Is multipath interference a layer 1 or layer 2 concern?

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LEARNING OBJECTIVES

By the end of this chapter, you should be able to:

▪ Explain basic radio signal propagation concepts, including frequencies, antennas,

and wireless propagation problems.

▪ Explain the frequency spectrum, service bands, channels, bandwidth, licensed

versus unlicensed service bands, and the type of spread spectrum transmission

used in 802.11 Wi-Fi LANs.

▪ Describe 802.11 Wi-Fi WLAN operation with access points and a switched Ethernet

distribution system to link the access points. Distinguish between BSSs, ESSs,

and SSIDs. Discuss communication between access points.

▪ If you read the box, compare the CSMA/CA+ACK and RTS/CTS media access

control disciplines.

▪ Compare and contrast the 802.11g, 802.22a, 802.11n, and 802.11ac

transmission standards. Discuss emerging trends in 802.11 operation,

including channels with much wider bandwidth, MIMO, beamforming,

and multiuser MIMO.

▪ Briefly discuss the key points of wireless mesh networking.

Wireless LANs I

Chapter 6

ISBN 1-323-07906-8

Business Data Networks and Security, Tenth Edition, by Raymond R. Panko and Julia L. Panko. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.

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INTRODUCTION

OSI Standards

In Chapter 5, we looked at wired switched Ethernet networks. Technologies for these

networks require both physical and data link layer standards. Consequently, they

use OSI standards. In this chapter and in Chapter 7, we will look at wireless LANs. Like

wired LANs, wireless LANs are also single networks, which require physical and DLL

standards. They too use OSI standards.

Test Your Understanding

1. a) At what layers do wireless LANs operate? b) Do wireless LAN standards come

from OSI or TCP/IP? Explain.

802.11 versus Wi-Fi

Having discussed wireless transmission briefly, we will look at wireless networking’s

widest application, wireless local area networks. Wireless LANs (WLANs) use radio

for physical layer transmission on the customer premises.

Ethernet 802.3 LANs

Require standards at Layer 1 (physical) and Layer 2 (data link)

Therefore, use OSI standards

The 802.3 Working Group of the IEEE 802 Committee creates standards

Wireless LANs

Operate at Layers 1 and 2

Therefore, they are OSI standards

802.11 Wireless LAN Technology

The dominant WLAN technology today

Standardized by the 802.11 Working Group of the IEEE 802 Committee

Wi-Fi Alliance

Industry association of 802.11 equipment manufacturers

Purpose

802.11 standards have many options

Wi-Fi Alliance selects subsets of standards as profiles

Does interoperability testing among vendors on these profiles

Only products that pass can display the Wi-Fi logo on their products

However, sometimes develops new standards

Two have been security nightmares

FIGURE 6-1 802.11 / Wi-Fi Wireless LAN (WLAN) Technology (Study Figure)

ISBN 1-323-07906-8

Business Data Networks and Security, Tenth Edition, by Raymond R. Panko and Julia L. Panko. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.

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Wireless LANs (WLANs) use radio for physical layer transmission on the customer premises.

In the last chapter, we saw that the 802.3 Working Group of the IEEE’s 802 LAN/

MAN Standards Committee creates Ethernet standards. Other working groups create

other standards. The dominant WLAN standards today are the 802.11 standards, which

are created by the IEEE 802.11 Working Group.

It is common to call the 802.11 standards “Wi-Fi” standards. In fact, the terms have

become almost interchangeable, and we will use them that way in this book. However,

as an IT professional, you should understand the technical difference between

802.11 and Wi-Fi. The term Wi-Fi stems from the Wi-Fi Alliance, which is an industry

consortium of 802.11 product vendors. When the 802.11 Working Group creates

standards, it often creates many options. The Wi-Fi Alliance creates subsets of 802.11

standards with selected options. The Alliance conducts interoperability tests among

products that claim to meet these “profiles.” Only products that pass interoperability

tests may display the Wi-Fi Logo on their products. Products that do not pass are rarely

sold, so when someone picks up a box containing an 802.11 product, they almost always

see the Wi-Fi logo.

Test Your Understanding

2. a) Distinguish between 802.3 standards and 802.11 standards. b) Distinguish

between 802.11 and Wi-Fi.

Wireless LAN Operation

It is possible to have a purely wireless LAN. In organizations today, however, the

normal situation is to have a hybrid switched/wireless single network. Figure 6-2 shows that

corporations already have comprehensive Ethernet switched LANs. These wired LANs

reach almost everywhere on the corporate premises. Wireless clients (wireless devices

Ethernet Wired LAN

Notebook

Client

Radio

Transmission

UTP

Access

Point A

Ethernet

Switch

Server

Needed by

Client

Communication

Router

for Internet

Access

Access

Point B

The Internet

FIGURE 6-2 Hybrid Switched/Wireless 802.11 Network

ISBN 1-323-07906-8

Business Data Networks and Security, Tenth Edition, by Raymond R. Panko and Julia L. Panko. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.

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are almost always clients) communicate wirelessly (by radio) to an 802.11 wireless

access point, which is typically simply called an access point.

Wi-Fi clients rarely communicate with other 802.11 clients. Instead, they usually

need to reach resources on the main Ethernet LAN. Obviously, clients need to reach

servers, and corporate servers are on the firm’s Ethernet network. In addition, of

course, clients need to reach the Internet, and the firm’s border router that connects

it to the Internet is also on the firm’s Ethernet network. In addition to orchestrating

radio transmissions between itself and the wireless clients it serves, an access point

connects the wireless devices to the firm’s main Ethernet LAN.

In addition to orchestrating radio transmissions between itself and the wireless clients

it serves, an access point connects the wireless devices to the firm’s main Ethernet LAN.

Only small firms can get by with a single access point. Larger firms disperse access

points around their premises so that a wireless client can connect to another access point

when it is moved to a different location.

Test Your Understanding

3. a) Why do wireless clients need access to the firm’s main wired switched Ethernet

network? b) How can firms provide WLAN coverage throughout a large building?

