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Introduction to Wireless Networking
Module-5Physical Layer Access Methods and
Spread Spectrum
CCRI ENGINEERING AND TECHNOLOGY
Jerry Bernardini
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• 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.sss-mag.com/primer.html
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Chapter Objectives
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• 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
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• 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
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• 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
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• 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
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• 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
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• 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
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• 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
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• 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 Channel
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• 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
Bandwidth
Amplitude
FrequencyChannel
CenterChannel Frequency
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RF Bands for Wireless Networks
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• 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
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• 14 Channels available• 11 Channels in the United States
2.401 GHz 2.473 GHz
1 2 3 4 5 6 7 8 9 10 11
Amplitude
Freq.
Channels
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DSSS 3 Non-overlap Channels
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2.401 GHz 2.473 GHz
Ch 1 Ch 6 Ch 11(2.412 GHz) (2.437GHz) (2.462 GHz)
Amplitude
Freq.
2401 MHz
22 MHz
3MHz
2426 MHz2423 MHz
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5 GHz Band and Channels
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Introduction to Spread Spectrum
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• Spread Spectrum – a telecommunications technique in which a signal is transmitted in a bandwidth considerably greater than the frequency content of the original information.
Frequency
Amplitude Narrowband
Wideband
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Narrow Band and Spread Spectrum Communications
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• 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
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• 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
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• 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
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• 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)
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• FHSS - Acronym for frequency-hopping spread spectrum. 802.11, Bluetooth, & HomeRF.
Freq.
Amp.
1 2 3 4
Frequency Hop Sequence: 1, 3, 2, 4
Wide BandChannel
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Frequency Hopping Spread Spectrum – Simplistic View
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FHSS System Block Diagram
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Frequency Synthesizer
CarrierFrequency
DataBuffer
SequenceGenerator
MixerMod
AntennaFHSS
1 23 4
1 23 4
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Direct Sequence Spread Spectrum (DSSS)
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Freq.
Amp.
1 2 3 4
DSSS BandChannel
1 Signal
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DSSS System Block Diagram
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CarrierFrequency
DataBuffer
Pseudo –Noise
Generator
Antenna
CarrierGenerator
11-bit Barker Code
Mixer
Encoder
DSSS
Chipping Code
Mod
10110111000
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Comparing FHSS & DSSS
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Frequency HoppingSpread Spectrum,
FHSS802.11
Direct SequenceSpread Spectrum,
DSSS802.11b
Dwell Time400 mS Higher Cost No
Dwell Time Lower Cost
LowerThroughput
(2 or 3 Mbps)
LowerInteroperabili
ty
HigherThroughput (11 Mbps)
HigherInteroperabili
ty
Better Immunity to Interference
More User Density (79)
Poorer Immunity to Interference
Less User Density (3)
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Orthogonal Frequency Division Multiplexing (OFDM)
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• 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 sidebands
f1f3
f2
f4
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OFDM Features
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• 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)
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• 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
REFERENCES
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Multiple Input/Multiple Output (MIMO) Channels
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• 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)
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• 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 adjacent-channel overlap
REFERENCES
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MIMO Throughput and Features
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• 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
REFERENCES
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IEEE 802.11n Features
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• 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
REFERENCES
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Co-Location WLAN systems
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• Co-location of IEEE 802.11b HR/DSSS and IEEE 802.11a/g/n OFDM Systems
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Co-Location WLAN systems
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• Adjacent-channel and Co-channel Interference– Adjacent-channel and co-channel interference
• WLAN/WPAN Coexistence – IEEE 802.11 wireless LANs can be affected when co-
located with WPAN devices
REFERENCES
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Encoding and Modulation
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• 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
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• 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. Digital Transmissions
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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
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• 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
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Analog Modulation
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• 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
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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
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• 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)
REFERENCES
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Frequency vs. Phase Shift Key Modulation
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Frequency shift keying (FSK)
Phase shift keying (PSK)
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Throughput vs. Data Rate
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• 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 11 Mbps Data Rate 5 Mbps Throughput
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Analog vs. Digital Bandwidth
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• 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)
Frequency
Amplitude
Bandwidth
Digital Bandwidth (Average Bit Rate)
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Quadrature phase shift keying (QPSK)
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16-QAM Modulation
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64-QAM - 64-level Quadrature Amplitude Modulation
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REFERENCES
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Spread Spectrum Comparisons
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PHY Data Rates Frequency Band
Standards Max Colocated WLANs
Max Total Service Area
Data Rate
FHSS 1 or 2 Mbps 2.4 GHz ISM IEEE 802.11 1997
79 max,12 practical
24 Mbpspractical
DSSS 1 or 2 Mbps 2.4 GHz ISM IEEE 802.11 1997
2 or 3 6 Mbps
HR/DSSS
1, 2, 5.5, or11 Mbps
2.4 GHz ISM IEEE802.11b1999
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
REFERENCES
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Data Rates and Throughput Estimates
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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
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• 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