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CSCD 433/533 Wireless Networks and Security

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CSCD 433/533 Wireless Networks and Security Lecture 8 Physical Layer, and 802.11 b,g,a Differences Fall 2012 – PowerPoint PPT presentation

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Title: CSCD 433/533 Wireless Networks and Security


1
CSCD 433/533Wireless Networks and Security
  • Lecture 8
  • Physical Layer, and 802.11 b,g,a Differences
  • Fall 2012

2
Topics
  • Differences between 802.11 b,g,a and n
  • Frequency ranges
  • Speed
  • Spread Spectrum Techniques
  • DSSS Spread Spectrum, 802.11b

3
Introduction
  • Today, discuss the physical layer of the 802.11
    standard
  • Many flavors and techniques that help to increase
    throughput via various techniques
  • We will start with the slowest and end with the
    fastest

4
Introduction
  • General question we will address is how do we
    share the bandwidth at the physical wireless
    level?
  • Look at the wireless characteristics of the
    signals and the FCC regulations that govern
    sharing of the unlicensed bands

5
FCC Regulation
  • In 1995, Federal Communications Commission
    allocated several bands of wireless spectrum for
    use without a license
  • The FCC stipulated that the use of spread
    spectrum technology would be required
  • In 1990, the IEEE began exploring a standard
  • In 1997 the 802.11 standard was ratified and is
    now obsolete
  • July 1999 the 802.11b standard was ratified

6
Spread Spectrum Transmission
  • Spread Spectrum Transmission
  • You are required by law to use spread spectrum
    transmission in unlicensed bands
  • Spread spectrum transmission reduces propagation
    problems
  • Especially multipath interference
  • Spread spectrum transmission is NOT used for
    security in WLANs
  • Although the military does use spread spectrum
    transmission to make signals hard to detect
  • This requires a different spread spectrum
    technology

5-6
7
Frequency Band
ISM Industry, Science, Medicine
unlicensed frequency spectrum 900Mhz, 2.4Ghz,
5.1Ghz, 5.7Ghz
8
IEEE 802.11 Frequency Band
Wavelength
9
802.11 Physical Channels
  • The 802.11b standard defines 14 frequency
    channels in the 2.4GHz range
  • Only eleven are allowed for unlicensed use by the
    FCC in the US
  • Each channel uses "Direct Sequence Spread
    Spectrum" (DSSS) to spread the data over the
    channel that extends 11MHz on each side of the
    center frequency
  • The channels overlap, but there are three out of
    11 channels that don't

10
802.11b/g Channels
Channel Width 22 MHz
Channels 12 14, not sanctioned by FCC
2400 2483 Each channel spaced 5 MHz
apart Only non-overlapping channels are 1, 6 and
11
11
Comparisons of 802.11 Physical Layer
  • 3 Flavors of 802.11
  • 802.11a OFDM
  • 802.11b High Rate DS or DSSS
  • 802.11g Extended Rate or ERP
  • Newest one
  • 802.11n MIMO PHY High Throughput PHY

12
Radio Communications
  • How do you transmit Radio Signals reliably?
  • Classic approach .
  • Confine information carrying signal to a narrow
    frequency band and pump as much power as possible
    into signal
  • Noise is naturally occurring distortion in
    frequency band
  • Overcome noise
  • Ensure power of signal gt noise

13
Radio Communications
  • Legal authority must impose rules on how RG
    spectrum is used
  • FCC in US
  • European Radiocommunications Office (ERO)
  • European Telecommunications Standards Institute
    (ETSI)
  • Ministry of Internal Communications (MIC) in
    Japan
  • Worldwide harmonization work done under
  • International Telecommunications Union (ITU)
  • Must have license to transmit at given frequency
    except for certain bands

14
Radio Communications
  • There are unlicensed bands
  • 802.11 Networks operate in bands which are
    license free, Industrial, Scientific and Medical
    (ISM)
  • Does require FCC oversight, requires manufacturer
    to file information with the FCC
  • Competing devices have been developed in 2.4 GHz
    range
  • 802.11 products
  • Bluetooth
  • Cordless phones
  • X10 Protocol for home automation