RADIO SIGNAL PROPAGATION

Chapter 5 discussed propagation effects in wired transmission media (UTP and

optical fiber). Propagation effects in wired transmission can be well controlled by

respecting cord distance limits and taking other installation precautions. This is possible

because wired propagation is predictable. If you input a signal, you can estimate

precisely what it will be at the other end of a cord. A wired network is like a faithful,

obedient dog.

Propagation effects in wired transmission can be well controlled by respecting cord

distance limits and taking other installation precautions.

In contrast, radio propagation is very unreliable. Radio signals bounce off

obstacles, fail to pass through walls and filing cabinets, and have other problems we

will look at in this section. Consequently, Wi-Fi networks, which use radio to deliver

signals, are more complex to implement than wired networks. They do not have a

few simple installation guidelines that can reduce propagation effects to nonissues.

Therefore, we will spend more time on wireless propagation effects than we did on

wired propagation effects.

Propagation effects in wireless networks are complex and difficult to implement.

ISBN 1-323-07906-8

Business Data Networks and Security, Tenth Edition, by Raymond R. Panko and Julia L. Panko. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.

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Test Your Understanding

4. a) In 802.3 Ethernet networks, can simple installation rules usually reduce propagation

effects to nonissues? b) In 802.11 Wi-Fi networks, can simple installation

rules usually reduce propagation effects to nonissues?

Frequencies

Radios for data transmission are called transceivers because they both transmit and

receive. When transceivers send, their wireless signals propagate as waves, as we saw

in Chapter 5. Figure 6-3 again notes that waves have amplitude and wavelength. While

optical fiber waves are described in terms of wavelength, radio waves are described in

terms of another wave characteristic, frequency.

Frequency is used to describe the radio waves used in WLANs.

In waves, frequency is the number of complete cycles per second. One cycle per

second is one hertz (Hz). Metric designations are used to describe frequencies. In the

metric system, frequencies increase by a factor of 1,000 rather than 1,024. The most common

radio frequencies for wireless transceivers range between about 500 megahertz

(MHz) and 10 gigahertz (GHz).

Amplitude

Amplitude

Wavelength

Wavelength

1 Second, 2 Cycles

Wavelength is the physical distance between comparable points on adjacent cycles.

Optical fiber transmission is described in terms of wavelength.

Frequency is the number of cycles per second.

In this case, there are two cycles in 1 second, so the frequency is two hertz (2 Hz).

Radio transmission is measured in terms of frequency.

Amplitude is the power of the wave.

FIGURE 6-3 Electromagnetic Wave

ISBN 1-323-07906-8

Business Data Networks and Security, Tenth Edition, by Raymond R. Panko and Julia L. Panko. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.

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Test Your Understanding

5. a) What is a transceiver? b) Is wireless radio transmission usually expressed in

terms of wavelength or frequency? c) What is a hertz? d) Convert 3.4 MHz to

a number without a metric prefix. (The use of metric prefixes was discussed in

a box in Chapter 1.) e) At what range of frequencies do most wireless systems

operate?

Antennas

A transceiver must have an antenna to transmit its signal. Figure 6-4 shows that there

are two types of radio antennas: omnidirectional antennas and dish antennas.

􀁲􀀁Omnidirectional antennas transmit signals equally strongly in all directions

and receive incoming signals equally well from all directions. Consequently, the

antenna does not need to point in the direction of the receiver. However, because

the signal spreads in all three dimensions, only a small fraction of the energy

transmitted by an omnidirectional antenna reaches the receiver. Omnidirectional

antennas are best for short distances, such as those found in a wireless LAN or a

cellular telephone network.

􀁲􀀁Dish antennas, in contrast, point in a particular direction, which allows them to

send stronger signals in that direction for the same power and to receive weaker

incoming signals from that direction. (A dish antenna is like the reflector in a

flashlight.) Dish antennas are good for longer distances because of their focusing

ability, although users need to know the direction of the other radio. In addition,

dish antennas are hard to use. (Imagine if you had to carry a dish with you whenever

you carried your cellular phone. You would not even know where to point

the dish!)

Omnidirectional Antenna

Signal spreads in all directions

Rapid signal attenuation

-----

No need to point at receiver

Dish Antenna

Focuses signal in a narrow range

Signals can travel longer distances

-----

Must point at receiver

FIGURE 6-4 Omnidirectional and Dish Antennas

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Test Your Understanding

6. a) Distinguish between omnidirectional and dish antennas in terms of operation.

b) Under what circumstances would you use an omnidirectional antenna?

c) Under what circumstances would you use a dish antenna? d) What type of

antenna normally is used in WLANs? Why?

Wireless Propagation Problems

We have already noted that, although wireless communication gives mobility, wireless

transmission is not very predictable, and there often are serious propagation problems.

Figure 6-5 illustrates five common wireless propagation problems.

Inverse Square Law Attenuation Compared to signals sent through wires

and optical fiber, radio signals attenuate very rapidly. When a signal spreads out

from any kind of antenna, its strength is spread over the area of a sphere. (In omnidirectional

antennas, power is spread equally over the sphere, while in dish antennas,

power is concentrated primarily in one direction on the sphere.)

The area of a sphere is proportional to the square of its radius, so signal strength

in any direction weakens by an inverse square law (1/r2), as Equation 6–1 illustrates.

Here, S1 is the signal strength at distance r1, and S2 is the signal strength at a farther

distance r2.

S2 = S1 * (r1/r2)2 (Equation 6–1)

If you triple the distance (r1/r2 = 1/3), the final signal strength (S2) falls to only

one-ninth (1/32) of its original strength (S1). With radio propagation, you have to be

relatively close to your communication partner unless the signal strength is very high,

a dish antenna is used, or both.

To give a specific example, at 10 meters, the signal strength is 30 milliwatts (mW).

How strong will the signal be at 30 meters?

Transmission Antenna

Laptop

Dead Zone:

Worse at

Higher

Frequencies

Multipath

Interference

Inverse Square Law

Attenuation

1/r^2

Electromagnetic

Interference

(EMI)

Direct Signal

Reflected Signal

Absorptive

Attenuation

Worse at

Higher

Frequencies

FIGURE 6-5 Wireless Propagation Problems

ISBN 1-323-07906-8

Business Data Networks and Security, Tenth Edition, by Raymond R. Panko and Julia L. Panko. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.

