CWNA Guide to Wireless LANs, Second Edition
Chapter 4
IEEE 802.11 Physical Layer Standards
At a Glance
Instructor’s Manual Table of Contents

Overview

Objectives

Teaching Tips

Quick Quizzes

Class Discussion Topics

Additional Projects

Additional Resources

Key Terms
4-1
CWNA Guide to Wireless LANs, Second Edition
4-2
Lecture Notes
Overview
In this chapter and the next, the students will see how a popular conceptual model of
networking, the OSI model, relates to wireless LANs and the IEEE wireless LAN
standards.
In this chapter, the students will learn about IEEE 802.11 wireless LAN functions at the
lowest layer of the OSI reference model, the Physical layer. Because the Physical layer
primarily deals with turning packets into electrical impulses for transmission, students
begin by exploring the different wireless modulation schemes that are used. Then, the
students will examine each of the IEEE WLAN standards, 802.11b, 802.11a, and
802.11g, to see how the standards are implemented at the Physical layer.
Chapter 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
Teaching Tips
Introduction
1. Provide an overview/review of the OSI model. Illustrate with Figures 4-1 and 4-2.
Using Table 4-1 as a guide, discuss the purpose of each OSI model layer.
Wireless Modulation Schemes
1. Explain that there are four primary wireless modulation schemes: narrowband
transmission, frequency hopping spread spectrum, direct sequence spread spectrum, and
orthogonal frequency division multiplexing. Mention that the last three are used by
WLANS.
Narrowband Transmission
1. Briefly discuss the concept of narrowband transmission. Explain that narrowband
transmission is used primarily by radio stations. Illustrate with Figure 4-3. Stress that
this type of transmission is not included in the IEEE 802.11 standards.
CWNA Guide to Wireless LANs, Second Edition
Teaching
Tip
4-3
The reason why broadcast radio stations’ narrowband transmissions work
efficiently is because each station is allowed to transmit on only one frequency in
a geographical area. The Federal Communications Commission (FCC) regulates
those broadcast radio frequencies.
Spread Spectrum Transmission
1. Explain that spread spectrum is a technique that takes a narrow, weaker signal and
spreads it over a broader portion of the radio frequency band. Illustrate with Figure 4-4.
2. Using the list on pages 116 and 117 of the text as a guide, discuss the advantages of
spread spectrum over narrowband transmission.
3. Provide an introduction to the two spread spectrum methods used to spread a signal
over a wider area.
Teaching
Tip
Spread-spectrum and narrowband signals can occupy the same band, with little
or no interference.
Frequency Hopping Spread Spectrum (FHSS)
1. Discuss the history of FHSS.
2. Explain that frequency hopping uses a range of frequencies that change during the
transmission.
3. Using Figure 4-5 to illustrate, discuss how FHSS works. Define the terms hop time and
hopping code.
Teaching
Tip
Make sure the students understand that both the sending and receiving stations
must know the hopping code in order to correctly transmit and receive the
transmission.
4. Explain that if interference is encountered on a particular frequency then that part of the
signal will be retransmitted on the next frequency of the hopping code. Illustrate with
Figure 4-6. Mention that, because FHSS transmits short bursts over a wide range of
frequencies, the extent of any interference will be very small and can easily be corrected
by error checking.
5. Discuss the restrictions on FHSS established by the FCC at various frequencies.
6. Mention that due to speed limitations FHSS is not widely implemented in today’s
WLAN systems, but Bluetooth technology does use FHSS.
Direct Sequence Spread Spectrum (DSSS)
1. Explain that DSSS uses an expanded redundant code to transmit each data bit. Define
the term chipping code. Illustrate with Figure 4-7.
CWNA Guide to Wireless LANs, Second Edition
Teaching
Tip
4-4
The term “chipping code” is used because a single radio bit is commonly referred
to as a “chip.”
2. Explain how the bits in the signal are used with the chipping code to get the final signal
sent. Mention that the mathematical operation used in this process is XOR. Figure 4-7
can be used to illustrate this process. There are also two examples on pages 121 and 122
of the text that could be used to illustrate.
3. Using the list on page 122 of the text as a guide, discuss the advantages of using DSSS
with a chipping code.
4. Briefly discuss the FCC regulations on DSSS.
Orthogonal Frequency Division Multiplexing (OFDM)
1. Briefly review the concept of multipath distortion. Provide a description of the issue of
waiting for the entire transmission to arrive under these conditions. Explain how this
puts a “speed limit” on a WLAN.
2. Provide an introduction to OFDM. Stress that its primary role is to split a high-speed
digital signal into several slower signals running in parallel.