15
Radio Communications
  • 2.4 GHz is Unlicensed but
  • Must obey FCC limitations on power, band use and
    purity of signal
  • No regulations specify coding or modulation
  • Thus, there is contention between devices
  • Solve the problems
  • Stop using device, amplify its power or move it
  • Cant rely on FCC to step in

16
Radio Communications
  • Given multiple devices compete in ISM bands, how
    do you reliably transmit data?
  • Spread Spectrum is one of the answers
  • Radio signals are sent with as much power as
    allowed over a narrow band of frequency
  • Spread Spectrum
  • Used to transform radio for data
  • Uses math functions to diffuse signal over large
    range of frequencies
  • Makes transmissions look like noise to narrowband
    receiver

17
Radio Communications
  • Spread Spectrum continued
  • On receiver side, signal is transformed back to
    narrow-band and noise is removed
  • Spread spectrum is a requirement for unlicensed
    devices
  • Minimize interference between unlicensed devices,
    FCC imposes limitations on power of transmissions

18
Radio Communications
  • Trivia Question
  • Who patented spread spectrum transmission and
    when was it patented?

19
Hedy Lamarr
  • Austrian actress Hedy Lamarr became a pioneer
    in the field of wireless communications following
    her emigration to the United States
  • With co-inventor George Anthiel, developed a
    "Secret Communications System" to help combat the
    Nazis in World War II
  • By manipulating radio frequencies at irregular
    intervals between transmission and reception, the
    invention formed an unbreakable code to prevent
    classified messages from being intercepted by
    enemy personnel
  • Patented in 1941

20
Spread Spectrum
  • 802.11 uses three different Spread Spectrum
    technologies
  • FH Frequency Hopping (FHSS)
  • Jumps from one frequency to another in random
    pattern
  • Transmits a short burst at each subchannel
  • 2 Mbps FH or FHSS is the original spread spectrum
    technology developed in 1997 with the 802.11
    standard
  • However, it was quickly bypassed by more
    sophisticated spread spectrum technologies
  • We wont cover it, not enough time
  • FHSS is covered in,
  • http//www.cs.clemson.edu/westall/851
    /spread-spectrum.pdf

21
Spread Spectrum
  • 802.11 uses three different Spread Spectrum
    technologies
  • DS or DSSS Direct Sequence
  • Took over from FHSS and allowed for faster
    throughput
  • Used in 802.11b
  • Spreads out signal over a wider path
  • Uses frequency coding functions
  • OFDM Orthogonal Frequency Division Multiplexing
  • Divides channel into several subchannels and
    encode a portion of signal across each subchannel
    in parallel
  • 802.11a and 802.11g uses this technology
  • Allows for even faster throughput than DSSS

22
RF Propagation
  • As radio signals travel through space, they
    degrade over distance
  • Performance determined by signal to noise ratio
    (SNR)
  • Says how strong is my signal compared to noise?
  • Degradation of signal will limit signal to noise
    ratio of receiver
  • Noise floor stays the same over 802.11 network
  • But, as station gets further from Access Point,
    signal level drops and SNR will be lower

23
RF Propagation
AP1
Received Signal
Noise
Distance
24
RF Propagation
  • Signal Degradation
  • When no obstacles, signal degradation can be
    calculated by following equation
  • Depends on distance and frequency
  • Path loss (dB) 32.5 20 log F log d
  • where F GHz , d distance in meters
  • Higher F leads to more path loss at equal
    distances
  • Explains why 802.11a has a shorter range
  • It operates in the 5 GHz frequency range

25
  • 802.11 Signal Propagation Techniques

26
Spread Spectrum Code Techniques
  • Spread-spectrum is a signal propagation technique
  • Employs several methods
  • Decrease potential interference to other
    receivers while achieving privacy
  • Generally makes use of noise-like signal
    structure to spread normally narrowband
    information signal over a relatively wideband
    (radio) band of frequencies
  • Receiver correlates received signals to retrieve
    original information signal