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􀁲􀀁􀀵􀁉􀁆􀀁􀁅􀁊􀁔􀁕􀁂􀁏􀁄􀁆􀀁􀁕􀁓􀁊􀁑􀁍􀁆􀁔􀀁􀀉􀁔􀁐􀀁r1/r2 is 1/3).

􀁲􀀁􀀴􀁐􀀁􀁘􀁆􀀁􀁎􀁖􀁍􀁕􀁊􀁑􀁍􀁚􀀁􀁕􀁉􀁆􀀁􀁔􀁊􀁈􀁏􀁂􀁍􀀁􀁔􀁕􀁓􀁆􀁏􀁈􀁕􀁉􀀁􀁂􀁕􀀁􀀒􀀑􀀁􀁎􀁆􀁕􀁆􀁓􀁔􀀁􀁃􀁚􀀁􀀒􀀐􀀚􀀁􀀉􀀒􀀐􀀔􀀁􀁔􀁒􀁖􀁂􀁓􀁆􀁅􀀊􀀏

􀁲􀀁􀀔􀀑􀀁􀁎􀀸􀀁􀁎􀁖􀁍􀁕􀁊􀁑􀁍􀁊􀁆􀁅􀀁􀁃􀁚􀀁􀀒􀀐􀀚􀀁􀁊􀁔􀀁􀀔􀀏􀀔􀀔􀀁􀁎􀀸􀀏

􀁲􀀁􀀴􀁐􀀁􀁕􀁉􀁆􀀁􀁔􀁕􀁓􀁆􀁏􀁈􀁕􀁉􀀁􀁐􀁇􀀁􀁕􀁉􀁆􀀁􀁔􀁊􀁈􀁏􀁂􀁍􀀁􀁂􀁕􀀁􀀔􀀑􀀁􀁎􀁆􀁕􀁆􀁓􀁔􀀁􀁘􀁊􀁍􀁍􀀁􀁃􀁆􀀁􀀔􀀏􀀔􀀔􀀁􀁎􀀸􀀏

Absorptive Attenuation As a radio signal travels, it is partially absorbed by the

air molecules, plants, and other things it passes through. This absorptive attenuation is

especially bad in moist air and office plants because water is an especially good absorber

of radio signals.

Absorptive attenuation can be confusing because we have already seen inverse

square law attenuation. Yes, wireless propagation suffers from two forms of attenuation.

Inverse square law attenuation is due to the signal spreading out as a sphere and

so becoming weaker at each point on the sphere. Absorptive attenuation is signal loss

through energy absorption.

Wireless transmission suffers from two forms of attenuation—inverse square law attenuation

and absorptive attenuation.

Dead Zones To some extent, radio signals can go through and bend around objects.

However, if there is a dense object (e.g., a thick wall) blocking the direct path between the

sender and the receiver, the receiver may be in a dead zone, also called a shadow zone or

dead spot. In these zones, the receiver cannot get the signal. If you have a mobile phone

and often try to use it within buildings, you may be familiar with this problem.

Multipath Interference In addition, radio waves tend to bounce off walls,

floors, ceilings, and other objects. As Figure 6-7 shows, this may mean that a receiver

will receive two or more signals—a direct signal and one or more reflected signals.

The Situation

Signals spread over the surface of a sphere

As the radius of the sphere increases with distance, the signal weakens

Weakens as the square of the distance

S2 = S1 * (r1/r2)2 (Equation 6–1)

Example

At 10 meters, the signal strength is 30 mW

How strong will it be at 30 m?

The distance triples (so r1/r2 is 1/3).

So we multiply the signal strength at 10 meters by 1/9 (1/3 squared)

30 mW multiplied by 1/9 is 3.33 mW.

So the strength of the signal at 30 meters will be 3.33 mW.

FIGURE 6-6 Inverse Square Law Attenuation (Study Figure)

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Business Data Networks and Security, Tenth Edition, by Raymond R. Panko and Julia L. Panko. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.

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The direct and reflected signals will travel different distances and so may be out of

phase when they reach the receiver. For example, one may be at its highest amplitude

while the other is at its lowest, giving an average of zero. If their amplitudes

are the same, they will completely cancel out. In real situation, multiple signals

travelling different paths will interfere, so we call this type of interference multipath

interference.

Multipath interference may cause the signal to range from strong to nonexistent

within a few centimeters. If the difference in time between the direct and reflected signal

is large, some reflected signals may even interfere with the next direct signal. Multipath

interference is the most serious propagation problem at WLAN frequencies.

Multipath interference is the most serious propagation problem at WLAN frequencies.

Electromagnetic Interference (EMI) A final common propagation problem in

wireless communication is electromagnetic interference (EMI). Many devices produce

EMI at frequencies used in wireless data communications. Among these devices are

cordless telephones, microwaves, and nearby access points. Consequently, placing

access points so that they give good coverage without creating excessive mutual

interference is difficult.

Frequency-Dependent Propagation Problems To complicate matters, two

wireless propagation problems get worse as frequency increases.

􀁲􀀁􀀧􀁊􀁓􀁔􀁕􀀍􀀁􀁉􀁊􀁈􀁉􀁆􀁓􀀎􀁇􀁓􀁆􀁒􀁖􀁆􀁏􀁄􀁚􀀁􀁘􀁂􀁗􀁆􀁔􀀁􀁔􀁖􀁇􀁇􀁆􀁓􀀁􀁎􀁐􀁓􀁆􀀁􀁓􀁂􀁑􀁊􀁅􀁍􀁚􀀁􀁇􀁓􀁐􀁎􀀁􀁂􀁃􀁔􀁐􀁓􀁑􀁕􀁊􀁗􀁆􀀁􀁂􀁕􀁕􀁆􀁏􀁖􀁂􀁕􀁊􀁐􀁏􀀁􀁕􀁉􀁂􀁏􀀁

lower-frequency waves because they are absorbed more rapidly by moisture in the

air. Consequently, as we will see in this chapter, WLAN signals around 5 GHz attenuate

more rapidly than signals around 2.4 GHz.

