CWNA Guide to Wireless LANs, Second Edition

1
CWNA Guide to Wireless LANs, Second Edition

• Chapter Four
• IEEE 802.11 Physical Layer Standards

2
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

3
Introduction
Figure 4-2 OSI data flow
4
Introduction (continued)
Table 4-1 OSI layers and functions
5
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.

Center Channel Frequency
Amplitude
Bandwidth
Frequency
Channel
6
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
fH
7
Noise Floor

• Noise A disturbance, especially a random and
  persistent disturbance, that obscures or reduces
  the clarity of a signal. Anything you dont want.

Amplitude
Channel
Signal
Noise Floor
Shot
Thermal
Freq.
8
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.

Narrowband
Amplitude
Wideband
Frequency
9
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

10
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.

11
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

12
Narrowband Transmission (continued)
Figure 4-3 Narrowband transmission
13
Spread Spectrum Transmission
Figure 4-4 Spread spectrum transmission
14
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

15
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 todays WLAN systems
• Bluetooth does use FHSS

16
Frequency Hopping Spread Spectrum (continued)
Figure 4-6 FHSS error correction
17
FHSS

• FHSS - Acronym for frequency-hopping spread
  spectrum. Bluetooth HomeRF.

Amp.
1
2
3
4
Freq.
Channel
Wide Band
Frequency Hop Sequence 1, 3, 2, 4
18
FHSS Timing
Time
Data
Amplitude
Hop Time
Dwell Time
Hop Sequence
1
2
3
4
Channels
Frequency
19
FHSS System Block Diagram
Antenna
FHSS
Data Buffer
1
2
3
4
Mixer
Carrier Frequency
Sequence Generator
1
2
3
4
Frequency Synthesizer
20
FHSS Channel Allocation
2.480 GHz
2.403 GHz
2.479 GHz
2.402 GHz
CH 79
CH 80
CH 2
CH 3
Amplitude
1 MHz
1 MHz
2.401.5 GHz
2.401.5 GHz
2.402.5 GHz
2.402.5 GHz
Freq.
2.4835 GHz
2.400 GHz
21
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

22
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

23
Direct Sequence Spread Spectrum (continued)
Figure 4-7 Direct sequence spread spectrum
(DSSS) transmission
24
DSSS

• DSSS - Acronym for direct-sequence spread
  spectrum. WLAN, 802.11.

Amp.
Signal
1
1
2
3
4
Freq.
Channel
DSSS Band
25
DSSS Channel Allocation
Amplitude
Channels
1
2
3
4
5
6
7
8
9
10
11
Freq.
2.401 GHz
2.473 GHz
26
DSSS 3 Non-overlap Channels
Amplitude
Ch 1
Ch 6
Ch 11
(2.412 GHz)
(2.437GHz)
(2.462 GHz)
Freq.
22 MHz
2.473 GHz
3MHz
2.401 GHz
2401 MHz
2423 MHz
2426 MHz
27
DSSS System Block Diagram
Carrier Frequency
Antenna
DSSS
Mixer
Carrier Generator
11-bit Barker Code
Pseudo Noise Generator
Data Buffer
Modulator
Chipping Code
28
Comparing FHSS and DSSS
Frequency Hopping Spread Spectrum, FHSS Frequency Hopping Spread Spectrum, FHSS Direct Sequence Spread Spectrum, DSSS 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)
29
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

30
Orthogonal Frequency Division Multiplexing
(continued)
Figure 4-8 Multiple channels
31
Orthogonal Frequency Division Multiplexing
(continued)
Figure 4-9 Orthogonal frequency division
multiplexing (OFDM) vs. single-channel
transmissions
32
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

33
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

34
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

35
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

36
IEEE 802.11 Physical Layer Standards (continued)
Figure 4-10 Data Link sublayers
37
IEEE 802.11 Physical Layer Standards (continued)
Figure 4-11 PHY sublayers
38
IEEE 802.11 Physical Layer Standards (continued)
Figure 4-12 PLCP sublayer reformats MAC data
39
IEEE 802.11 Physical Layer Standards (continued)
Figure 4-13 IEEE LANs share the same LLC
40
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

41
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
42
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

43
IEEE 802.11b Physical Layer Standards (continued)

• Physical Medium Dependent Standards PMD
  translates binary 1s and 0s 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

44
IEEE 802.11b Physical Layer Standards (continued)
Table 4-2 802.11b ISM channels
45
IEEE 802.11b Physical Layer Standards (continued)
Table 4-3 IEEE 802.11b Physical layer standards
46
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

47
U-NII Frequency Band
Table 4-4 ISM and U-NII WLAN characteristics
Table 4-5 U-NII characteristics
48
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

49
Channel Allocation
Figure 4-16 802.11a channels
50
Channel Allocation (continued)
Figure 4-17 802.11b vs. 802.11a channel coverage
51
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
  co-located for FHSS to achieve a throughput of 32
  Mbps.

52
Co-location Comparison
3 Mbps FHSS (sync)
40
11 Mbps DSSS
30
3 Mbps FHSS (no sync)
20
10
1
10
15
20
5
Number of Co-located Systems
53
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

54
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
55
Physical Layer Standards (continued)
Table 4-6 802.11a Rate field values
56
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
57
Physical Layer Standards (continued)
Figure 4-19 Phase shift keying (PSK)
58
Physical Layer Standards (continued)
Figure 4-20 Quadrature phase shift keying (QPSK)
59
Physical Layer Standards (continued)
Figure 4-21 16-level quadrature amplitude
modulation (16-QAM)
60
Physical Layer Standards (continued)
Figure 4-22 64-level quadrature amplitude
modulation (64-QAM)
61
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

62
IEEE 802.11g Physical Layer Standards (continued)
Table 4-8 IEEE 802.11g Physical layer standards
63
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

64
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

65
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

66
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