CWNA Guide to Wireless
LANs, Second Edition
Chapter Four
IEEE 802.11 Physical Layer Standards
Objectives
• List and describe the wireless modulation schemes
used in IEEE WLANs
• Tell the difference between frequency hopping
spread spectrum and direct sequence spread
spectrum
• Explain how orthogonal frequency division
multiplexing is used to increase network throughput
• List the characteristics of the Physical layer
standards in 802.11b, 802.11g, and 802.11a
networks
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Introduction
Figure 4-2: OSI data flow
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Introduction (continued)
Table 4-1: OSI layers and functions
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Telecommunication Channel
•
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
Channel
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Frequency
5
Narrow and Wide Band
• Narrow and Wide Band – a relative comparison of
a group or range of frequencies used in a
telecommunications system. Narrow Band would
describe a small range of frequencies as compared
to a larger Wide Band range.
Amplitude
NB
WB
Frequency
Freq. Range
fL
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fH
6
Noise Floor
• Noise –A disturbance, especially a random and
persistent disturbance, that obscures or reduces
the clarity of a signal. Anything you don’t want.
Amplitude
Channel
Signal
Noise Floor
Thermal
Shot
Freq.
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Introduction to Spread Spectrum
• Spread Spectrum – a telecommunications
technique in which a signal is transmitted in a
bandwidth considerably greater than the frequency
content of the original information.
Amplitude
Narrowband
Wideband
Frequency
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Wireless Modulation Schemes
• Four primary wireless modulation schemes:
–
–
–
–
Narrowband transmission
Frequency hopping spread spectrum
Direct sequence spread spectrum
Orthogonal frequency division multiplexing
• Narrowband transmission used primarily by radio
stations
• Other three used in IEEE 802.11 WLANs
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Uses of Spread Spectrum
• 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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Narrowband Transmission
• Radio signals by nature transmit on only one radio
frequency or a narrow portion of frequencies
• Require more power for the signal to be transmitted
– Signal must exceed noise level
• Total amount of outside interference
• Vulnerable to interference from another radio signal
at or near same frequency
• IEEE 802.11 standards do not use narrowband
transmissions
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Narrowband Transmission (continued)
Figure 4-3: Narrowband transmission
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Spread Spectrum Transmission
Figure 4-4: Spread spectrum transmission
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Spread Spectrum Transmission
(continued)
• Advantages over narrowband:
–
–
–
–
–
–
–
Resistance to narrowband interference
Resistance to spread spectrum interference
Lower power requirements
Less interference on other systems
More information transmitted
Increased security
Resistance to multipath distortion
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Frequency Hopping Spread Spectrum
(FHSS)
• Uses range of frequencies
– Change during transmission
• Hopping code: Sequence of changing frequencies
– If interference encountered on particular frequency
then that part of signal will be retransmitted on next
frequency of hopping code
• FCC has established restrictions on FHSS to
reduce interference
• Due to speed limitations FHSS not widely
implemented in today’s WLAN systems
– Bluetooth does use FHSS
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Frequency Hopping Spread Spectrum
(continued)
Figure 4-6: FHSS error correction
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FHSS
• FHSS - Acronym for frequency-hopping spread
spectrum. Bluetooth & HomeRF.
Amp.
1
Channel
3
2
4
Freq.
Wide Band
Frequency Hop Sequence: 1, 3, 2, 4
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FHSS Timing
Amplitude
Hop
Time
Time
Data
Dwell
Time
Hop
Sequence
1
2
3
Channels
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Frequency
18
FHSS System Block Diagram
FHSS
Data
Buffer
Antenna
1 3 2 4
Mixer
Carrier
Frequency
Sequence
Generator
1
3
2
4
Frequency Synthesizer
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FHSS Channel Allocation
2.403 GHz
2.402 GHz
CH
2
Amplitude
CH
3
1 MHz
2.401.5 GHz
2.402.5 GHz
2.480 GHz
2.479 GHz
CH
79
CH
80
1 MHz
2.401.5 GHz
2.402.5 GHz
Freq.
2.400 GHz
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2.4835 GHz
20
FCC Rules for FHSS
• Prior to 8-31-00
–
–
–
–
Use 75 of the 79 channels
Output Powermax = 1 Watt
Bandwidthmax = 1 MHz
Data Ratemax = 2 Mbps
• After 8-31-00
–
–
–
–
Only 15 of the 79 channels required
Output Powermax = 125 mW
Bandwidthmax = 5 MHz
Data Ratemax = 10 Mbps
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Direct Sequence Spread Spectrum
(DSSS)
• Uses expanded redundant code to transmit data
bits
• Chipping code: Bit pattern substituted for original
transmission bits
– Advantages of using DSSS with a chipping code:
• Error correction
• Less interference on other systems
• Shared frequency bandwidth
– Co-location: Each device assigned unique
chipping code
• Security
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Direct Sequence Spread Spectrum
(continued)
Figure 4-7: Direct sequence spread spectrum (DSSS) transmission
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DSSS
• DSSS - Acronym for direct-sequence spread
spectrum. WLAN, 802.11.
