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CSCD 433/533 Wireless Networks and Security


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

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

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

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

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

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

Spread Spectrum Transmission
  • Spread Spectrum Transmission
  • You are required by law to use spread spectrum
    transmission in unlicensed bands
  • Spread spectrum transmission reduces propagation
  • 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

Frequency Band
ISM Industry, Science, Medicine
unlicensed frequency spectrum 900Mhz, 2.4Ghz,
5.1Ghz, 5.7Ghz
IEEE 802.11 Frequency Band
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

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
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

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

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

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

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

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

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
  • Minimize interference between unlicensed devices,
    FCC imposes limitations on power of transmissions

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

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

Spread Spectrum
  • 802.11 uses three different Spread Spectrum
  • FH Frequency Hopping (FHSS)
  • Jumps from one frequency to another in random
  • 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
  • However, it was quickly bypassed by more
    sophisticated spread spectrum technologies
  • We wont cover it, not enough time
  • FHSS is covered in,
  • http//

Spread Spectrum
  • 802.11 uses three different Spread Spectrum
  • DS or DSSS Direct Sequence
  • Took over from FHSS and allowed for faster
  • 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

RF Propagation
  • As radio signals travel through space, they
    degrade over distance
  • Performance determined by signal to noise ratio
  • 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

RF Propagation
Received Signal
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
  • Explains why 802.11a has a shorter range
  • It operates in the 5 GHz frequency range

  • 802.11 Signal Propagation Techniques

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

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

Spread Spectrum Code Techniques
  • Three characteristics of Spread Spectrum
  • 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
  • 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
  • - Use of an independent code and synchronous
    reception allows multiple users to access the
    same frequency band at the same time

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

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

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

Same code must be known in advance at both ends
of the transmission channel
Spread Spectrum Code Techniques
Codes are what DSSS uses talk about next
Spread Spectrum Code Techniques
  • Real advantage of SS
  • Intentional or un-intentional interference and
    jamming signals rejected do not contain the SS
  • Only desired signal, which has key, will be seen
    at receiver when despreading operation is
  • 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

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

  • 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

  • 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
  • Typical SS processing gains run from
  • 10dB to 60dB

  • 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

  • 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

DSSS Chipping Sequence
Encoded Data
Modulo 2 Subtract
Modulo 2 add
1 0
1 0
Spreading Code
Spreading Code
  • 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

  • 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
  • Ratio of noise to actual spread and data is less
  • Doubling spreading ratio requires doubling
    chipping rate and doubles required bandwidth too

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

  • 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

  • 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

  • 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

  • Complementary Code Keying (CCK)
  • Based on mathematical transforms allow use of
    8-bit sequences to encode 4 or 8 bits per code
  • Helped increase data throughput to 5.5 Mbps or 11
  • 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

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

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

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
  • This characteristic will be important in coming
    years as wireless networks dominate especially in
    enterprise environments

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)

  • 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
OFDM based on FDM
  • In OFDM, data divided among large number of
    closely spaced carriers
  • The "frequency division multiplex" part of the
  • 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

  • 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

Traditional View with Guards
Guard bands waste the spectrum
Receiver filter passband one signal selected
Traditional view of signals carrying modulation
  • 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

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

Others are have zero power
  • Shows parallel nature of subcarriers

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

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
  • 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
  • Will randomly boost or cancel out parts of the

Multipath Fading
Benefits of OFDM
  • Main way to prevent Intersymbol Interference
  • 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

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
  • Some guard interval is necessary for any wireless
  • Goal is to minimize that interval and maximize
    symbol transmission time

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

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

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
  • Include BPSK, QPSK, 16-QAM, and 64-QAM.

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
  • Disadvantage
  • However, high carrier frequency also brings
  • 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.

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
  • 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

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

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.

  • 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

  • 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

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

  • 802.11n A miracle or

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

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

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

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

  • 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

  • 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
  • MIMO-enabled access points use spatial
    multiplexing to transmit different bits of a
    message over separate antennas
  • Provide greater data throughput

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

  • Performance gain is result of MIMO smart antenna
  • 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

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
  • 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

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

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

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
  • 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

802.11 Comparison