Teaching
Tip
OFDM is also the technology behind consumer-based Digital Subscriber Line
(DSL) service, which provides home Internet access over standard telephone
lines.
3. Explain how OFDM splits, sends, and recombines data. Illustrate with Figure 4-8.
4. Describe how OFDM avoids problems caused by multipath distortion by sending the
message slowly enough that any delayed copies (refracted or diffracted signals) are late
by a much smaller amount of time than a standard transmission. Illustrate with Figure 49.
5. Mention that OFDM is used in IEEE 802.11a networks.
Comparison of Wireless Modulation Schemes
1. Compare DSSS and FHSS. Explain that FHSS transmissions are less prone to
interference and WLAN systems that use FHSS have the potential for a higher number
of co-location units than DSSS.
2. Define the term throughput. Explain that DSSS supports much greater throughput than
FHSS. Mention that DSSS systems are preferred over FHSS for 802.11b WLANs.
3. Discuss why OFDM is currently the preferred modulation scheme due to its high
throughput.
CWNA Guide to Wireless LANs, Second Edition
Teaching
Tip
4-5
Packet size on average in FHSS systems is only one fifth that of a DSSS packet,
which means that packets will be fragmented more frequently. Also, FHSS
suffers from higher MAC latencies.
Quick Quiz 1
1. The ____________________ layer of the OSI Model picks the route packets take and
handles addressing of packets for delivery.
Answer: Network
2. ____________________ is a technique that takes a narrow, weaker signal and spreads it
over a broader portion of the radio frequency band.
Answer: Spread spectrum
3. With ____________________, a short burst is transmitted at one frequency, then a
short burst is transmitted at another frequency, and so on, until the entire transmission is
completed.
Answer: Frequency hopping spread spectrum (FHSS)
4. ____________________ uses an expanded redundant code to transmit each data bit.
Answer: Direct sequence spread spectrum (DSSS)
5. ____________________ sends a transmission in parallel across several lower-speed
channels.
Answer: Orthogonal frequency division multiplexing (OFDM)
6. The amount of data that a channel can send and receive is known as
____________________.
Answer: throughput
IEEE 802.11 Physical Layer Standards
1. Explain that the IEEE wireless standards follow the OSI model, with some
modifications.
2. Explain that the Data Link layer has been divided into two sublayers: the LLC sublayer
and the MAC sublayer. Discuss the purpose that each of these sublayers serve. Illustrate
with Figure 4-10.
3. Explain that the Physical layer is also divided into two sublayers: the PMD and PLCP
sublayers. Discuss the purpose of each of these sublayers. Illustrate with Figure 4-11.
4. Discuss the PLCP sublayer’s two basic functions. Explain that it reformat the data
received from the MAC layer into a frame that the PMD sublayer can transmit.
Illustrate with Figure 4-12.
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5. Stress that the IEEE WLAN standards specify that the features of a WLAN must be
transparent to the upper layers of the IEEE model. Explain that this makes the PHY and
MAC layers of the IEEE 802.11 wireless LANs function like those of other IEEE
network standards, such as Ethernet and Token Ring, in sharing the same LLC layer.
Illustrate with Figure 4-13.
Legacy WLANs
1. Explain that, at this writing, there are at least two “obsolete” WLAN standards: the
original IEEE 802.11 (with no letter) and HomeRF.
2. Describe the reasons why IEEE 802.11 is obsolete. Mention that IEEE 802.11 can be
used either FHSS or DSSS for RF transmissions, but that all WLAN devices have to use
the same transmission option.
3. Discuss the functionality of HomeRF, and mention that it never gained popularity
because it is slow and because the IEEE 802.11x standards gained industry acceptance
much more quickly.
IEEE 802.11b Physical Layer Standards
1. Reiterate that the basic purpose of the 802.11 PHY layer is to send the signal to the
network and receive the signal from the network.
Physical Layer Convergence Procedure Standards
1. Stress that the PLCP standards for 802.11b are based on DSSS. Reiterate that PLCP
must reformat the data received from the MAC layer (when transmitting) into a frame
that the PMD sublayer can transmit.
2. Illustrate a PLCP frame with Figure 4-14.
3. Describe the three components of a PLCP frame. Using the list on pages 128 and 129 of
the text as a guide, discuss the fields contained within the data portion of a PLCP frame.
4. Mention that the PLCP frame preamble and header are always transmitted at 1 Mbps,
and explain why.
Teaching
Tip
An advantage of the slower PLCP preamble and header transmission speed is
that the slower signal can cover a larger area than a faster signal can.
