REFERENCES
Introduction to
Wireless Networking
Module-5
Physical Layer Access Methods and
Spread Spectrum
CCRI
ENGINEERING AND TECHNOLOGY
Jerry Bernardini
CCRI
CCRIEngineering
Engineeringand
andTechnology
Technology Jbernardini
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11
REFERENCES
• CWTS Certified Wireless Technology Specialist
Official Study Guide , Chapter-5
• CWNA Certified Wireless Network
Administration Official Study Guide (PWO-104),
David Coleman, David Westcott, 2009,
Chapter-6
• The California Regional Consortium for
Engineering Advances in Technological
Education (CREATE) project
• Spread Spectrum Scene http://www.sssmag.com/primer.html
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Chapter
REFERENCES
Objectives
• Define concepts which make up the functionality of
RF and spread spectrum technology
• Define and differentiate between the following
physical layer (PHY) wireless technologies
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IEEE 802.3 CSMA/CD
vs. IEEE 802.11 CSMA/CA
REFERENCES
•
•
•
•
•
•
CSMA/CD is for wired collision handling
CSMA/CA is for wireless collision handling
CSMA = Carrier Sense Multiple Access
CD = Collision Detection
CA = Collision Avoidance
Why do collisions occur?
– Answer = Two or more stations transmit at the same time
• Why is it important to detect or avoid collisions?
– Answer = Because there is data loss and retransmission is
necessary
• Wired networks are designed for the transmitting station to
detect most collisions
• Many collisions will not be detected by Wireless networks –
therefore avoid collisions
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IEEE 802.11 Collision
Handling CSMA/CA
REFERENCES
• In CSMA/CA a Wireless node that wants to transmit
performs the following sequence:
1. Listen on the desired channel.
2. If channel is idle (no active transmitters) it sends a packet.
3. If channel is busy the node waits random time until
transmission stops and then waits an additional time period.
4. If the channel is still idle at the end of the time period the
node transmits its packet otherwise it repeats the process
defined in 3 above until it gets a free channel.
5. Additional support mechanisms such as ACK, RTS/CTS can be
used but increase overhead noticeably.
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CSMA/CA
and ACK
REFERENCES
• CSMA/CA also reduces collisions via explicit frame
acknowledgment
• Acknowledgment frame (ACK): Sent by receiving device to
sending device to confirm data frame arrived intact
• If ACK not returned, transmission error assumed
• CSMA/CA does not eliminate collisions and does not solve hidden
node problem
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Two Kinds of Carrier
Sensing Mechanisms
REFERENCES
•
Physical Carrier Sense
–
–
–
–
–
•
Uses Clear Channel Assessment (CCA)
Is the RF energy on the channel above a threshold?
If CCA>threshold --->wait for CCA< threshold before transmitting
Checks received signal strength using RSSI
RF energy from a hidden node could be missed
Virtual Carrier Sense
– Uses the Network Allocation Vector (NAV) in each station
– NAV is a timer that determines if station can contend for RF medium
– NAV >0 --->wait for count down to NAV=0
– NAV=0 --->use CCA to check for RF energy on medium
– IF NAV=0 and CCA > threshold --->station resets NAV>0 and waits
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Network
Access Methods
REFERENCES
• Reserving Time for Data Transmission Using
Distributed Coordination Function (DCF)
– Employs a contention period for devices competing to
send data on the wireless network
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CSMA/CA Request
to Send/Clear to Send
REFERENCES
• Request to Send/Clear to Send (RTS/CTS) protocol:
Option used to solve hidden node problem
– Significant overhead upon the WLAN with transmission of
RTS and CTS frames
• Especially with short data packets
– RTS threshold: Only packets that longer than RTS
threshold transmitted using RTS/CTS
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IEEE 802.11 -Half
Duplex Communication
REFERENCES
• Effects of Half Duplex on Wireless Throughput
– Half Duplex: two way communication that occurs in only
one direction at a time
• Effective halves the max bit rate
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Telecommunication
REFERENCESChannel
• Channel - a path along which information in the
form of an electrical signal passes.