􀁲􀀁􀀴􀁆􀁄􀁐􀁏􀁅􀀍􀀁􀁅􀁆􀁂􀁅􀀁􀁛􀁐􀁏􀁆􀀁􀁑􀁓􀁐􀁃􀁍􀁆􀁎􀁔􀀁􀁈􀁓􀁐􀁘􀀁􀁘􀁐􀁓􀁔􀁆􀀁􀁘􀁊􀁕􀁉􀀁􀁇􀁓􀁆􀁒􀁖􀁆􀁏􀁄􀁚􀀏􀀁􀀢􀁔􀀁􀁇􀁓􀁆􀁒􀁖􀁆􀁏􀁄􀁚􀀁􀁊􀁏􀁄􀁓􀁆􀁂􀁔􀁆􀁔􀀍􀀁

radio waves become less able to go through and bend around objects.

Direct Wave

Low

Amplitude

Reflected Wave

High

Amplitude

Signals Cancel Each Other

Total Amplitude = 0

FIGURE 6-7 Multipath Interference

ISBN 1-323-07906-8

Business Data Networks and Security, Tenth Edition, by Raymond R. Panko and Julia L. Panko. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.

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Test Your Understanding

7. a) If the signal strength from an omnidirectional radio source is 8 mW at 30 meters,

how strong will it be at 120 meters, ignoring absorptive attenuation? Show your

work. b) Contrast inverse square law attenuation and absorptive attenuation. c) How

are dead zones created? d) Why is multipath interference very sensitive to location?

e) What is the most serious propagation problem in WLANs? f) List some sources of

EMI. g) What two propagation problems become worse as frequency increases?

RADIO BANDS, BANDWIDTH, AND SPREAD

SPECTRUM TRANSMISSION

Service Bands

The Frequency Spectrum The frequency spectrum is the range of all possible

frequencies from zero hertz to infinity, as Figure 6-8 shows.

Service Bands Regulators divide the frequency spectrum into contiguous spectrum

ranges called service bands that are dedicated to specific services. For instance, in

the United States, the AM radio service band lies between 535 kHz and 1,705 kHz. The

FM radio service band, in turn, lies between 88 MHz and 108 MHz. The 2.4 GHz service

band that we will see later in this chapter extends from 2.4 GHz to 2.4835 GHz. There

are also service bands for police and fire departments, amateur radio operators, communication

satellites, and many other purposes.

Channels Service bands are subdivided further into smaller frequency ranges

called channels. A different signal can be sent in each channel because signals in

different channels do not interfere with one another. This is why you can receive different

television channels successfully.

Channel 5

Channel 4

Channel 3

Channel 2

Channel 1

Service

Band

0 Hz

Frequency

Spectrum

(0 Hz to Infinity)

The frequency spectrum is the

range of all possible frequencies

from 0 Hz to infinity.

A service band is a (usually)

contiguous range of the frequency

spectrum dedicated to a specific

purpose, such as FM radio,

emergency response, GPS, etc.

Service bands are divided further

into channels. Signals sent in

different channels do not interfere

with one another.

FIGURE 6-8 The Frequency Spectrum, Service Bands, and Channels

ISBN 1-323-07906-8

Business Data Networks and Security, Tenth Edition, by Raymond R. Panko and Julia L. Panko. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.

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Test Your Understanding

8. a) Distinguish among the frequency spectrum, service bands, and channels. b) In

radio, how can you send multiple signals without the signals interfering with one

another?

Signal and Channel Bandwidth

􀀧􀁊􀁈􀁖􀁓􀁆􀀁􀀗􀀎􀀔􀀁􀁔􀁉􀁐􀁘􀁆􀁅􀀁􀁂􀀁􀁘􀁂􀁗􀁆􀀁􀁐􀁑􀁆􀁓􀁂􀁕􀁊􀁏􀁈􀀁􀁂􀁕􀀁􀁂􀀁􀁔􀁊􀁏􀁈􀁍􀁆􀀁􀁇􀁓􀁆􀁒􀁖􀁆􀁏􀁄􀁚􀀏􀀁􀀪􀁏􀀁􀁄􀁐􀁏􀁕􀁓􀁂􀁔􀁕􀀍􀀁􀀧􀁊􀁈􀁖􀁓􀁆􀀁􀀗􀀎􀀚􀀁􀁔􀁉􀁐􀁘􀁔􀀁

that real signals do not operate at a single frequency. Rather, real signals spread over a

range of frequencies. This range is called the signal’s bandwidth. Signal bandwidth is

measured by subtracting the lowest frequency from the highest frequency.

A channel also has a bandwidth. For instance, if the lowest frequency of an FM

􀀁􀁄􀁉􀁂􀁏􀁏􀁆􀁍􀀁􀁊􀁔􀀁􀀙􀀚􀀏􀀑􀀁􀀮􀀩􀁛􀀁􀁂􀁏􀁅􀀁􀁕􀁉􀁆􀀁􀁉􀁊􀁈􀁉􀁆􀁔􀁕􀀁􀁇􀁓􀁆􀁒􀁖􀁆􀁏􀁄􀁚􀀁􀁊􀁔􀀁􀀙􀀚􀀏􀀓􀀁􀀮􀀩􀁛􀀍􀀁􀁕􀁉􀁆􀁏􀀁􀁕􀁉􀁆􀀁channel bandwidth

is 0.2 MHz (200 kHz). AM radio channels are 10 kHz wide, FM channels are

200 kHz wide, and television channels are 6 MHz wide. How wide must the channel

bandwidth be? The channel bandwidth must be wide enough for a signal’s bandwidth.

Claude Shannon discovered a remarkable thing about signal transmission.

A signal carrying X bits per second only needs half the bandwidth of a signal carrying

2X bits per second.1 Looked at the other way, if you want to transmit twice as many

bits per second, you need to double your bandwidth. More generally, if you want to be

able to transmit N times as fast, you need N times as much channel bandwidth. High

bandwidth brings high radio transmission speed.

To transmit N times as fast, you need N times as much channel bandwidth.

1 Speaking more precisely, Shannon also found that the signal-to-noise ratio (the ratio of single power to

noise) also affects propagation speed. However, engineers find it far easier to increase speed by increasing

bandwidth than by increasing the signal-to-noise ratio.

