
www.britishlibrary.net
42
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.

43
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 dont need to license your
  microwave oven or portable phone.

44
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.

45
Fourier synthesis
Go to interactive activity 3.3.3

• 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.

46
http//www.sfu.ca/sonic-studio/handbook/Fourier_Sy
nthesis.html
Sound Example Addition of the first 14 sine
wave harmonics resulting in the successive
approximation of a sawtooth wave.
47
802.11 Physical Layer Technologies

• PLCP
• PMD
• Note The information presented here is just to
  introduce these terms and concepts. Many of the
  hows and whys are beyond the scope of this
  material. Dont get lost in the detail!

48
802.11 Physical Layer Technologies

• 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.

49
802.11 Physical Layer Technologies

• 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)

50
802.11 Physical Layer Technologies

• 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.

51
PLCP (Physical Layer Convergence Procedure)
General 802.11 Frame
IP Packet
LLC
PDSU

• 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

52
802.11 Physical Layer Technologies

• 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
• Lets 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.

53
PMD (Physical Medium Dependent)
Original Data Bits
Scrambler
Scrambled Data Bits
Transmission Medium
Original Data Bits
Descrambler
Scrambled Data Bits

• 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 0s
  or 1s.
• The data is scrambled by the transmitter and
  descrambled by the receiver.

54
PMD - Coding
Noise
Spread Signal of coded bits
Frequency

• 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
55
802.11 Chipping Sequence Barker Sequence
Scrambled Data Bit
Expanded Data Bit
Transmitted Chipped Sequence
1
11111111111
XOR
01001000111
10110111000
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.

56
802.11 Chipping Sequence Barker Sequence
Original Data Bit
XOR 0 XOR 0 -gt 0 1 XOR 1 -gt 0 0 XOR 1 -gt 1
1
Either one
Scrambled Data Bit
Expanded Data Bit
Transmitted Chipped Sequence
1
11111111111
XOR
01001000111
10110111000
Barker Sequence
Scrambled Data Bit
Expanded Data Bit
Transmitted Chipped Sequence
0
00000000000
XOR
10110111000
10110111000
Barker Sequence
57
PMD Concepts and Building Blocks
Original Data Bits
Scrambler
Block Coder

• 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 cant 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 wont go into the detail here.

Block Interleaver
Modulated over Transmission Medium
Block Interleaver
Original Data Bits
Descrambler
Block Decoder
58
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.
• 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.

59
802.11 Physical Layer Technologies

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

60
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

61
802.11 Physical Layer Technologies
Original 802.11

• 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

62
802.11 - Frequency-hopping spread-spectrum (FHSS)
63
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

64
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.

65
802.11 - Frequency-hopping spread-spectrum (FHSS)

• 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.

• 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.

66
802.11 - Frequency-hopping spread-spectrum (FHSS)

• 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.

67
802.11 - Frequency-hopping spread-spectrum (FHSS)
DSSS (Spread Spectrum) Signal (22 MHz)
FHSS Signal (1 MHz)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
20 21 22
Frequency MHz
68
802.11 - Direct-sequence spread-spectrum (DSSS)
69
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).

70
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

71
802.11 - Direct-sequence spread-spectrum (DSSS)
DSSS (Spread Spectrum) Signal (22 MHz)
FHSS Signal (1 MHz)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
20 21 22
Frequency MHz
72
802.11 - Direct-sequence spread-spectrum (DSSS)
General 802.11 Frame
IP Packet
LLC
PDSU

• 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.

73
802.11 - Direct-sequence spread-spectrum (DSSS)
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.

74
PLCP and MAC Error Statistics
75
802.11 - Direct-sequence spread-spectrum (DSSS)

• 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.

76
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.

77
802.11b - High-Rate Direct-sequence
spread-spectrum (HR/DSSS)
78
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

79
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).

80
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).

81
802.11b - High-Rate Direct-sequence
spread-spectrum (HR/DSSS)
82
802.11b - High-Rate Direct-sequence
spread-spectrum (HR/DSSS)
Long
Short

• 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

83
ACU
HELP Information

• 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.

84
802.11b - High-Rate Direct-sequence
spread-spectrum (HR/DSSS)

• 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).

85
802.11b - High-Rate Direct-sequence
spread-spectrum (HR/DSSS)

• 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.

86
802.11a OFDM (Orthogonal Frequency Division
Multiplexing)
87
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).

88
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.

89
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

90
802.11a OFDM (Orthogonal Frequency Division
Multiplexing)
4
8

• 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

91
802.11a OFDM (Orthogonal Frequency Division
Multiplexing)
www.networkcomputing.com/1201/1201ws1.html

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

92
802.11a OFDM (Orthogonal Frequency Division
Multiplexing)

• 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.

93
802.11a OFDM (Orthogonal Frequency Division
Multiplexing)
www.networkcomputing.com/1201/1201ws1.html

• 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.

94
802.11a OFDM (Orthogonal Frequency Division
Multiplexing)
www.networkcomputing.com/1201/1201ws1.html

• 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.

95
802.11a OFDM (Orthogonal Frequency Division
Multiplexing)
www.networkcomputing.com/1201/1201ws1.html

• 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.

96
802.11a OFDM (Orthogonal Frequency Division
Multiplexing)
48 subchannels for data

• 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.

97
802.11a OFDM (Orthogonal Frequency Division
Multiplexing)
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

98
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 channels 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.

99
802.11g Extended Rate Physical (ERP) layer
100
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

101
802.11g Extended Rate Physical (ERP) layer
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

• 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.)

102
802.11g / 802.11b Compatibility
Cant hear 802.11g OFDM messages during CCA
(Clear Channel Assessment), so will transmit and
may cause collisions
802.11g
802.11b

• 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.

103
802.11g / 802.11b Compatibility
RTS/CTS
CTS-to-self
CTS
RTS
CTS
802.11g
802.11b
802.11g
802.11b

• 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.

104
802.11g Extended Rate Physical (ERP) layer

• 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
105
802.11g Extended Rate Physical (ERP) layer
802.11b compatibility Backwards compatibility
with 802.11
Long PPDU
Short PPDU
802.11b compatibility Minimizes overhead
802.11g Higher data rates
106
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) - (802.11 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.

107
Comparing 802.11a, 802.11b, 802.11g
108
(No Transcript)
109
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
110
Relative Ranges
Broadband.com

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

111
Expected Throughputs
Broadband.com

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

112
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.

113
ACU and various client adapters

• Cisco ACU works with all adapters.

114
ACU and various client 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.

115
ACU and various client adapters

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

116
PLCP and MAC Error Statistics
117
http//www.cisco.com/en/US/products/hw/wireless/ps
4555/products_data_sheet09186a00801ebc29.html
118
Ch. 2 802.11 and NICsPart 3 802.11 PHY

• Cisco Fundamentals of Wireless LANs version 1.1
• Rick Graziani
• Cabrillo College