27
Spread Spectrum Code Techniques
  • Typical applications include
  • Satellite-positioning systems (GPS)
  • 3G mobile telecommunications
  • W-LAN (IEEE802.11a, IEEE802.11b, IEE802.11g)
  • Bluetooth

28
Spread Spectrum Code Techniques
  • Three characteristics of Spread Spectrum
    techniques
  • 1. Signal occupies bandwidth much greater than
    that which is necessary to send the information
  • - Many benefits, immunity to interference,
    jamming and multi-user access talk about this
    later
  • 2. Bandwidth is spread by means of code
    independent of data
  • - Independence of code distinguishes this from
    standard modulation schemes in which data
    modulation will always spread spectrum somewhat
  • 3. Receiver synchronizes to code to recover the
    data
  • - Use of an independent code and synchronous
    reception allows multiple users to access the
    same frequency band at the same time

29
Spread Spectrum Code Techniques
  • Transmitted signal takes up more bandwidth than
    information signal that is being modulated
  • Name 'spread spectrum' comes from fact that
    carrier signals occur over full bandwidth
    (spectrum) of a device's transmitting frequency
  • Military has used Spread Spectrum for many years
  • Worry about signal interception and jamming
  • SS signals hard to detect on narrow band
    equipment because the signal's energy is spread
    over a bandwidth of maybe 100 times information
    bandwidth

30
Spread Spectrum Techniques
  • In a spread-spectrum system, signals spread
    across wide bandwidth, making them difficult to
    intercept and demodulate

31
Spread Spectrum Code Techniques
  • Spread Spectrum signals use fast codes
  • These special "Spreading" codes are called
    "Pseudo Random" or "Pseudo Noise" codes
  • Called "Pseudo" because they are not truly random
    distributed noise
  • Will look at an example of this later

32
Same code must be known in advance at both ends
of the transmission channel
Spread Spectrum Code Techniques
Spreading
de-Spreading
Codes are what DSSS uses talk about next
33
Spread Spectrum Code Techniques
  • Real advantage of SS
  • Intentional or un-intentional interference and
    jamming signals rejected do not contain the SS
    key
  • Only desired signal, which has key, will be seen
    at receiver when despreading operation is
    exercised
  • Practically can ignore interference if it does
    not include key used in the despreading operation
  • That rejection also applies to other SS signals
    not having right key
  • Allows different SS communications to be active
    simultaneously in the same band
  • Each will have their own PN code

34
Spread Spectrum Code Techniques
  • Can see results of interference attempts,
    interferer signals are not recovered

35
DSSS and HR/DSSS
36
DSSS
  • DSSS is a spread spectrum technique
  • Modulation scheme used to transmit signal over
    wider frequency bandwidth
  • Modulation is the altering of carrier wave in
    order to transmit a data signal (text, voice,
    audio, video, etc.) from one location to another
    via a discrete channel
  • Phase-modulates a sine wave pseudorandomly
  • Continuous string of pseudonoise (PN) code
    symbols called "chips
  • Each of which has a much shorter duration than an
    information bit
  • Each information bit is modulated by a sequence
    of much faster chips

37
DSSS
  • DSSS Techniques
  • To a narrowband receiver, transmitted signal
    looks like noise
  • Original signal can be recovered through
    correlation that reverses the process
  • The ratio (in dB) between the spread baseband and
    the original signal is called processing gain
  • It is the ratio by which unwanted signals or
    interference can be suppressed relative to the
    desired signal when both share the same frequency
    channel
  • Typical SS processing gains run from
  • 10dB to 60dB

38
DSSS
  • How DSSS works
  • Apply something called a chipping sequence to
    the data stream
  • Chip is a binary digit
  • But, spread-spectrum developers make distinction
    to separate encoding of data from the data itself
  • Talk about data is bits
  • Talk about encoding is chips or chipping sequence

