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CWNA guide to wireless LANs 2nd ch04

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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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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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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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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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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