Signal

Power

Frequency

Bandwidth

Signal

Lowest

Frequency

Highest

Frequency Frequency is

measured in hertz (Hz)

Signals spread over a range of frequencies.

Faster signals spread over a wider range of frequencies.

This range of frequencies is called the signal’s bandwidth.

Channel bandwidth must be wide enough for the signal’s bandwidth.

FIGURE 6-9 Signal Bandwidth

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Radio channels with large bandwidths are called broadband channels. They

can carry data very quickly. Although the term broadband technically refers only

to the width of a channel, broadband has come to mean “fast,” whether or not radio

is used.

Transmission systems that are very fast are usually called broadband systems even when

they do not use radio channels.

Test Your Understanding

9. a) Does a signal usually travel at a single frequency, or does it spread over a range

of frequencies? b) If the lowest frequency in a channel is 1.22 MHz and the highest

frequency is 1.25 MHz, what is the channel bandwidth? (Use proper metric

notation.) c) If you want to transmit seven times as fast, how much wider must

the channel be? d) Why is large channel bandwidth desirable? e) What do we

call a system whose channels are wide? f) What other types of system do we call

broadband?

The 2.4 GHz and 5 GHz Service Bands

802.11 Wi-Fi WLANs today use two service bands. One is the 2.4 GHz band. The other

is the 5 GHz band.

The 2.4 GHZ Service Band The 2.4 GHz service band is the same in most

countries in the world, stretching from 2.4 GHz to 2.4835 GHz. Radio propagation is

better in the 2.4 GHz service band than it is in the higher-frequency 5 GHz band, where

absorptive attenuation is higher and dead zones are deader. Consequently, propagation

differences are somewhat shorter.

Unfortunately, the 2.4 GHz band only has 83.5 MHz of bandwidth. Traditionally,

each 802.11 channel was 20 MHz wide, although 40 MHz bandwidth channels were

introduced in 802.11n. Furthermore, due to the way channels are allocated, there are

only three possible non-overlapping 20 MHz 802.11 channels, which are centered at

Required Transmission Speed and Required Channel Bandwidth

There is a direct relationship between required transmission speed and required channel bandwidth

Doubling bandwidth doubles the possible transmission speed

Multiplying bandwidth by N makes possible N times the transmission speed

Broadband Channels

Broadband means wide radio channel bandwidth and therefore high speed

Popularly, fast systems are called “broadband” even if they are not radio systems

FIGURE 6-10 Channel Bandwidth and Transmission Speed (Study Figure)

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222􀀁 􀀤􀁉􀁂􀁑􀁕􀁆􀁓􀀁􀀗􀀁 􀁲􀀁 􀀸􀁊􀁓􀁆􀁍􀁆􀁔􀁔􀀁􀀭􀀢􀀯􀁔􀀁􀀪

Channels 1, 6, and 11.2 If nearby access points operate in the same channel, their signals

will interfere with each other unless the access points are far apart. This is called cochannel

interference. If an 802.11n station finds itself in a crowded area, it will drop

back from 40 MHz channels to 20 MHz channels to reduce interference. Of course,

speed roughly drops in half when this happens.

If you have only three access points that can all hear each other, there is no problem

with having only three channels. You simply run each on a different channel, and there

will be no co-channel interference. However, when you have multiple access points that

can all hear each other, Figure 6-12 shows that there is no way to avoid having some

2 Channel numbers were defined for the 2.4 GHz band when channels were narrower. A 20 MHz 802.11

channel overlaps several initially defined channels. Channels 1, 6, and 11 operate in the 2.402 GHz to

2.422 GHz, 2.427 GHz to 2. 447 GHz, and 2.452 GHz to 2.472 GHz frequency ranges, respectively. Note that

there are unused 5 MHz “guard bands” between the channels to prevent inter-channel interference.

The 2.4 GHz Service Band

2.4 GHz to 2.485 GHz

Propagation characteristics are good

For 20 MHz 802.11 channels, only three nonoverlapping channels are possible

Channels 1, 6, and 11

This creates co-channel interference between nearby access points transmitting in the same

channel

Except in very small networks, difficult or impossible to put nearby access points on different channels

(Figure 6-12)

The 5 GHz Service Band

More bandwidth, so between 11 and 24 non-overlapping 20 MHz channels

Makes it easy to have nearby access points operate on non-overlapping channels

Increasing channel bandwidth in newer standards reduces the number of possible channels

FIGURE 6-11 The 2.4 GHz and 5 GHz Service Bands (Study Figure)

Access Point A

Channel 1

Access Point B

Channel 6

Access Point C

Channel 6

Access Point D

Channel 6

Access Point E

Channel 6

Access Point F

Channel 11

OK

OK

OK

OK

Interference

Interference

Interference

In 802.11g,

nonoverlapping

channels are

1, 6, and 11

FIGURE 6-12 Co-Channel Interference in the 2.4 GHz Service Band

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co-channel interference. You can minimize co-channel interference somewhat by giving

the shared channel to the two access points that are farthest apart, but this will only

reduce interference somewhat.

The 5 GHZ Service Band Wi-Fi can also operate in the 5 GHz service band.

The big advantage of the 5 GHz band is that it is far wider than the 2.4 GHz band. In

contrast to the 2.4 GHz band’s mere three channels, the 5 GHz band provides between

11 and 24 non-overlapping 20 MHz channels today, depending on the frequencies

allocated to this service band in a particular country. In addition, while the 2.4 GHz

band is extremely crowded almost everywhere, it is only recently that companies have

begun to use the 5 GHz band extensively.

The problem with the 5 GHz band has been simple economics. Radio transceivers

in this band are inherently more expensive than they are in the 2.4 GHz band. However,

technological advances have brought 5 GHz radio transceivers down to the price range

that companies and households can now afford. Given the room in the 5 GHz band, this

has led to a gold rush for vendors and users moving into this uncrowded service band.

Adding to the attractiveness of the 5 GHz band, regulators in several countries

have been extending it to add more total bandwidth and therefore more channels. The

United States added more bandwidth in 2003. In 2013, the Federal Communications

Commission announced that it would add 35% more. In contrast, the 2.4 GHz band has

no expansion potential because it is bordered by services that cannot be moved.