39
DSSS
  • Chipping sequence
  • Also called Pseudorandom Noise Codes (PNC)
  • Must run at a higher rate than underlying data
  • At left, is a data bit 0 or 1
  • For each bit, chip sequence is used
  • Originally, the chip was an 11 bit code combined
    with a data bit to produce an 11 bit code
  • This gets transmitted to receiver

40
DSSS Chipping Sequence
Encoded Data
Correlation
Data
Spreading
Modulo 2 Subtract
Modulo 2 add
1 0
01001000111
1 0
10110111000
10110111000
10110111000
Spreading Code
Spreading Code
41
DSSS
  • Chipping stream
  • Receiver uses correlation recovers bits by
    looking at each 11 bit segment of stream
  • Compares it to chipping sequence which is static
  • If it matches, bit is a zero
  • If it doesnt match, bit is a one
  • Result of using a high chip-to-bit signal if
    signal is spread out over a wider bandwidth

42
DSSS
  • Chipping stream
  • DS system is concerned with Spreading Ratio
  • Number of chips used to transmit a single bit
  • Higher spreading ratios improve ability to
    recover transmitted signal
  • Because, also, spreading out noise over a larger
    area
  • Ratio of noise to actual spread and data is less
  • Doubling spreading ratio requires doubling
    chipping rate and doubles required bandwidth too

43
DSSS
  • Chipping stream
  • Two costs to increased chipping ratio
  • Direct cost of more expensive RF components that
    operate at higher frequencies
  • Amount of bandwidth required

44
DSSS
  • Encoding DS
  • 802.11 originally adopted an 11-bit Barker word
  • Each bit encoded using entire Barker word or
    chipping sequence
  • Key attribute of Barker words
  • Have good autocorrelation properties
  • High signal recovery possible when signal
    distorted by noise
  • Correlation function operates over wide range of
    environments and is tolerant of propagation delay

45
DSSS
  • Encoding DS
  • Why 11 bits?
  • Regulatory authorities require a 10 dB processing
    gain in DS systems
  • Using an 11 bit spreading code for each bit let
    802.11 meet regulatory requirements
  • Recall
  • The ratio (in dB) between the spread baseband and
    the original signal is processing gain

46
DSSS
  • Complementary Code Keying (CCK)
  • Different modulation scheme used to encode more
    bits per code word
  • In 1999, CCK was adopted to replace the Barker
    code in wireless digital networks
  • CCK divides chip stream up into 8-bit code
    symbols so underlying transmission based on
    series of 1.375 million code symbols/sec

47
DSSS
  • Complementary Code Keying (CCK)
  • Based on mathematical transforms allow use of
    8-bit sequences to encode 4 or 8 bits per code
    word
  • Helped increase data throughput to 5.5 Mbps or 11
    Mbps
  • CCK selected over competing modulation techniques
    as it utilized same bandwidth and could utilize
    same header as pre-existing 1 and 2 Mbit/s
    wireless networks
  • Guarantee interoperability

48
Intro to 802.11a
  • 802.11a was approved in September 1999, two years
    after 802.11 standard approved
  • Operates in 5 GHz unlicensed national information
    infrastructure (UNII) band
  • Spectrum is divided into three domains, each
    having restrictions imposed on the maximum
    allowed output power
  • First 100 MHz in the lower frequency portion is
    restricted to a maximum power output of 50 mW
  • Second 100 MHz has a higher 250 mW maximum
  • Third 100 MHz, which is mainly intended for
    outdoor applications, has a maximum of 1.0 W
    power output

49
Intro to 802.11a
  • 802.11a
  • Offered an alternative to the overcrowded band
    2.4 GHz, 5GHz
  • The 5GHz ISM bandwidth is not continuous
  • There are two areas 5.15GHz - 5.35GHz and
  • 5.725GHz - 5.825Ghz
  • More details about 802.11a later

