CWNA Guide to Wireless LANs, Second Edition 4-13

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

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.

Teaching

Tip / 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.
  1. Using the list on pages 116 and 117 of the text as a guide, discuss the advantages of spread spectrum over narrowband transmission.
  1. 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.
  1. Explain that frequency hopping uses a range of frequencies that change during the transmission.
  1. 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.
  1. 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.
  1. Discuss the restrictions on FHSS established by the FCC at various frequencies.
  1. 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.

Teaching

Tip / The term “chipping code” is used because a single radio bit is commonly referred to as a “chip.”
  1. 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.
  1. Using the list on page 122 of the text as a guide, discuss the advantages of using DSSS with a chipping code.
  1. 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.
  1. 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.
  1. Explain how OFDM splits, sends, and recombines data. Illustrate with Figure 4-8.
  1. 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 4-9.
  1. 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.
  1. 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.
  1. Discuss why OFDM is currently the preferred modulation scheme due to its high throughput.

Teaching

Tip / 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

  1. ______is a technique that takes a narrow, weaker signal and spreads it over a broader portion of the radio frequency band.

Answer: Spread spectrum

  1. 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)

  1. ______uses an expanded redundant code to transmit each data bit.

Answer: Direct sequence spread spectrum (DSSS)

  1. ______sends a transmission in parallel across several lower-speed channels.

Answer: Orthogonal frequency division multiplexing (OFDM)

  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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.

  1. 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.
  1. 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.
  1. Discuss the DBPSK and DQPSK modulation techniques, and explain under what conditions each technique is used, as specified by the 802.11b standard.
  1. 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.
  1. 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 4-5.

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.
  1. 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.
  1. 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.

Teaching

Tip / 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. Using Table 4-6 as a guide, discuss the possible rate field values.
  1. 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 4-22.
  1. Discuss the concept of 2X modes, and explain how vendors may implement these modes.

IEEE 802.11g Physical Layer Standards