In addition, we will see that 802.11n and 802.11ac are using channels much wider

than 20 MHz—up to 160 MHz. Wider channels mean fewer channels in the service

band. Without growth in 5 GHz bandwidth, there would be too little bandwidth in the

service band to permit enough very wide channels.

Test Your Understanding

10. a) In what two service bands does 802.11 operate? b) Which band dominated use

initially? c) How many 20 MHz non-overlapping channels does the 2.4 GHz band

support? d) Why is this a problem? e) Why are companies moving rapidly into

the 5 GHz band? f) How many non-overlapping channels does the 5 GHz band

support? g) Why is it important that governments to add more bandwidth to the

5 GHz band? h) If you triple channel bandwidth, what happens to the number of

channels in the service band?

NORMAL AND SPREAD SPECTRUM TRANSMISSION

Spread Spectrum Transmission

At the frequencies used by WLANs, there are numerous and severe propagation problems.

In these service bands, regulators mandate the use of a form of transmission called

spread spectrum transmission. Spread spectrum transmission is transmission that uses

far wider channels than transmission speed requires.

Spread spectrum transmission is transmission that uses far wider channels than transmission

speed requires.

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Regulators mandate the use of spread spectrum transmission to minimize propagation

problems—especially multipath interference. (If the direct and reflected signals

cancel out at some frequencies within the band, they will be double at other frequencies

and will average out over a wide enough frequency range.)

In commercial spread spectrum transmission, security is not a benefit. The

military uses spread spectrum transmission for security, but it does so by keeping

certain parameters of its spread spectrum transmission secret. Commercial spread

spectrum transmission must make these parameters publicly known to allow parties to

communicate easily.

In wireless LANs, spread spectrum transmission is used to reduce propagation problems,

not to provide security.

Test Your Understanding

11. a) In Wi-Fi service bands, what type of transmission method is required by

regulators? b) What is the benefit of spread spectrum transmission for business

communication? c) Is spread spectrum transmission done for security reasons in

commercial WLANs?

Licensed and Unlicensed Radio Bands

If two nearby transceivers send at the same frequency, their signals will interfere with

each other. To prevent chaos, governments regulate how radio transmission is used. The

International Telecommunications Union, which is a division of the United Nations,

creates worldwide rules that define service bands and specify how individual radio

service bands are to be used. Individual countries enforce these rules but are given

discretion over how to implement controls.

Licensed Radio Bands In licensed radio bands, transceivers must have a government

license to operate. They also need a license change if they move. Commercial

television bands are licensed bands, as are AM and FM radio bands. Government agencies

control who may have licenses in these bands. By doing so, the government limits

interference to an acceptable level. In some licensed bands, the rules allow mobile

hosts to move about while only central transceivers are regulated. This is the case for

mobile telephones.

Unlicensed Radio Bands However, for companies that have wireless access

points and mobile computers, even the requirement to license central antennas (in

this situation, access points) is an impossible burden. Consequently, the International

Telecommunications Union has created a few unlicensed radio bands. In these bands,

a company can add or drop access points any time it chooses. It can also have as many

wireless hosts as it wishes. All 802.11 Wi-Fi networks operate in these unlicensed

radio bands.

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The downside of unlicensed radio bands is that companies must tolerate interference

from others. If your neighbor sets up a wireless LAN next door to yours, you have

no recourse but to negotiate with him or her over such matters as which channels each

of you will use. At the same time, the law prohibits unreasonable interference by using

illegally high transmission power.

Test Your Understanding

12. a) Do WLANs today use licensed or unlicensed bands? b) What is the advantage

of using unlicensed bands? c) What is the downside?

Implementing Spread Spectrum Transmission

Normal versus Spread Spectrum Transmission As noted earlier in our discussion

of the bandwidth and speed, if you need to transmit at a given speed, you must have

a channel whose bandwidth is sufficiently wide.

To allow as many channels as possible, channel bandwidths in normal radio transmission

are limited to the speed requirements of the user’s signal, as Figure 6-14 illustrates.

For a service that operates at 10 kbps, regulators would allocate only enough channel

bandwidth to handle this speed. Adding more channel bandwidth would not increase

speed. It would be pure waste.

In contrast to normal radio transmission, which uses channels just wide enough

for transmission speed requirements, spread spectrum transmission takes the original

signal, called a baseband signal, and spreads the signal energy over a much broader

channel than is required by the transmission speed.

Licensed Radio Bands

If two nearby radio hosts transmit in the same channel, their signals will interfere

Most radio bands are licensed bands, in which hosts need a license to transmit

The government limits licenses to reduce interference

Television bands, AM radio bands, etc. are licensed

In cellular telephone bands, which are licensed, only the central antennas are licensed, not the mobile

phones

Unlicensed Radio Bands

Some bands are set aside as unlicensed bands

Hosts do not need to be licensed to be turned on or moved

802.11 Wi-Fi operates in unlicensed radio bands

This allows access points and hosts to be moved freely

However, there is no legal recourse against interference from other nearby users

Your only recourse is to negotiate

At the same time, you may not cause unreasonable interference by transmitting at illegally high power

FIGURE 6-13 Licensed and Unlicensed Radio Bands (Study Figure)

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Orthogonal Frequency Division Multiplexing There are several spread spectrum

transmission methods. The 802.11 Working Group’s current standards all use

orthogonal frequency division multiplexing (OFDM), which Figure 6-15 illustrates.

In OFDM, each broadband channel is divided into many smaller subchannels

called subcarriers. OFDM transmits part of a frame in each subcarrier. OFDM sends

data redundantly across the subcarriers, so if there is impairment in one or even a few

subcarriers, all of the frame will usually still get through.

Why use subcarriers instead of simply spreading the signal over the entire channel?

The problem is that sending data over a very wide channel reliably is very difficult.

It is much easier to send many slow signals in many small subcarriers.