50
Intro to OFDM
  • 802.11a and 802.11g based on OFDM
  • Orthogonal Frequency Division Multiplexing
  • Revolutionized Wi-Fi and other cellular products
    by allowing faster throughput and more robustness
  • OFDM makes highly efficient use of the available
    spectrum
  • This characteristic will be important in coming
    years as wireless networks dominate especially in
    enterprise environments

51
OFDM Based on FDM
  • Recall
  • Frequency division multiplexing (FDM) is a
    technology that transmits multiple signals
    simultaneously over a single transmission path,
    such as a cable or wireless system
  • Each signal travels within its own unique
    frequency range (carrier)

52
FDM
  • Comment
  • FDM Access transmissions are the least efficient
    networks since each analog channel can only be
    used one user at a time

Each User has their own channel
53
OFDM based on FDM
  • In OFDM, data divided among large number of
    closely spaced carriers
  • The "frequency division multiplex" part of the
    name
  • The entire bandwidth is filled from a single
    source of data
  • Instead of transmitting data serially, data is
    transferred in a parallel
  • Divided among multiple subcarriers
  • Only a small amount of the data is carried on
    each carrier, which besides the obvious benefit
    of being parallel
  • Provides benefits related to the radio nature of
    wireless

54
OFDM
  • An OFDM signal consists of
  • Several closely spaced modulated carriers
  • When modulation of any form - voice, data, etc.
    is applied to a carrier
  • Sidebands spread out on either side
  • A receiver must be able to receive the whole
    signal to be able to demodulate the data
  • So, when signals are transmitted close to one
    another they must be spaced with a guard band
    between them

55
Traditional View with Guards
Guard bands waste the spectrum
Receiver filter passband one signal selected
Guards
Traditional view of signals carrying modulation
56
OFDM
  • Making the subcarriers mathematically orthogonal
  • Breakthrough for OFDM
  • Enables OFDM receivers to separate subcarriers
    via an Fast Fourier Transform
  • Eliminate the guard bands
  • OFDM subcarriers can overlap to make full use of
    the spectrum
  • Peak of each subcarrier spectrum, power in all
    the other subcarriers is zero

57
OFDM
  • OFDM offers higher data capacity in a given
    spectrum while allowing a simpler system design

Power
Others are have zero power
58
OFDM
  • Shows parallel nature of subcarriers

59
Benefits of OFDM
  • Radio signals are imperfect
  • General challenges of RF signals include
  • Signal-to-noise ratio
  • Self-interference (intersymbol interference or
    ISI)
  • Fading owing to multipath effects
  • Same signal arrives at a receiver via different
    paths
  • Briefly look at multipath fading

60
Multipath Fading
  • The mobile or indoor radio channel is
    characterized by multipath reception
  • Sent signal contains not only a direct
    line-of-sight radio wave, but also a large number
    of reflected radio waves
  • Even worse in urban areas, the line-of-sight is
    often blocked by obstacles, and collection of
    differently delayed waves is received by a mobile
    antenna
  • These reflected waves interfere with direct wave,
    causes significant degradation link performance
  • Reason is that waves arrive at slightly different
    times, so they are out of phase with original
    wave
  • Will randomly boost or cancel out parts of the
    signal

61
Multipath Fading
62
Benefits of OFDM
  • Main way to prevent Intersymbol Interference
    errors
  • Transmit a short block of data (a symbol)
  • Wait until all the multipath echoes fade before
    sending another symbol
  • Waiting time often referred to as guard interval

63
Benefits of OFDM
  • Longer the guard intervals - more robust system
    to multipath effects
  • But during guard interval, system gets no use
    from the available spectrum
  • Longer the wait, the lower the effective channel
    capacity
  • Some guard interval is necessary for any wireless
    system
  • Goal is to minimize that interval and maximize
    symbol transmission time

64
Benefits of OFDM
  • OFDM meets this challenge by dividing
    transmissions among multiple subcarriers.
  • Same guard interval can then be applied to each
    subcarrier, while the symbol transmission time is
    multiplied by the number of subcarriers
  • With 802.11a, there are 52 channels, so the
    system has 52 times the transmission capacity
    compared to single channel

