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
Introduction
OSI data flow
Introduction (continued)
OSI layers and functions
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
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
Narrowband Transmission (continued)
Narrowband transmission
Spread Spectrum Transmission
Spread spectrum transmission
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
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
Frequency Hopping Spread Spectrum
(continued)
FHSS error correction
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
Direct Sequence Spread Spectrum
(continued)
Direct sequence spread spectrum (DSSS) transmission
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
Orthogonal Frequency Division
Multiplexing (continued)
Multiple channels
Orthogonal Frequency Division
Multiplexing (continued)
Orthogonal frequency division multiplexing (OFDM) vs. singlechannel transmissions
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
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
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
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
IEEE 802.11 Physical Layer Standards
(continued)
Data Link sublayers
IEEE 802.11 Physical Layer Standards
(continued)
PLCP sublayer reformats MAC data
IEEE 802.11 Physical Layer Standards
(continued)
IEEE LANs share the same LLC
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
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
802.11b PLCP frame
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
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
IEEE 802.11b Physical Layer
Standards (continued)
802.11b ISM channels
IEEE 802.11b Physical Layer
Standards (continued)
IEEE 802.11b 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
U-NII Frequency Band
ISM and U-NII WLAN characteristics
U-NII characteristics
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
Channel Allocation
802.11a channels
Channel Allocation (continued)
802.11b vs. 802.11a channel coverage
Channel Setup
There are two critical steps for a good WLAN deployment:
1. Determine placement of access points or bridges –
• This includes determining where they should be placed and
deciding how many are required for the desired coverage.
• Very few gaps in the coverage should be left.
• These gaps are essentially dead air and the client will lack
connectivity in these locations.
• As discussed before, bandwidth requirements have an
impact on the coverage areas.
2. Map out the channel assignments –
• There should be as little overlap as possible between
channels that use the same frequency.
35
Multiple Overlapping Networks
Coverage Current Thinking:
Ch
1
2
3
4
5
6
7
8
9
10
11
Start Fqy Mid Fqy
End Fqy
2401
2412
2423
2406
2417
2428
2411
2422
2433
2416
2427
2438
2421
2432
2443
2426
2437
2448
2431
2442
2453
2436
2447
2458
2441
2452
2463
2446
2457
2468
2451
2462
2473
1.
Only three of the 11 channels
used by wireless hubs in the US
can be allocated simultaneously.
2.
Reason:
1.
In North America, the
802.11b spectrum ranges
form 2411 MHz to 2473
MHz, and is divided up into
11 channels. Channels are
spaced 5 MHz apart from the
center.
2.
However, each channel is 22
MHz wide, so there is a great
overlap
Channels 1, 6, and 11 are the safe channels to use.
36
Burton’s Analysis
An entire 22MHz is not simply swallowed up in a
rectangular pattern with power on the vertical axis and
frequency on the horizontal – instead it's more of a parabola,
centered around the midpoint of the frequency. Thus, as you
get further away from the center, the power drops off
substantially.
According to Burton's analysis, when three channels
separate 802.11b frequencies, there is only about 4% of
interference. This is the case between frequencies 1 and 4,
and 8 and 11. Between 4 and 8, the interference is
substantially less than 1%.
37
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
Physical Layer Standards
• PLCP for 802.11a based on OFDM
• Three basic frame components: Preamble, header,
and data
802.11a PLCP frame
Physical Layer Standards (continued)
802.11a Rate field values
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
802.11a characteristics
Physical Layer Standards (continued)
Phase shift keying (PSK)
Physical Layer Standards (continued)
Quadrature phase shift keying (QPSK)
Physical Layer Standards (continued)
Figure 4-21: 16-level quadrature amplitude modulation (16-QAM)
Physical Layer Standards (continued)
Figure 4-22: 64-level quadrature amplitude modulation (64-QAM)
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
IEEE 802.11g Physical Layer
Standards (continued)
Table 4-8: IEEE 802.11g Physical layer standards
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
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
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
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