Test Your Understanding

13. a) In normal radio operation, how does channel bandwidth relate to the bandwidth

required to transmit a data stream of a given speed? b) How does this change in

Note: Height of box indicates bandwidth of channel

Channel bandwidth

required for signal speed

Normal Radio: Transmission:

Bandwidth is

no wider than required

for the signal's speed

Spread Spectrum

Transmission:

Channel bandwidth is

much wider than required

for the signal's speed

Commercial spread spectrum transmission reduces certain propagation effects,

especially multipath interference

Commercial spread spectrum transmission does not provide security as a

military spread spectrum transmission does

FIGURE 6-14 Normal Radio Transmission and Spread Spectrum Transmission

Subcarrier 1 (part of frame)

Subcarrier 2 (another part of frame)

Subcarrier 3 (yet another part of frame)

Bandwidth of

Spread Spectrum Channel

Subcarriers are subchannels

More Subcarriers

FIGURE 6-15 Orthogonal Frequency Division Multiplexing (OFDM)

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spread spectrum transmission? c) What spread spectrum transmission method

dominates today? d) Why does it use subcarriers instead of simply spreading the

data over the entire channel?

802.11 WLAN OPERATION

As Figure 6-16 shows, an 802.11 Wi-Fi LAN typically connects a small number

of mobile devices to a large wired Ethernet LAN because the servers and Internet

access routers that mobile hosts need to use usually are on the wired LAN.3 In 802.11

terminology, the wired Ethernet LAN to which access points connect is a distribution

system (DS).

The wired LAN to which access points connect is a distribution system (DS).

Test Your Understanding

14. In Figure 6-16, what is the distribution system?

Wireless Access Points

When a wireless host wishes to send a frame to a server, it transmits the frame to a wireless

access point.

3 There is a rarely used 802.11 ad hoc mode, in which no wireless access point is used. In ad hoc mode, computers

communicate directly with other computers without using an access point. (In contrast, when an access point

is used, this is called 802.11 infrastructure mode.) In addition, 802.11 can create point-to-point transmission

over longer distances than 802.11 normally supports. This approach, which normally is used to connect nearby

buildings, uses dish antennas.

Distribution System

(Wired Ethernet LAN)

Notebook

Client

Radio

Access Transmission

Point A

Removes packet from

incoming frame,

places it in ongoing frame

Ethernet

Switch

Server

Needed by

Client

Packet

802.11 Frame

Containing Packet

Packet

802.3 Frame

Containing Packet

802.3 Frame

Containing Packet

Packet

Notebook client sends

a packet to the server

on the distribution

system (wired LAN)

FIGURE 6-16 Typical 802.11 Wi-Fi Operation

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As Figure 6-16 shows, when a wireless host transmits to a server on the wired

LAN, it puts the packet in an 802.11 frame.4 An 802.11 frame cannot travel over the 802.3

LAN. Wi-Fi has an entirely different frame organization, and Ethernet switches have no

idea how to handle 802.11 frames. To address this problem, the access point removes

the packet from the 802.11 frame and places the packet in an 802.3 Ethernet frame. The

access point sends this 802.3 frame to Ethernet network, which delivers the frame to

the server. Later, when the server replies, the wireless access point receives the 802.3

frame, removes the packet from the Ethernet frame, and forwards the packet to the

wireless host in a Wi-Fi frame.5

The packet goes all the way from the wireless host to a server. The 802.11 frame travels

only between the wireless host and the wireless access point. The 802.3 frame travels

only between the wireless access point and the server.

Test Your Understanding

15. a) Why must an access point remove an arriving packet from the frame in which

the packet arrives and place the packet in a different frame when it sends the

packet back out?

Basic Service Sets (BSSs)

We need to introduce a bit of jargon at this point. First, a basic service set (BSS) consists

of an access point and the wireless hosts it serves. In Figure 6-17, there are two

BSSs. The basic service set of Access Point A has two wireless hosts, while the BSS of

Access Point B has one. Of course, most BSSs serve many more wireless hosts.

A basic service set (BSS) consists of an access point and the wireless hosts it serves

The access point in a BSS has an identifier called the service set identifier (SSID).

(Note that the term basic is not in the name.) Wireless hosts must know the SSID to associate

with the access point. Fortunately, this information is very easy to learn.

Test Your Understanding

16. a) What is a BSS? (Do not just spell out the acronym.) b) What is an SSID? (Do not

just spell out the acronym.) c) Does the access point have an SSID? d) Why must

wireless devices know the access point’s SSID?

4 802.11 frames are much more complex than 802.3 Ethernet frames. Much of this complexity is needed to counter

wireless propagation problems.

5 This sounds like what a router does. However, a router can connect any two single networks. Access points

are limited to connecting 802.3 and 802.11 networks.

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Extended Service Sets (ESSs), Handoffs, and Roaming

If a mobile host travels too far from a wireless access point, its signal will become too

weak to reach the access point. However, if there is a closer access point, the host can

be handed off to that access point for service. In WLANs, the ability to use handoffs is

also called roaming.6

Roaming requires that both access points belong to the same extended service set. An

extended service set (ESS) is a group of BSSs that 1) are connected to the same distribution

system and 2) in which all access points have the same SSID.

An extended service set (ESS) is a group of BSSs that 1) are connected to the same distribution

system and 2) in which all access points have the same SSID.

We said earlier in this section that one function of access points is to work together

to coordinate service, and we gave roaming as an example of this. In roaming, the two

access points involved have to coordinate the handoff. They do this by communicating

over the distribution system. Specifically, they coordinate via 802.11r messages, which

are nicely named because they deal with roaming.

6 In cellular telephony, which we will see in Chapter 10, the terms handoff and roaming mean different things.

Large Wired LAN

Distribution System (DS)

Access

Point A

(SSID = abc)

Basic

Service

Set

(BSS)

Basic

Service

Set

(BSS)

Extended Service Set (ESS)

Access

Point B

(SSID = abc)

Roaming/

Handoff

A basic service set (BSS) is an

access point and its wireless hosts.

Service set ID (SSID) identifies an

access point

Extended service set (ESS) is a

group of BSSs with the same SSID

that connect via a distribution

system. (In this case, SSID = abc.)

Traveling hosts can be handed off

(roam) to a different BSS in the

same ESS.

FIGURE 6-17 Basic Service Sets, Extended Service Set, Handoff, and Roaming

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Access points also need to contact one another via the distribution system. In roaming,

they coordinate using the 802.11r protocol.