65
Benefits of OFDM
  • Using multiple subcarriers also makes OFDM
    systems more robust to fading
  • Fading typically decreases received signal
    strength at particular frequencies, so problem
    affects only a few of the subcarriers at any
    given time and
  • Error-correcting codes provide redundant
    information that enables OFDM receivers to
    restore information lost in these few erroneous
    subcarriers

66
802.11a OFDM
  • 802.11a specifies eight non-overlapping 20 MHz
    channels in the lower two bands
  • Each divided into 52 sub-carriers (four of which
    carry pilot data) of 300-kHz bandwidth each
  • Four non-overlapping 20 MHz channels are
    specified in the upper band
  • The receiver processes the 52 individual bit
    streams, reconstructing the original high-rate
    data stream
  • Four complex modulation methods are employed,
    depending on the data rate that can be supported
    by channel conditions between the transmitter and
    receiver.
  • Include BPSK, QPSK, 16-QAM, and 64-QAM.

67
Trying to Use 802.11a
  • Advantage
  • Since 2.4 GHz band is heavily used, using 5 GHz
    band gives 802.11a the advantage of less
    interference
  • Disadvantage
  • However, high carrier frequency also brings
    disadvantages
  • It restricts use of 802.11a to almost line of
    sight, necessitating use of more access points
  • It also means that 802.11a cannot penetrate as
    far as 802.11b since it is absorbed more readily,
    other things (such as power) being equal.

68
Trying to Use 802.11a
  • 802.11a products started shipping in 2001
  • Lagged 802.11b products slow availability of the
    5 GHz components needed to implement products
  • 802.11a was not widely adopted because 802.11b
    was already widely adopted
  • Because of 802.11a's disadvantages, poor initial
    product implementations, making its range even
    shorter, and because of regulations
  • Manufacturers of 802.11a equipment responded to
    lack of market success by improving the
    implementations
  • Plus making technology that can use more than one
    802.11 standard.
  • There are dual-band, or dual-mode or tri-mode
    cards that can automatically handle 802.11a and
    b, or a, b and g, as available
  • Similarly, there are mobile adapters and access
    points which can support all these standards
    simultaneously

69
Comparing 802.11a and 802.11b
  • The throughput of 802.11a is 2 to 4.5 times
    better than 802.11b up to a certain range
  • Example At 225 ft, 802.11a averages yielded 5.2
    Mbps compared to 1.6 Mbps for 802.11b
  • Next slide shows this as a graph

70
Throughput Range Performance
  • Averaged throughput performance for 1500 byte
    packets 802.11a thoughputs always better by 2
    to 4.5 times up to 225 ft.

71
802.11g
  • June 2003, a third modulation standard ratified
  • 802.11g
  • Works in 2.4 GHz band (like 802.11b) but operates
    at a maximum raw data rate of 54 Mbit/s, or about
    24.7 Mbit/s net throughput like 802.11a
  • 802.11g hardware will work with 802.11b hardware
  • Older networks, 802.11b node significantly
    reduces the speed of an 802.11g network

72
802.11g
  • The modulation scheme used in 802.11g
  • OFDM for data rates of 6, 9, 12, 18, 24, 36, 48,
    and 54 Mbit/s, and reverts to CCK, like 802.11b
    for 5.5 and 11 Mbit/s
  • DBPSK/DQPSKDSSS for 1 and 2 Mbit/s
  • Even though 802.11g operates in same frequency
    band as 802.11b
  • Achieve higher data rates because of its
    similarities to 802.11a
  • The maximum range of 802.11g devices is slightly
    greater than that of 802.11b devices
  • Range in which a client can achieve full (54
    Mbit/s) data rate speed is much shorter than that
    of 802.11b

73
Beyond 802.11a and b, 802.11g
  • Despite its major acceptance, 802.11g suffers
    from same interference as 802.11b in the already
    crowded 2.4 GHz range

