com nuter
`
`A TOP-DOWN APPROACH
`FEATURING THE INTERNET
`
`
`
`networ ing
`
`third edition
`
`James F. Kurose
`
`Keith W. Ross
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`1of63
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`Fed
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`Ex Exhibit 1020
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`FedEx v. N
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`|PR2017-00741
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`THIRD EDITION
`
`Computer Networking
`A Top-Down Approach Featuring the Internet
`
`James F. Ku rose
`University of Massachusetts, Amherst
`0
`
`Keith W. Ross
`
`Polytechnic University, Brooklyn
`
`AW
`
`PEARSON
`fl.
`Addison
`Wesley
`
`Boston SanFrancisco NewYork
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`1 ;_
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`Managing Editor
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`Library of Congress Cataloging-in-Publication Data
`Kurose, James F
`
`Computer networking : a top-down approach featuring the Internet / James F. Kurose,
`Keith W. Ross.—3rd ed.
`p. cm.
`
`Includes bibliographical references and index.
`ISBN 0-321-22735-2
`
`1. Internet. 2. Computer networks I. Ross, Keith W., 1956— 11. Title.
`
`TK5105.875.157 K88 2005
`
`004.67'8—dc22
`
`Copyright © 2005 by Pearson Education, Inc.
`
`2004044284
`
`All rights reserved. No part of this publication may be reproduced, stored in a retrieval
`system, or transmitted, in any form or by any means, electronic, mechanical, photo-
`copying, recording, or otherwise, without the prior written permission of the publisher.
`Printed in the United States of America.
`
`ISBN 0-321-22735-2
`
`123456789lO-CRW—0807060504
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`

`Wireless and
`
`Mobile
`
`Networks
`
`
`
`
`In the telephony world, the past 10 years have arguably been the decade of cellular
`telephony. The number of worldwide mobile cellular subscribers increased from 34
`million in 1993 to more than 1 billion in2003,’ with the number of cellular sub-
`scribers now surpassing the number of main telephone lines [ITU StatistiCs 2004].
`The many advantages of cell phones are evident to all—anywhere, anytime, unteth~
`ered access to the global telephone network via a highly portable lightweight device.
`With the advent of laptops, palmtops, PDAs and their promise of anywhere, any-
`time, untethered access to the global Internet, is a similar explosion in the use of
`wireless Internet devices just around the corner?
`Regardless of the future growth of wireless Internet devices, it's already clear
`that wireless networks and the mobility-related services they enable are here to stay.
`From a networking standpoint, the challenges posed by these networks, particularly
`at the data link and network layers, are so different from traditional wired computer
`networks that an individual chapter devoted to the study of wireless and mobile net,-
`works (i.e., this chapter) is appropriate.
`We’ll begin this chapter with a discussion of mobile users, Wireless links and
`networks, and their relationship to the larger (typically wired) networks to which
`they connect. We’ll draw a distinction between the challenges posed by the wireless
`nature of the communication links in such networks, and by the mobility that these
`wireless links enable. Making this important distinction—between wireless and
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`mobility—will allow us to better isolate, identify, and master the key concepts in
`each area. Note that there are indeed many networked environments in which the
`network nodes are wireless but not mobile (e. g., wireless home or office networks
`with stationary workstations and large displays), and limited forms of mobility that
`do not require wireless links (e. g., a worker who uses a wired laptop at home, shuts
`down the laptop, drives to work, and attaches the laptop to the company’s wired net-
`work). Of course, many of the most exciting networked environments are those in
`which users are both wireless and mobile—for example, a scenario in 'which a
`mobile user (say in the back seat of car) maintains a voice-over-IP call and multiple
`ongoing TCP connections while racing down the autobahn at 160 kilometers per
`hour. It is here, at the intersection of wireless and mobility, that we’ll find the most
`interesting technical challenges!