Amp.
Signal
1
1
Channel
3
2
4
Freq.
DSSS Band
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DSSS Channel Allocation
Amplitude
Channels
1
2
3
4
5
6
7
8
9
10 11
Freq.
2.401 GHz
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2.473 GHz
25
DSSS 3 Non-overlap Channels
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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DSSS System Block Diagram
Carrier
Frequency
Carrier
Generator
DSSS
Mixer
Pseudo – 11-bit Barker Code
Noise
Modulator
Chipping Code
Generator
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Antenna
Data
Buffer
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Comparing FHSS and DSSS
Frequency Hopping
Spread Spectrum, FHSS
Direct Sequence
Spread Spectrum, DSSS
Dwell Time
400 mS
Higher Cost
No
Dwell Time
Lower Cost
Lower
Throughput (2 or
3 Mbps)
Lower
Interoperability
Higher
Throughput (11
Mbps)
Higher
Interoperability
Better NB
Immunity to
Interference
More User
Density (79)
Poorer NB
Immunity to
Interference
Less User
Density (3)
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Orthogonal Frequency Division
Multiplexing (OFDM)
• With multipath distortion, receiving device must
wait until all reflections received before transmitting
– Puts ceiling limit on overall speed of WLAN
• OFDM: Send multiple signals at same time
– Split high-speed digital signal into several slower
signals running in parallel
• OFDM increases throughput by sending data more
slowly
• Avoids problems caused by multipath distortion
• Used in 802.11a networks
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Orthogonal Frequency Division
Multiplexing (continued)
Figure 4-8: Multiple channels
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Orthogonal Frequency Division
Multiplexing (continued)
Figure 4-9: Orthogonal frequency division multiplexing (OFDM)
vs. single-channel transmissions
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Comparison of Wireless Modulation
Schemes
• FHSS transmissions less prone to interference
from outside signals than DSSS
• WLAN systems that use FHSS have potential for
higher number of co-location units than DSSS
• DSSS has potential for greater transmission
speeds over FHSS
• Throughput much greater for DSSS than FHSS
– Amount of data a channel can send and receive
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Comparison of Wireless Modulation
Schemes (continued)
• DSSS preferred over FHSS for 802.11b WLANs
• OFDM is currently most popular modulation
scheme
– High throughput
– Supports speeds over 100 Mbps for 802.11a WLANs
– Supports speeds over 54 Mbps for 802.11g WLANs
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IEEE 802.11 Physical Layer Standards
• IEEE wireless standards follow OSI model, with
some modifications
• Data Link layer divided into two sublayers:
– Logical Link Control (LLC) sublayer: Provides
common interface, reliability, and flow control
– Media Access Control (MAC) sublayer: Appends
physical addresses to frames
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IEEE 802.11 Physical Layer Standards
(continued)
• Physical layer divided into two sublayers:
– Physical Medium Dependent (PMD) sublayer:
Makes up standards for characteristics of wireless
medium (such as DSSS or FHSS) and defines
method for transmitting and receiving data
– Physical Layer Convergence Procedure (PLCP)
sublayer: Performs two basic functions
• Reformats data received from MAC layer into frame
that PMD sublayer can transmit
• “Listens” to determine when data can be sent
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IEEE 802.11 Physical Layer Standards
(continued)
Figure 4-10: Data Link sublayers
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IEEE 802.11 Physical Layer Standards
(continued)
Figure 4-11: PHY sublayers
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IEEE 802.11 Physical Layer Standards
(continued)
Figure 4-12: PLCP sublayer reformats MAC data
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IEEE 802.11 Physical Layer Standards
(continued)
Figure 4-13: IEEE LANs share the same LLC
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Legacy WLANs
• Two “obsolete” WLAN standards:
– Original IEEE 802.11: FHSS or DSSS could be used
for RF transmissions
• But not both on same WLAN
– HomeRF: Based on Shared Wireless Access
Protocol (SWAP)
• Defines set of specifications for wireless data and
voice communications around the home
• Slow
• Never gained popularity
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IEEE 802.11b Physical Layer
Standards
• Physical Layer Convergence Procedure
Standards: Based on DSSS
– PLCP must reformat data received from MAC layer
into a frame that the PMD sublayer can transmit
Figure 4-14: 802.11b PLCP frame
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IEEE 802.11b Physical Layer
Standards (continued)
• PLCP frame made up of three parts:
– Preamble: prepares receiving device for rest of
frame
– Header: Provides information about frame
– Data: Info being transmitted
•
•
•
•
•
•
•
Synchronization field
Start frame delimiter field
Signal data rate field
Service field
Length field
Header error check field
Data field
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IEEE 802.11b Physical Layer
Standards (continued)
• Physical Medium Dependent Standards: PMD
translates binary 1’s and 0’s of frame into radio
signals for transmission
– Can transmit at 11, 5.5, 2, or 1 Mbps
– 802.11b uses ISM band
• 14 frequencies can be used
– Two types of modulation can be used
• Differential binary phase shift keying (DBPSK): For
transmissions at 1 Mbps
• Differential quadrature phase shift keying
(DQPSK): For transmissions at 2, 5.5, and 11 Mbps
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IEEE 802.11b Physical Layer
Standards (continued)
Table 4-2: 802.11b ISM channels
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IEEE 802.11b Physical Layer
Standards (continued)
Table 4-3: IEEE 802.11b Physical layer standards
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IEEE 802.11a Physical Layer
Standards
• IEEE 802.11a achieves increase in speed and
flexibility over 802.11b primarily through OFDM
– Use higher frequency
– Accesses more transmission channels
– More efficient error-correction scheme
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U-NII Frequency Band
Table 4-4: ISM and U-NII WLAN characteristics
Table 4-5: U-NII characteristics
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U-NII Frequency Band (continued)
• Total bandwidth available for IEEE 802.11a WLANs
using U-NII is almost four times that available for
802.11b networks using ISM band
• Disadvantages:
– In some countries outside U.S., 5 GHz bands
allocated to users and technologies other than
WLANs
– Interference from other devices is growing
• Interference from other devices one of primary
sources of problems for 802.11b and 802.11a
WLANs
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Channel Allocation
Figure 4-16: 802.11a channels
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Channel Allocation (continued)
Figure 4-17: 802.11b vs. 802.11a channel coverage
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Co-location
• FHSS has many more frequencies / channels
then DSSS which only has 3 co-location channels.