Physical Medium Dependent Standards
1. Reiterate that the job of the PMD is to translate the binary 1’s and 0’s of the frame into
radio signals that can be used for transmission. Mention that the PMD can transmit the
data at 11, 5.5, 2, or 1 Mbps.
2. Explain that the 802.11b standard uses the ISM band for its transmissions. Using Table
4-2 as a guide, discuss the 14 frequencies that can be used by the IEEE 802.11b
standard.
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3. Discuss the DBPSK and DQPSK modulation techniques, and explain under what
conditions each technique is used, as specified by the 802.11b standard.
4. Explain that the 802.11b standard also outlines the type of DSSS coding to be used.
Mention that the Barker code is used when 802.11b is transmitting at 1 Mbps or 2
Mbps, but CCK is used at higher rates. Summarize all of this information with the aid of
Table 4-3.
Teaching
Tip
The United States and Canada use channels 1-11; Europe, France, and Japan use
channels 12-14.
IEEE 802.11a Physical Layer Standards
1. Explain that IEEE 802.11a achieves its increase in speed and flexibility over 802.11b
primarily through its use of OFDM multiplexing. Mention that it uses a higher
frequency, access more transmission channels, and use a more efficient error-correction
scheme.
U-NII Frequency Band
1. Explain that the 802.11a standard uses the U-NII band. Using Table 4-4 as a guide,
compare and contrast 802.11b and 802.11a in terms of the band used, frequency, and
total bandwidth.
2. Mention that the FCC has segmented the 300 MHz of the U-NII spectrum into four
bands, and that each of these bands has a maximum power limit. Illustrate with Table 45.
Teaching
Tip
In 2003 the FCC added an additional 255 MHz of spectrum in the 5.470-5.725
band, which increased the amount of spectrum for 802.11a devices by almost
80%. This was an attempt to better harmonize the U.S. spectrum with that of
other nations.
Teaching
Tip
The U-NII High Band is more commonly used for building-to-building wireless
transmissions.
3. Explain that the total bandwidth available for IEEE 802.11a WLANs using U-NII is
almost four times that available for 802.11b networks using the ISM band.
4. Discuss the two main disadvantages to using the U-NII band, as described on page 132
of the text. Stress that interference from other devices is one of the primary problems
for both 802.11b and 802.11a WLANs.
CWNA Guide to Wireless LANs, Second Edition
Teaching
Tip
4-8
When encountering interference from cordless telephones, you can try changing
the frequency (channel) of the phone, lowering its antenna, or moving the AP as
far away as possible.
Channel Allocation
1. Explain that a reason for the faster speed of an 802.11a WLAN is increased channel
allocation. Revisit the channel allocation used in 802.11b, illustrating with Figure 4-15.
2. Explain that, with 802.11a, eight frequency channels can overlap and operate
simultaneously in the Lower Band and Middle Band. Illustrate with Figure 4-16.
Mention the number of carrier signals that each channel can support.
3. Discuss the differences in channel coverage between 802.11b and 802.11a. Illustrate
with Figure 4-17. Stress that, regardless of the number of available channels, an AP
uses only one channel at a time.
4. Discuss the additional advantages of having more channels. Explain that, when multiple
APs are used, more users can be supported by assigning specific channels to users
associated with specific APs. Mention that with more available channels there is the
ability to set the AP to use a different channel to reduce or eliminate interference.
Error Correction
1. Describe the reasons why 802.11a WLANs experience fewer errors than 802.11b
WLANs.
2. Discuss the concept of FEC. Explain that the redundant transmissions sent in FEC can
be used to recovery lost data, eliminating the need for retransmission of data. Mention
that this system does not affect WLAN performance.
Physical Layer Standards
1. Explain that PLCP for 802.11a is based on OFDM. Using Figure 4-18 to illustrate,
discuss the frame format of an 802.11a PLCP frame.
2. Discuss the three major components of an 802.11a PLCP frame. Using the list on pages
135 and 136 of the text as a guide, discuss the fields contained in the data portion of a
PLCP frame.
3. Using Table 4-6 as a guide, discuss the possible rate field values.
4. Explain that the modulation techniques used to encode the 802.11a data vary depending
upon the speed. Table 4-7 summarizes the modulation techniques used at each speed.
Discuss each of these modulation techniques, illustrating with Figures 4-19 through 422.
5. Discuss the concept of 2X modes, and explain how vendors may implement these
modes.