• Usually a range of contiguous frequencies involved
in supporting information transmission
Amplitude
Center
Channel Frequency
Bandwidth
Frequency
Channel
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RF Bands REFERENCES
for Wireless Networks
• ISM- Industrial Scientific and Medical – Three Bands
– 900 MHz band
– 2.4 GHz band
– 5 GHz Band
• UNII- Unlicensed National Information Infrastructure
–
–
–
–
–
5 GHz band
UNII-1 (Lower)
UNII-2 (middle)
UNII2 Extended
UNII-3 (Upper)
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DSSS USA
Channel Allocation
REFERENCES
• 14 Channels available
• 11 Channels in the United States
Amplitude
Channels
1
2
3
4
5
6
7
8
9
10 11
Freq.
2.401 GHz
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DSSS 3 Non-overlap
REFERENCESChannels
Amplitude
Ch 1
(2.412 GHz)
Ch 6
(2.437GHz)
Freq.
22 MHz
2.401 GHz
2401 MHz
Ch 11
(2.462 GHz)
3MHz
2.473 GHz
2423 MHz
2426 MHz
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5 GHz REFERENCES
Band and Channels
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Introduction
to Spread Spectrum
REFERENCES
• Spread Spectrum – a telecommunications technique
in which a signal is transmitted in a bandwidth
considerably greater than the frequency content of
the original information.
Narrowband
Amplitude
Wideband
Frequency
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Narrow Band and Spread
Spectrum Communications
REFERENCES
• Narrowband Vs. Spread Spectrum Communication
– Narrowband and Spread Spectrum are two examples of
how devices can communicate using radio frequency
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4-Types
Spread Spectrum
REFERENCES
•
•
•
•
•
Time Hopping, (THSS)
Frequency Hopping, (FHSS)
Direct Sequence Spread Spectrum, (DSSS)
Hybrid, DSSS/FHSS
Original IEEE 802.11 wireless LAN standard:
– Frequency-hopping spread spectrum (FHSS)
– Direct-sequence spread spectrum (DSSS)
• High Rate/ Direct-Sequence Spread Spectrum
(HR/DSSS)
• DSSS and HR/DSSS Channels
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Uses of
Spread Spectrum
REFERENCES
• Military - For low probability of interception of
telecommunications.
• Civil/Military - Range and positioning
measurements. GPS – satellites.
• Civil Cellular Telephony.
• Civil Wireless Networks – 802.11 and Bluetooth.
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Frequency
Hopping Patent
REFERENCES
• Hedy Lamarr and composer George Antheil, patent
number 2,292,387 , circa 1942
• A Hollywood cocktail party with Navy officers
present
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Frequency Hopping
Spread Spectrum (FHSS)
REFERENCES
• FHSS - Acronym for frequency-hopping spread
spectrum. 802.11, Bluetooth, & HomeRF.
Amp.
1
3
2
4
Freq.
Channel
Wide Band
Frequency Hop Sequence: 1, 3, 2, 4
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Frequency Hopping Spread
Spectrum – Simplistic View
REFERENCES
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FHSS System
Block Diagram
REFERENCES
FHSS
Data
Buffer
Antenna
1 3 2 4
Mixer
Mod
Carrier
Frequency
Sequence
Generator
1
3
2
4
Frequency Synthesizer
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Direct Sequence
REFERENCES
Spread Spectrum (DSSS)
Amp.
Signal
1
1
3
2
4
Freq.
Channel
DSSS Band
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DSSS System
Block Diagram
REFERENCES
Carrier
Frequency
DSSS
Antenna
Mixer
Carrier
Generator
Mod
Pseudo – 11-bit Barker Code
Noise
Encoder
Chipping Code
Generator
10110111000
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Data
Buffer
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Comparing
FHSS & DSSS
REFERENCES
Frequency Hopping
Direct Sequence
Spread Spectrum, FHSS Spread Spectrum, DSSS
802.11
802.11b
Dwell Time
400 mS
Lower
Throughput (2
or 3 Mbps)
Better
Immunity to
Interference
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Higher Cost
No
Dwell Time
Lower Cost
Higher
Lower
Higher
Throughput (11
Interoperability
Interoperability
Mbps)
More User
Density (79)
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Poorer
Immunity to
Interference
Less User
Density (3)
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Orthogonal Frequency
Division Multiplexing (OFDM)
REFERENCES
• Frequency division multiplexing (FDM) is a
technology that transmits multiple signals
simultaneously over a single transmission path, such
as a cable or wireless system.