Test Your Understanding

17. a) What is a handoff in 802.11? b) What is the relationship between handoffs and

roaming in Wi-Fi? c) What is an ESS? (Do not just spell out the abbreviation.) d) What

characteristics do all access points in an ESS share? e) How can access points communicate

with each other? f) What is the purpose of the 802.11r standard?

Media Access Control

The access point and all of the wireless hosts it serves transmit and receive in a

single channel. Figure 6-18 shows that if two devices transmit in the same channel

at the same time, their signals will interfere with each other. When a wireless

host or the access point transmits, all other devices must wait. As the number of

hosts served by an access point increases, individual throughput falls because of

this waiting. The box “Media Access Control” discusses how media access control

(MAC) methods govern when hosts and access points may transmit so that collisions

are avoided.7

7 Yes, this is where the term MAC address comes from. Conceptually, Media Access Control is a sublayer of

the data link layer. It applies to Ethernet, Wi-Fi, and other 802.11 standards. Addresses are defined at this layer

so that all 802.11 standards use EUI-48 addresses.

Access

Point

Channel Sharing

The access point and all the hosts it serves transmit in a

single channel. If two devices transmit at the same time, their

signals will collide, becoming unreadable.

Media Access Control (MAC)

MAC methods govern when devices may transmit so that only

one device transmits at a time.

Collision! Laptop

FIGURE 6-18 Hosts and Access Points Transmit on a Single Channel

ISBN 1-323-07906-8

Business Data Networks and Security, Tenth Edition, by Raymond R. Panko and Julia L. Panko. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.

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Media access control (MAC) methods govern when hosts and access points may transmit

so that collisions can be avoided.

The access point and all of the wireless hosts it serves transmit and receive in a single

channel. When a wireless host or the access point transmits, all other devices must wait.

Test Your Understanding

18. All wireless hosts and the access point that serves them transmit on the same channel.

a) What problem does this cause? b) How does media access control address this

problem? c) Does media access control apply to wireless hosts, access points, or both?

BOX 1

Media Access Control (MAC)

The 802.11 standard has two mechanisms for media access control. The first, CSMA/CA+ACK, is

mandatory. Access points and wireless hosts must support it. The second, RTS/CTS, is optional.8

CSMA/CA+ACK Media Access Control

The mandatory method is Carrier Sense Multiple Access with Collision Avoidance and

Acknowledgement, which is mercifully shortened to CSMA/CA+ACK.

Carrier sense (CS) means to listen to (sense) traffic (the carrier, in radio parlance). Multiple

access (MA) means that this method uses listening to control how multiple hosts can access the

network to transmit. Quite simply, if another device is transmitting, the wireless host or access

point does not transmit.

Collision avoidance (CA) means that the method attempts to avoid two devices transmitting

at the same time. Most obviously, if one device has been sending for some time, two or

more others may be waiting to send. If they both send as soon as the current sender stops, they

will both transmit at the same time. This will cause a collision. Collision avoidance adds a random

delay time to decide which device may transmit first. This works, but it is inefficient because it

adds dead time when no one is transmitting.

ACK means that if the receiver receives a message correctly, it immediately sends an

acknowledgment to the sender, not waiting at all. This is another reason to require stations to

delay before sending when a sender stops transmitting.

If the sender does not receive an ACK, it retransmits the frame. Sending acknowledgments

and retransmissions makes 802.11 Wi-Fi transmission reliable because it provides both

error detection and error correction. CSMA/CA+ACK is the only reliable transmission method we

will see in this book other than TCP. Most early DLL protocols were reliable because transmission

then was unreliable, even in wired networks. Under these circumstances, error correction at the

data link layer made sense. This is no longer true today generally. Wired transmission protocols

such as Ethernet are unreliable. Doing error correction is simply not worth the effort when transmission

errors are rare. We have seen that wireless transmission, however, is encumbered with

propagation problems, and lost or damaged frames are far too common. It makes sense under

these conditions to make 802.11 (and many other wireless protocols) reliable.

8 Actually, if you have even a single host with older 802.11b equipment connected to an access point, RTS/CTS

becomes mandatory. However, 802.11b wireless hosts are almost never encountered anymore.

(continued)

ISBN 1-323-07906-8

Business Data Networks and Security, Tenth Edition, by Raymond R. Panko and Julia L. Panko. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.

232􀀁 􀀤􀁉􀁂􀁑􀁕􀁆􀁓􀀁􀀗􀀁 􀁲􀀁 􀀸􀁊􀁓􀁆􀁍􀁆􀁔􀁔􀀁􀀭􀀢􀀯􀁔􀀁􀀪

Thanks to CSMA/CA+ACK, 802.11 is a reliable protocol.

CSMA/CA+ACK works well, but it is inefficient. Waiting before transmission wastes valuable

time. Sending ACKs also is time consuming. Overall, an 802.11 LAN can only deliver throughput

(actual speed) of about half the rated speed of its standard—that is, the speed published in the

standard.

Test Your Understanding

19. a) What does CS mean? (Do not just spell out the abbreviation.) b) How is carrier sensing

used in multiple access? c) Why is CA desirable? d) Does a frame’s receiver transmit an ACK

immediately or after a random delay? e) Is CSMA/CA+ACK reliable or unreliable? f) Why

was 802.11 made reliable? g) Is CSMA/CA+ACK efficient?

Request to Send/Clear to Send (RTS /CTS)

Although CSMA/CA+ACK is mandatory, there is another control mechanism called request to

send/clear to send (RTS/CTS). Figure 6-20 illustrates RTS/CTS. As noted earlier, the RTS/CTS

(continued)

Carrier Sense Multiple Access with Collision Avoidance and Acknowledgement

Mandatory for 802.11 Wi-Fi Operation

Carrier Sensing with Multiple Access

Sender listens for traffic (senses the carrier)

If another device is transmitting, it waits

This controls access by multiple devices that must not transmit simultaneously

Collision Avoidance

When the current sender stops, two or more waiting devices may immediately want to transmit

This will cause a collision

Instead, the devices must wait a randomized amount of time before sending

This usually avoids collision, but it is inefficient