1 cscd 433/533 wireless networks and security lecture 8 physical layer, and 802.11 b,g,a,n,ac...
TRANSCRIPT
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CSCD 433/533Wireless Networks and Security
Lecture 8Physical Layer, and 802.11 b,g,a,n,ac
Differences
Fall 2013
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Topics
Differences between 802.11 b,g,a and n Frequency ranges Speed
Spread Spectrum Techniques DSSS Spread Spectrum, 802.11b OFDM and 802.11n
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Introduction
Today, discuss physical layer of 802.11 standard
Many flavors and techniques that help to increase throughput via various techniques
We will start with slowest and end with fastest
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Introduction
General question we will address is how do we share bandwidth at physical wireless level?
Look at wireless characteristics of signals and FCC regulations that govern sharing of unlicensed bands
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FCC Regulation
In 1995, Federal Communications Commission allocated several bands of wireless spectrum for use without license
FCC stipulated that use of spread spectrum technology would be required
In 1990, IEEE began exploring a standard July 1999 the 802.11b standard was
ratified
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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 military does use spread spectrum transmission to make signals hard to detect
• This requires a different spread spectrum technology
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Frequency BandISM: Industry, Science, Medicine
unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz
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IEEE 802.11 Frequency Band
and 802.11b/g 802.11a
Wavelength
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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 data over channel that extends 11MHz on each side of center frequency
Channels overlap, but there are three out of 11 channels that don't
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802.11b/g Channels
2400 – 2483 Each channel spaced 5 MHz apart
Only non-overlapping channels are 1, 6 and 11
Channel Width = 22 MHz Channels – 12 – 14, not sanctioned by FCC
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Frequency Assignments
Channel 12.412 GHz
Channel 62.437 GHz
Channel 112.462 GHz
25 MHz25 MHz
The Center frequencies of each channel are only 5 Mhz apart but each channel is 22 Mhz wide therefore adjacent channels will overlap.
DSSS systems with overlapping channels in the same physical space would cause interference between systems.
Co-located DSSS systems should have frequencies which are at least 5 channels apart, e.g., Channels 1 and 6, Channels 2 and 7, etc.
Channels 1, 6 and 11 are the only theoretically non-overlapping channels.
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Comparisons of 802.11 Physical Layer
3 Flavors of 802.11 802.11a 802.11b 802.11g
Newest ones 802.11n and 802.11ac
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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 occurs as distortion in frequency band Overcome noise
Ensure power of signal > noise Recall, SNR = Signal to Noise Ratio
» But, what if you include your own noise into the signal?
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Radio Communications Legal authority imposes 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 …
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Radio Communications
There are some 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
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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
Can’t rely on FCC to step in
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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
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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
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Radio Communications
Trivia Question Who patented spread spectrum transmission
and when was it patented?
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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
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Spread Spectrum 802.11 uses three different Spread
Spectrum technologies1. 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 won’t cover it, not enough time FHSS is covered in, http://www.cs.clemson.edu/~westall/851/spread-
spectrum.pdf
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Spread Spectrum
802.11 uses three different Spread Spectrum technologies2. 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
3. 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
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802.11 and Spread Spectrum
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Spread Spectrum Code Techniques
Spread-spectrum is a signal propagation technique Employs several methods
Decrease potential interference to other receivers Generally makes use of noise-like signal
structure to spread normally narrowband information signal over a relatively wideband (radio) band of frequencies
Receiver correlates (matches) received signals to retrieve original information signal
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Spread Spectrum Defined
Spread spectrum is a communication technique that spreads a narrowband communication signal over a wide range of frequencies for transmission then de-spreads it into the original data bandwidth at the receive.