• Orthogonal means to establish right angle
relationships between frequencies
• OFDM spread spectrum technique distributes the
data over a large number of carriers that are spaced
apart at precise frequencies and null out of channel
f2
sidebands
f3
f1
f4
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OFDM
REFERENCES
Features
• Used in IEEE 802.11a (OFDM) and IEEE 802.11g (ERP-OFDM)
and IEEE 802.11n (HT-OFDM) amendments
• Allows for much higher data rate transfers than DSSS and
HR/DSSS
• Up to 54 Mbps for OFDM, ERP-OFDM and 300-600 Mbps for
HT-OFDM
• OFDM functions in either the 2.4 GHz ISM or the 5 GHz UNII
bands
• The channel width is smaller than DSSS or HR/DSSS
• The width of an OFDM channel is only 20 MHz compared to
22 MHz for DSSS
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Orthogonal Frequency
Division Multiplexing (OFDM)
REFERENCES
• OFDM operates in either the 2.4 GHz ISM or the 5
GHz UNII bands
• The width of an OFDM channel is only 20 MHz
compared to 22 MHz for DSSS or HR/DSSS
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Multiple Input/Multiple
Output (MIMO) Channels
REFERENCES
• MIMO networks can operate in both the 2.4 GHz
ISM and 5 GHz UNII bands
• Capable of either 20 or 40 MHz–wide channels
• Wider channels mean more data can be transmitted
over the RF medium simultaneously
• In the 2.4 GHz ISM band, there is only one 40 MHz–
wide channel without any adjacent-channel overlap
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Multiple Input/Multiple
Output (MIMO)
REFERENCES
• Used by IEEE 802.11n devices
• Wider channels mean more data
can be transmitted over the RF
medium simultaneously
• In the 2.4 GHz ISM band, there
is only one 40 MHz–wide
channel without any adjacentchannel overlap
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MIMO Throughput
and Features
REFERENCES
•
•
•
•
•
Allows for data rates up to 600 Mbps
Current data rates up to 450 Mbps
More throughput, reliable, predictable
Lower latency for mobile communications
More consistent coverage and throughput for
mobile applications
• MIMO networks can operate in both the 2.4 GHz
ISM and 5 GHz UNII bands
• Capable of either 20 MHz or 40 MHz–wide channels
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IEEE REFERENCES
802.11n Features
• Uses three modes of
OFDM
– 20MHz and 40 MHz bands
– Data rates up to 600 Mbps
• Non-HT mode
– OFDM
– Backward compatibility to a, b, g
• HT mixed mode
– Supports OFDM and ERP-OFDM
• Greenfield mode
– Only ERP-OFDM
– Highest data rates
• Channel Bonding
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Co-Location
WLAN systems
REFERENCES
• Co-location of IEEE 802.11b HR/DSSS and IEEE
802.11a/g/n OFDM Systems
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Co-Location
WLAN systems
REFERENCES
• Adjacent-channel and Co-channel Interference
– Adjacent-channel and co-channel interference
• WLAN/WPAN Coexistence
– IEEE 802.11 wireless LANs can be affected when colocated with WPAN devices
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Encoding
and Modulation
REFERENCES
• Encoding - To change or translate one bit stream into
another.
• Modulation – Appling information on a carrier signal
by varying one or more of the signal's basic
characteristics - frequency, amplitude and phase.
DBPSK (Differential Binary Phase Shift Keying)
DQPSK (Differential Quaternary PSK)
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Modulation
REFERENCES
• Carrier signal is a continuous electrical signal
– Carries no information
• Three types of modulations enable carrier signals to
carry information
– Height of signal
– Frequency of signal
– Relative starting point
• Modulation can be done on analog or digital
transmissions
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Analog vs.REFERENCES
Digital Transmissions
Analog Signal = A signal that has continuously varying voltages, frequencies, or
phases. All amplitude values are present from minimum to maximum signal levels.
Digital Signal = A signal in which information is carried in a limited number of
different discrete states or levels; High/Low, One/Zero, 1/0
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Analog and
Digital Modulation
REFERENCES
• Analog Transmission use analog carrier signals and
analog modulation.
• Digital Transmission use analog carrier signals and
digital modulation.