CWNA Guide to Wireless LANs, Second Edition
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IEEE 802.11g Physical Layer Standards
1. Explain that IEEE 802.11g combines the best features of 802.11a and 802.11b.
Teaching
Tip
The 802.11g standard was a compromise based on input from several chip
manufacturers who had a major stake in the outcome. Although most major
commercial wireless networking product vendors will build and sell products
based upon whatever standard is approved, the same is not true for the chip
manufacturers. These businesses must make huge monetary investments in
designing, sampling, and manufacturing the silicon chips used in wireless
network products. They must then try to sell their chips to product vendors that
design and build commercial products based on the chip sets.
2. Mention that 802.11g operates entirely in the 2.4 GHz ISM frequency.
3. Discuss the two mandatory and one optional mode used in 802.11g. Use Table 4-8 to
facilitate the discussion.
4. Describe the advantages and disadvantages of the 802.11g standard compared to the
802.11a and 802.11b standards. Mention that, in an environment where both 802.11b
and 802.11g devices are transmitting, the 802.11g devices will drop to only 11 Mbps
speeds.
Teaching
Tip
Wireless NICs and APs are now available as “dual-band” or “multiple band,”
meaning they support two or more of the 802.11a, b, and g standards
simultaneously.
5. Describe how vendors are able to implement a proprietary higher speed, and discuss the
potential disadvantages of this technique. Explain how dynamic turbo attempts to
overcome these disadvantages.
Teaching
Tip
HiperLAN2 is an ETSI standard utilizing OFDM in the 5GHz frequency range. It
is nearly identical to IEEE 802.11a at the physical level, but very different at the
MAC layer. It is based largely on ATM technology, and is difficult to
implement, and consequently has become obsolete.
Quick Quiz 2
1. The ____________________ sublayer performs two basic functions: it reformats the
data received from the MAC layer (when transmitting) into a frame that the PMD
sublayer can transmit, and it “listens” to the medium to determine when the data can be
sent.
Answer: Physical Layer Convergence Procedure (PLCP)
2. The PLCP standards for 802.11b are based on ____________________.
Answer: direct sequence spread spectrum (DSSS)
CWNA Guide to Wireless LANs, Second Edition
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3. The 802.11b standard uses the ____________________ band for its transmissions.
Answer: Industrial, Scientific, and Medical (ISM)
4. In 802.11a there are ____________________ frequency channels can overlap and
operate simultaneously in the Lower Band (5.15 to 5.25 GHz) and Middle Band (5.25 to
5.35 GHz).
Answer: eight
5. ____________________ transmits a secondary copy along with the primary
information.
Answer: Forward Error Correction (FEC)
6. In 802.11a, transmitting at 24 Mbps requires a(n) ____________________ modulation
technique.
Answer: 16-level quadrature amplitude modulation (16-QAM)
7. The optional mode for 802.11g is known as ____________________.
Answer: PBCC-22 (Packet Binary Convolution Coding)
Class Discussion Topics
1. Have the students discuss the similarities and differences between various types of
802.11 data frames and data frames used in various “wired” types of networks (such as
Ethernet).
2. Given what the students have learned so far, do they think that 802.11a and 802.11b
will eventually become obsolete? Why or why not?
3. Do the students believe that the FCC will receive political or industry pressure to
expose a greater portion of the electromagnetic spectrum for use in wireless
communications? Why or why not?
Additional Projects
1. The students have probably heard of “jamming” a signal. Have the students first discuss
and postulate how signal jamming might be accomplished. Then have them research
online to find out how jamming actually occurs, and write a short report on it. Do they
think that jamming a WLAN signal is feasible? Why or why not? What types of signals
could be jammed and how?
2. When working with most any type of electromagnetic signaling, the signal-to-noise
ratio is an important diagnostic quantity. Have the students do research online or in the
library to determine what exactly the signal-to-noise ratio is, how it is measured, what
diagnostic information it can provide. Have them write a short report containing this
information and any other information about the signal-to-noise ratio that they can find.
3. There are a number of versions of OFDM. Have the students do research online to find
information about some of these versions, and the applications that they are intended
for.