Spread spectrum is characterized by:
wide bandwidth and
low power
Jamming and interference have less effect on Spread spectrum because it
Resembles noise
Hard to detect
Hard to intercept
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Spread Spectrum Code Techniques
Typical applications include Satellite-positioning systems (GPS) 3G mobile telecommunications W-LAN (IEEE802.11a, IEEE802.11b,
IEE802.11g) Bluetooth
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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
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Spread Spectrum (SS)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 They 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
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Spread Spectrum Techniques
In a spread-spectrum system, signals spread across wide bandwidth, making them difficult to intercept and demodulate
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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
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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
Spreading de-Spreading
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General Model of Spread Spectrum System
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Spread Spectrum Code Techniques
• Real advantage of SS– If Intentional or un-intentional interference and
jamming ----> Signal rejected … does not contain SS key
• Only desired signal, which has key, will be seen at receiver when despreading operation
• Practically can ignore all other signals if it does not include key used in despreading operation
Allows different SS communications to be active simultaneously in same band
• Each will have their own PN code
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Spread Spectrum Code Techniques
Can see results of interference attempts, interferer signals are not recovered
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DSSS and HR/DSSS
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DSSS
DSSS is a spread spectrum technique Modulation is altering carrier wave in order to
transmit a data signal (text, voice, audio, video, etc.)
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
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DSSS
Why this works ... 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
Typical SS processing gains run from 10dB to 60dB
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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
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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 Chip is an 11 bit code combined with a data bit
to produce an 11 bit code This gets transmitted to receiver
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DSSS Chipping Sequence
Data SpreadingEncoded Data Correlation
1
0
1
0
Modulo 2
add
Spreading Code
10110111000
10110111000
01001000111Modulo 2
Subtract
10110111000
Spreading Code
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Figure 6.33 DSSS example
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Direct Sequence Spread Spectrum Example
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Code Division Multiple Access (CDMA)
A multiplexing technique used with spread spectrumGiven a data signal rate DBreak each bit into k chips according to a fixed chipping code specific to each userResulting new channel has chip data rate kD chips per secondCan have multiple channels superimposed
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CDMA Example
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DSSS
• Chipping stream– Two costs to increased chipping ratio
1. Direct cost of more expensive RF components that operate at higher frequencies
2. Amount of bandwidth required
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DSSS
Encoding DSSS 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
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DSSS
Encoding DSSS Why 11 bits?
Regulatory authorities require a 10 dB processing gain in DSSS 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
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OFDM
Orthogonal Frequency Division Multiplexing
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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 available spectrum
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OFDM Based on FDM
• Recall …– Frequency division multiplexing (FDM) is
technology that transmits multiple signals simultaneously over single transmission path, such as cable or wireless system
– Each signal travels within its own unique frequency range (carrier)
– What do you recall about the efficiency of this technique?
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FDM• Comment
– FDM transmissions are least efficient since each analog channel can only be used one user at a time
Each User has their own channel
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OFDM based on FDM
• OFDM, data divided among large number of closely spaced carriers– "frequency division multiplex" part of name– Entire bandwidth is filled from single source of
data– Instead of transmitting data serially, data is
transferred in parallel– Divided among multiple subcarriers– Only small amount of data is carried on each
carrier
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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 whole signal
to be able to demodulate data• So, when signals are transmitted close to one
another they typically spaced with guard band between them
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Traditional View FDM with Guards
Traditional view of signals carrying modulation
Receiver filter passband: one signal selected
Guards
Guard bands waste the spectrum
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OFDM
• Making Subcarriers Mathematically Orthogonal – Breakthrough for OFDM– Enables OFDM receivers to separate
subcarriers via Fast Fourier Transform (FFT)• Eliminates guard bands• OFDM subcarriers can overlap to make full use of
spectrum• Peak of each subcarrier spectrum, power in all
other subcarriers is zero
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OFDM• OFDM offers higher data capacity in a given
spectrum while allowing a simpler system design
Others have zero power
Power
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OFDM
• Shows parallel nature of subcarriers
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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 …
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Multipath Fading
• Indoor and Outdoor 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– Outdoors line-of-sight often blocked by obstacles, and
collection of differently delayed waves received by mobile antenna
– These reflected waves interfere with direct wave, causes significant degradation link performance
- Waves arrive at slightly different times, so they are out