• Modem (MOdulator/DEModulator): Used when
digital signals must be transmitted over analog
medium
– On originating end, converts distinct digital signals into
continuous analog signal for transmission
– On receiving end, reverse process performed
• WLANs use digital modulation of analog signals
(carrier signal)
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Frequency
and Period
REFERENCES
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Analog
Modulation
REFERENCES
• Amplitude: Height of carrier wave
• Amplitude modulation (AM): Changes amplitude so
that highest peaks of carrier wave represent 1 bit
while lower waves represent 0 bit
• Frequency modulation (FM): Changes number of
waves representing one cycle
– Number of waves to represent 1 bit more than number of
waves to represent 0 bit
• Phase modulation (PM): Changes starting point of
cycle
– When bits change from 1 to 0 bit or vice versa
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Analog
Modulation
REFERENCES
Amplitude modulation (AM) – Carrier frequency varies in amplitude
Frequency modulation (FM) – Carrier frequency varies in frequency
Phase modulation (PM) – Carrier varies in phase
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Digital
Modulation
REFERENCES
• Advantages over analog modulation:
–
–
–
–
Better use of bandwidth
Requires less power
Better handling of interference from other signals
Error-correcting techniques more compatible with other
digital systems
• Unlike analog modulation, changes occur in discrete
steps using binary signals
– Uses same three basic types of modulation as
Amplitude shift keying (ASK)
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Frequency vs. Phase
REFERENCES
Shift Key Modulation
Frequency shift keying (FSK)
Phase shift keying (PSK)
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Throughput
vs. Data Rate
REFERENCES
•
•
•
•
Data Rate = Total Data Rate through system
Throughput = Data Payload Rate
Data Rate = Data Payload Rate + Overhead
Overhead = Coding + Modulation+ Bandwidth +
Hardware + Software + Retransmission(errors)
5 Mbps Throughput
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Analog vs.
REFERENCES
Digital Bandwidth
• Analog Bandwidth – Frequency in Khz,Mhz (1 Mhz)
• Digital Bandwidth – bits per second (11 Mbps)
• Wireless Bandwidth – Frequency Space made
available to network devices (22 Mhz)
Bandwidth
Amplitude
Digital Bandwidth
(Average Bit Rate)
Frequency
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Quadrature phase
shift keying (QPSK)
REFERENCES
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16-QAM
Modulation
REFERENCES
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64-QAM - 64-level Quadrature
Amplitude Modulation
REFERENCES
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Spread Spectrum
REFERENCES
Comparisons
PHY
Data Rates
Frequency
Band
Standards
Max Colocated
WLANs
FHSS
1 or 2 Mbps
2.4 GHz ISM
IEEE 802.11
1997
79 max,
12 practical
DSSS
1 or 2 Mbps
2.4 GHz ISM
IEEE 802.11
1997
2 or 3
6 Mbps
HR/
DSSS
1, 2, 5.5, or
11 Mbps
2.4 GHz ISM
IEEE802.11b
1999
3
33 Mbps
ERP
1-54 Mbps
2.4 GHz ISM
IEEE 802.11g
2003
3
162 Mbps
OFDM
6-54 Mbps
5 GHz U-NII
IEEE 802.11a
1999
23
648 Mbps
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Max Total
Service Area
Data Rate
24 Mbps
practical
50
Data Rates and
Throughput Estimates
REFERENCES
PHY
Standards
Data Rate
Throughput
FHSS
IEEE 802.11-1997
1–2 Mbps
0.7–1 Mbps
DSSS
IEEE 802.11-1997
1–2 Mbps
0.7–1 Mbps
HR/DSSS
IEEE 802.11b-1999
1, 2, 5.5, and 11 Mbps
3–6 Mbps
ERP
IEEE 802.11g-2003
1, 2, 5.5, 11, 6, 9, 12, 18, 24, 36,
48, 54 Mbps
3–29 Mbps
OFDM
IEEE 802.11a-1999
6, 9, 12, 18, 24, 36, 48, 54 Mbps
3–29 Mbps
HT
IEEE 802.11n-2009
1–600 Mbps (with 4 spatial
streams)
~ 100 Mbps
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Review
REFERENCES
• Access methods
• WLANs have no way of detecting collisions, so they
use CSMA/CA
• Wireless LANs use half-duplex communication
• Physical Layer Specifications
• DSSS and HR/DSSS channels
• OFDM
• MIMO
• WLAN Co-location
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