CWNA Guide to Wireless LANs, Second Edition
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Additional Resources
1. OSI Model Practice Quiz:
http://gocertify.com/quizzes/osi/
2. Open Systems Interconnection (OSI) Protocols:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/osi_prot.htm
3. Tutorial on Spread Spectrum Technology:
http://www.commsdesign.com/design_corner/showArticle.jhtml?articleID=16501183
4. A comparison of 802.11a and 802.11b Wireless LAN Standards (White Paper from
Linksys):
http://www.linksys.com/products/images/wp_802.asp
5. DSSS vs. FHSS narrowband interference performance issues:
http://rfdesign.com/mag/radio_dsss_vs_fhss/
6. DSSS vs. FHSS (pdf):
http://wireless.ittoolbox.com/browse.asp?c=WirelessPeerPublishing&r=%2Fpub%2FV
K042601a.pdf
7. OFDM Tutorials:
http://www.palowireless.com/ofdm/tutorials.asp
8. Interactive OFDM Tutorial:
http://www.see.ed.ac.uk/~acmc/OFDMTut.html?http://oldeee.see.ed.ac.uk/~acmc/OFD
MTut.html
9. A Technical Tutorial on the IEEE 802.11 Protocol (pdf):
http://www.sss-mag.com/pdf/802_11tut.pdf
10. 802.11b Physical Layer Revealed:
http://www.wi-fiplanet.com/tutorials/article.php/2107261
11. IEEE 80211.a White Paper:
http://www.vocal.com/data_sheets/80211a5.html
Key Terms
 16-level quadrature amplitude modulation (16-QAM): A modulation technique that
sends 16 different signals simultaneously.
 2X mode: A proprietary transmission scheme to double the effective rate of an 802.11a
network.
 64-level quadrature amplitude modulation (64-QAM): A modulation technique that
can transmit 1.125 Mbps over 48 subchannels each.
 Barker code: The bit pattern used in direct sequence spread spectrum (DSSS)
modulation.
 Channel: A single frequency.
 Channel bonding: An 802.11g proprietary technique in which two channels are
combined to provide frequency for higher speeds.
 Chipping code: The bit pattern used in direct sequence spread spectrum (DSSS)
modulation.
 Co-location: Sharing a frequency band between similar devices.
 Complementary code keying (CCK): A coding technique used in 802.11b networks
that consists of a set of 64 8-bit code words.
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 Differential binary phase shift keying (DBPSK): A two-level phase shift key used in
802.11b networks.
 Differential quadrature phase shift keying (DQPSK): A four-level phase change
used in 802.11b networks.
 Direct sequence spread spectrum (DSSS): A wireless modulation technique that uses
an expanded redundant code to transmit each data bit.
 Dwell time: The amount of time that a transmission occurs on a specific frequency in a
frequency hopping spread spectrum (FHSS) transmission.
 Dynamic turbo: An 802.11g proprietary technique that automatically checks for
available frequencies before implementing channel bonding.
 Forward Error Correction (FEC): An IEEE 802.11a error correction technique that
transmits a secondary copy along with the primary information.
 Frequency hopping spread spectrum (FHSS): A modulation technique that uses a
range of frequencies that change during the transmission.
 HomeRF: A legacy WLAN technology based on a protocol known as the Shared
Wireless Access Protocol (SWAP).
 Hop time: The amount of time that a transmission stays on a specific frequency in a
frequency hopping spread spectrum (FHSS) transmission.
 Hopping code: The sequence of changing frequencies in a frequency hopping spread
spectrum transmission.
 Microseconds (μs): One millionth of a second.
 Millisecond (ms): One thousandth of a second.
 Multiplexing: A technique for sending multiple signals simultaneously over one
channel.
 Narrowband transmission: Radio signals that are sent on only one radio frequency or
a very narrow portion of the frequencies.
 Noise: Outside interference on a radio signal.
 Noise level: The total amount of outside interference.
 Open Systems Interconnection (OSI) reference model: A seven-layer model that
conceptually illustrates the process of network communication.
 Orthogonal frequency division multiplexing (OFDM): A wireless LAN modulation
technology that splits a high-speed digital signal into several slower signals running in
parallel.
 PBCC-22 (Packet Binary Convolution Coding): An optional 802.11g technique for
transmitting at 22 Mbps.
 Physical Layer Convergence Procedure (PLCP): A Physical layer sublayer that
reformats the data received from the MAC layer (when transmitting) into a frame that
the PMD sublayer can transmit and “listens” to the medium to determine when the data
can be sent.
 Physical Medium Dependent (PMD): A Physical layer sublayer that defines the
standards for both the characteristics of the wireless medium and the method for
transmitting and receiving data through that medium.
 Quadrature phase shift keying (QPSK): An IEEE 802.11a modulation technique that
increases the amount of data encoded to 250 Kbps per channel.
 Shared Wireless Access Protocol (SWAP): A protocol that defines a set of
specifications for wireless data and voice communications around the home; no longer
widely used.
 Spread spectrum transmission: A technique that takes a narrow, weaker signal and
spreads it over a broader portion of the radio frequency band.
CWNA Guide to Wireless LANs, Second Edition
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 Throughput: The amount of data that a channel can send and receive.
 Turbo mode: A proprietary transmission scheme to double the effective rate of an
802.11a network.