74
  • 802.11n A miracle or

75
802.11n Introduction
  • 802.11n is long anticipated update to WiFi
    standards 802.11a/b/g
  • 4x increase in throughput
  • Improvement in range
  • 802.11n ratified by IEEE 2009

76
802.11n Features
  • 802.11n utilizes larger number of antennas
  • The number of antennas relates to the number of
    simultaneous streams
  • Two receivers and two transmitters (2x2) or four
    receivers and four transmitters (4x4)
  • The standards requirement is a 2x2 with a maximum
    two streams, but allows 4x4

77
802.11n Features
  • 802.11n standard operates in the 2.4-GHz, the
    5-GHz radio band, or both
  • Backward compatibility with preexisting
    802.11a/b/g deployment
  • Majority of devices and access points deployed
    are dual-band
  • Operate in both 2.4-GHz and 5-GHz frequencies

78
802.11n Features
  • Wireless solutions based on 802.11n standard
    employ several techniques to improve throughput,
    reliability, and predictability of wireless
  • Three primary innovations are
  • Multiple Input Multiple Output (MIMO) technology
  • Channel bonding (40MHz Channels)
  • Packet aggregation
  • Techniques allow 802.11n solutions to achieve an
    approximate fivefold performance increase over
    802.11a/b/g networks

79
MIMO
  • 802.11n builds on previous
  • standards by adding
  • multiple-input multiple-output (MIMO)
  • MIMO uses multiple transmitter and receiver
    antennas to improve the system performance
  • MIMO uses additional signal paths from each
    antenna to transmit more information, recombine
    signals on the receiving end

80
MIMO
  • 802.11n access points and clients transmit two or
    more spatial streams
  • Use multiple receive antennas and advanced signal
    processing to recover multiple transmitted data
    streams
  • MIMO-enabled access points use spatial
    multiplexing to transmit different bits of a
    message over separate antennas
  • Provide greater data throughput

81
MIMO Technology
  • Multiple independent streams are transmitted
    simultaneously to increase the data rate

82
MIMO
  • Performance gain is result of MIMO smart antenna
    technology
  • Allows wireless access points to receive signals
    more reliably over greater distances than with
    standard diversity antennas
  • Example, distance from access point at which an
    802.11a/g client communicating with a
    conventional access point might drop from 54 Mbps
    to 48 Mbps or 36 Mbps
  • Same client communicating with a MIMO access
    point may be able to continue operating at 54 Mbps

83
Channel Bonding
  • Most straightforward way to increase capacity of
    a network is to increase the operating bandwidth
  • However, conventional wireless technologies
    limited to transmit over one of several 20-MHz
    channels
  • 802.11n networks employ technique called channel
    bonding to combine two adjacent 20-MHz channels
    into a single 40-MHz channel
  • Technique more than doubles the channel bandwidth

84
Channel Bonding
  • Channel bonding most effective in 5-GHz frequency
    given greater number of available channels
  • 2.4-GHz frequency has only 3 non-overlapping
    20-MHz channels
  • Thus, bonding two 20-MHz channels uses two thirds
    of total frequency capacity
  • So, IEEE has rules on when a device can operate
    in 40MHz channels in 2.4GHz space to ensure
    optimal performance

85
Packet Aggregation
  • In conventional wireless transmission methods
  • Amount of channel access overhead required to
    transmit each packet is fixed, regardless of the
    size of the packet itself
  • As data rates increase, time required to transmit
    each packet shrinks
  • Overhead cost remains same

86
Packet Aggregation
  • 802.11n technologies increase efficiency by
    aggregating multiple packets of application data
    into a single transmission frame
  • 802.11n networks can send multiple data packets
    with the fixed overhead cost of just a single
    frame
  • Packet aggregation is more beneficial for certain
    types of applications such as file transfers
    because can aggregate packet content
  • Real-time applications (e.g. voice) dont benefit
    from packet aggregation because its packets would
    be interspersed at regular intervals
  • And combining packets into larger payload would
    introduce unnecessary latency

87
802.11 Comparison
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