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CWNA Guide to Wireless LANs, Second Edition Chapter Four IEEE 802.11 Physical Layer Standards Modified. 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

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CWNA Guide to Wireless LANs, Second EditionChapter FourIEEE 802.11 Physical Layer StandardsModified

objectives
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

introduction
Introduction

Figure 4-2: OSI data flow

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

introduction continued
Introduction (continued)

Table 4-1: OSI layers and functions

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

wireless modulation schemes
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

narrowband transmission
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

spread spectrum transmission
Spread Spectrum Transmission
  • 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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

frequency hopping spread spectrum fhss
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

Dwell Time (ms)

Hop Time (us)

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

direct sequence spread spectrum dsss
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

orthogonal frequency division multiplexing ofdm
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

comparison of wireless modulation schemes
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
  • Throughputmuch greater for DSSS than FHSS
    • Amount of data a channel can send and receive 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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11 physical layer standards
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
  • 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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11 physical layer standards13
IEEE 802.11 Physical Layer Standards

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11 physical layer standards continued
IEEE 802.11 Physical Layer Standards (continued)

Figure 4-12: PLCP sublayer reformats MAC data

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11 physical layer standards15
IEEE 802.11 Physical Layer Standards

Figure 4-13: IEEE LANs share the same LLC

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

legacy wlans
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11b physical layer standards
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11b physical layer standards continued
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11b physical layer standards continued19
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
      • Differentialbinary phase shift keying (DBPSK): For transmissions at 1 Mbps
      • Differential quadrature phase shift keying (DQPSK): For transmissions at 2, 5.5, and 11 Mbps

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11b physical layer channels
IEEE 802.11b Physical Layer Channels

Table 4-2: 802.11b ISM channels

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11b modulation
IEEE 802.11b Modulation

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11a physical layer standards
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

u nii frequency band
U-NII Frequency Band

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

u nii frequency band continued
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

channel allocation
Channel Allocation

Figure 4-16: 802.11a channels

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

channel allocation continued
Channel Allocation (continued)

Figure 4-17: 802.11b vs. 802.11a channel coverage

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

error correction
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

physical layer standards
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

physical layer standards continued
Physical Layer Standards (continued)

Table 4-6: 802.11a Rate field values

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

802 11a characteristics
802.11a characteristics
  • Modulation techniques used to encode 802.11a data vary depending upon speed
  • Speeds higher than 54 Mbps may be achieved using 2X modes

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

digital modulation
Digital Modulation

PSK Phase Domain

– One Phase per Baud

Phase shift keying (PSK) Time Domain

PSK Phase Domain –

Two Phases per Baud

= QPSK

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

quadrature phase shift keying qpsk
Quadrature phase shift keying (QPSK)

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

sine waves for phase modulation
Sine Waves for Phase Modulation

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

16 level quadrature amplitude modulation 16 qam
16-level Quadrature Amplitude Modulation (16-QAM)

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

64 level quadrature amplitude modulation 64 qam
64-level Quadrature Amplitude Modulation (64-QAM)

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11g physical layer standards
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11g physical layer standards37
IEEE 802.11g Physical Layer Standards

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

ieee 802 11g physical layer standards continued
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

summary
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

summary continued
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

summary continued41
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

CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

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