of phase with original wave• Randomly boosts or cancels out parts of signal
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Multipath Fading
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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
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Benefits of OFDM
• Longer guard intervals - more robust system to multipath effects– But during guard interval, system gets no use
from 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
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Benefits of OFDM
• OFDM meets this challenge by• Dividing transmissions among multiple
subcarriers– Symbol transmission time is multiplied by
number of subcarriers– For example: With 802.11a, there are 52
channels, so the system has 52 times transmission capacity compared to single channel
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OFDM vs. Single Channel
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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
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802.11a
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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 has restrictions imposed on maximum
allowed output power
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ISM vs. U-NII
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802.11a OFDM
• 802.11a specifies• 8 non-overlapping 20 MHz channels in lower two
bands– Each divided into 52 sub-carriers (four of which carry
pilot data) of 300-kHz bandwidth each
• 4 non-overlapping 20 MHz channels are specified in upper band
• Receiver processes 52 individual bit streams, reconstructing original high-rate data stream– Four complex modulation methods are employed,
depending on data rate that can be supported by conditions between transmitter and receiver
– Include BPSK, QPSK, 16-QAM, and 64-QAM
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802.11a Channels
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Trying to Use 802.11a
• Advantage– Since 2.4 GHz band is heavily used, using 5 GHz
band gives 802.11a 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
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802.11b vs 802.11a Path Loss
Free Space Path Loss in dB for 2.4 and 5 GHz Spectrums
Distance (miles) 2.4 GHz 5 GHz
0.5 98.36 104.56
5 104.38 110.58
1.5 107.91 114.10
3 113.93 120.12
4 116.42 122.62
5 118.36 124.56
10 124.38 130.58
Loss = 32.4 X 20Log(MHz) X 20Log(distance)
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Range and Data Rate
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802.11g
• June 2003, a third modulation standard ratified– 802.11g– Works in 2.4 GHz band (like 802.11b) – Maximum data rate of 54 Mbit/s– 802.11g hardware works with 802.11b
hardware– Older networks, 802.11b node significantly
reduces the speed of an 802.11g network
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802.11g
• Modulation schemes used in 802.11g– OFDM for data rates of 6, 9, 12, 18, 24, 36, 48, and
54 Mbit/s, and – Reverts to CCK – Complimentary Code Keyinglike 802.11b for 5.5 and 11 Mbit/s – DBPSK/DQPSK+DSSS for 1 and 2 Mbit/s
• Even though 802.11g operates in same frequency band as 802.11b– Achieve higher data rates because it uses
OFDM and better modulation
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802.11g Rates, Transmission, Modulation
Data Rate Mbps Trans Type Modulation
54 OFDM 64 QAM
48 OFDM 64 QAM
11 DSSS QPSK1
6 OFDM BPSK
5.5 DSSS CCK
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802.11n … A miracle or …
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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
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802.11n Features
• 802.11n utilizes larger number of antennas
• Number of antennas relates to 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
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802.11n Features
• 802.11n standard operates in 2.4-GHz, the 5-GHz radio band, or both – more flexibility– Backward compatibility with preexisting
802.11a/b/g deployment– Majority of devices and access points
deployed are dual-band• Operates in both 2.4-GHz and 5-GHz frequencies
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802.11n Features
• Wireless solutions based on 802.11n standard use 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 fivefold performance increase over 802.11a/b/g networks
• How does this work? Anyone?
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MIMO
• 802.11n builds on previous standards by adding multiple-input multiple-output (MIMO)
– MIMO uses multiple transmitter and receiver antennas to improve system performance
– MIMO uses additional signal paths from each antenna to transmit more information, recombine signals on the receiving end
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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
– Provides greater data throughput
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MIMO Technology• Multiple independent streams are transmitted
simultaneously to increase the data rate
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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 MIMO access point may be able to continue operating at 54 Mbps
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Channel Bonding
• Most straightforward way to increase capacity of network is to increase 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 channel bandwidth
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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
– 5 GHz has larger number of channels available for bonding
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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
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Packet Aggregation
• 802.11n technologies increase efficiency by aggregating multiple data packets into a single transmission frame
• 802.11n networks can send multiple data packets with fixed overhead cost of just a single frame
• Packet aggregation is more beneficial for certain types of applications such as file transfers – Real-time applications (e.g. voice) don’t benefit
from packet aggregation because its packets would need to be interspersed at regular intervals
– And combining packets into larger payload would introduce unnecessary latency
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802.11 Comparison
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Summary
From 1999 until 2013 … 14 years amazing changes in wireless LAN technology
From 5.5 Mbps to 300 + Mbps and beyond How?
Parallelism of data streams Increased number of antennas Resolving interference through math and
multiplexing Cramming more data within limited
frequencies Better modulation techniques
Future – More of the same !!!
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End
Small Assignment 2a – Wireless Questions
due Monday, October 21