• However 3 DSSS access points co-located at 11
Mbps each would result in a maximum throughput
of 33 Mbps. It would require 16 access points colocated for FHSS to achieve a throughput of 32
Mbps.
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Co-location Comparison
40
3 Mbps FHSS
(sync)
11 Mbps DSSS
30
3 Mbps FHSS
(no sync)
20
10
1
5
10
15
20
Number of Co-located Systems
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Error Correction
• 802.11a has fewer errors than 802.11b
– Transmissions sent over parallel subchannels
– Interference tends to only affect one subchannel
• Forward Error Correction (FEC): Transmits
secondary copy along with primary information
– 4 of 52 channels used for FEC
– Secondary copy used to recover lost data
• Reduces need for retransmission
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Physical Layer Standards
• PLCP for 802.11a based on OFDM
• Three basic frame components: Preamble, header,
and data
Figure 4-18: 802.11a PLCP frame
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Physical Layer Standards (continued)
Table 4-6: 802.11a Rate field values
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Physical Layer Standards (continued)
• Modulation techniques used to encode 802.11a
data vary depending upon speed
• Speeds higher than 54 Mbps may be achieved
using 2X modes
Table 4-7: 802.11a characteristics
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Physical Layer Standards (continued)
Figure 4-19: Phase shift keying (PSK)
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Physical Layer Standards (continued)
Figure 4-20: Quadrature phase shift keying (QPSK)
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Physical Layer Standards (continued)
Figure 4-21: 16-level quadrature amplitude modulation (16-QAM)
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Physical Layer Standards (continued)
Figure 4-22: 64-level quadrature amplitude modulation (64-QAM)
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IEEE 802.11g Physical Layer
Standards
• 802.11g combines best features of 802.11a and
802.11b
• Operates entirely in 2.4 GHz ISM frequency
• Two mandatory modes and one optional mode
– CCK mode used at 11 and 5.5 Mbps (mandatory)
– OFDM used at 54 Mbps (mandatory)
– PBCC-22 (Packet Binary Convolution Coding):
Optional mode
• Can transmit between 6 and 54 Mbps
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IEEE 802.11g Physical Layer
Standards (continued)
Table 4-8: IEEE 802.11g Physical layer standards
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IEEE 802.11g Physical Layer
Standards (continued)
• Characteristics of 802.11g standard:
–
–
–
–
–
Greater throughput than 802.11b networks
Covers broader area than 802.11a networks
Backward compatible
Only three channels
If 802.11b and 802.11g devices transmitting in same
environment, 802.11g devices drop to 11 Mbps
speeds
– Vendors can implement proprietary higher speed
• Channel bonding and Dynamic turbo
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Summary
• Three modulation schemes are used in IEEE
802.11 wireless LANs: frequency hopping spread
spectrum (FHSS), direct sequence spread
spectrum (DSSS), and orthogonal frequency
division multiplexing (OFDM)
• Spread spectrum is a technique that takes a
narrow, weaker signal and spreads it over a
broader portion of the radio frequency band
• Spread spectrum transmission uses two different
methods to spread the signal over a wider area:
FHSS and DSSS
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Summary (continued)
• OFDM splits a single high-speed digital signal into
several slower signals running in parallel
• IEEE has divided the OSI model Data Link layer
into two sublayers: the LLC and MAC sublayers
• The Physical layer is subdivided into the PMD
sublayer and the PLCP sublayer
• The Physical Layer Convergence Procedure
Standards (PLCP) for 802.11b are based on DSSS
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Summary (continued)
• IEEE 802.11a networks operate at speeds up to 54
Mbps with an optional 108 Mbps
• The 802.11g standard specifies that it operates
entirely in the 2.4 GHz ISM frequency and not the
U-NII band used by 802.11a
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