`We’ll begin by first illustrating the setting in which we’ll consider wireless com-
`munication and mobility—a network in which wireless (and possibly mobile) users are
`connected into the larger network infrastructure by a wireless link at the network’s
`edge. We’ll then consider the characteristics of this wireless link in Section 6.2. We
`include a brief introduction to Code Division Multiple Access (CDMA), a shared—
`medium access protocol that is often used in wireless networks, in Section 6.2. In Sec-
`tion 6.3, we’ll examine the link-level aspects of the IEEE 802.11 (Wi—Fi) wireless LAN
`standard in some depth; we’ll also say a few words about Bluetooth. In Section 6.4 we
`provide an overview of cellular Internet access, including the emerging 3G cellular
`technologies that provide both voice and high—speed Internet access. In Section 6.5,
`we’ll turn our attention to mobility, focusing on the problems of locating a mobile user,
`routing to the mobile user, and “handing off” the mobile user who dynamically moves
`from one point of attachment to the network to another. We’ll examine how these
`mobility services are implemented in the mobile IP standard and in GSM, in Sections
`6.6 and 6.7, respectively. Finally, we’ll consider the impact of wireless links and mobil-
`ity on transport-layer protocols and networked applications in Section 6.8.
`
`6 . 1 Introduction
`
`phone, or desktop computer. The hosts themselves may or may not be mobile.
`
`Figure 6.1 shows the setting in which we’ll consider the topics of wireless data com-
`munication and mobility. We’ll begin by keeping our discussion general enough to
`cover a wide range of networks, including both wireless LANs such as IEEE 802.11
`and cellular networks Such as a 3G network; we’ll dive down into a more detailed
`
`discussion of specific wireless architectures in later sections. We can identify the
`following elements in a wireless network:
`
`9 Wireless hosts. As in the case of wired networks, hosts are the end-system
`devices that run applications. A wireless host might be a laptop, palmtop, PDA,
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`o WIRELESS AND MOBILE NETWORKS
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`6.1
`
`.
`
`INTRODUCTION
`
`505
`
`
`
`CASE'HIISTORY
`
`PUBLIC Wl-Fl ACCESS: COMING SOON TO A CORNER NEAR YOU?
`
`Only five years ago, wireless computer networks were somewhat of an oddity. Although
`massive investment was pouring into licensing radio spectrum for 36 systems (see Case~
`History: 3G Cellular Mobile Versus Wireless LANS), 30 systems were (and still are) only
`at early stages of deployment. At the time, a few early adopters were beginning to try
`out the iust—standardized IEEE 802.] 1 wireless LAN technology. What a difference five
`years can make] Today many corporations, universities, and homes have their own wire-
`less lEEE 802.] l LANs. Even more remarkably, the number of wireless hot spots—public
`locations where users can find 802.l 1 wireless access—is rapidly expanding. The
`Gartner Group estimates there were 71,000 public hot spots in 2003, a nearly fifty-fold
`increase since 200i. In the United States, eateries such as Starbucks and McDonalds
`offer Wi—Fi access in a number of locations. In New York City, Verizon Communications
`has located Wi—Fi access points at over one thousand of its public phone booths and has
`connected the phone booths to the lnternet [Verizon 2004], providing Wi-Fi access to
`passersby and nearby businesses. In early 2004, T-Mobile [f-Mobile 2004] provided
`more that 4,000 public Wi-Fi hotspots in locations such as airports, restaurants, and
`bookstores. A recent startup, Cometa, announced plans in 2003 to set up 20,000 com-
`mercial Wi-Fi hotspots in 50 metropolitan areas by 2005. With this level of activity, the
`dream of nearly ubiquitous, anytime, untethered access to the global Internet may be
`closer than we thinkl
`
`
`
`6 Wireless links. A host connects to a base station (defined below) or to another
`wireless host through a wireless communication link. Different wireless link
`technologies have different transmission rates and can transmit over different
`distances. Figure 6.2 shows a few of the key characteristics of the more popular
`wireless link standards. We’ll cover these standards later in the first half of this
`
`chapter; we’ll also consider other wireless link characteristics (such as their bit
`error rates and their causes) in Section 6.2.
`
`9 In Figure 6.], wireless links connect hosts located at the edge of the network into
`the larger network infrastructure. We hasten to add that wireless links are also
`sometimes used within a network and to connect routers, switches, and other net-
`
`work equipment. However, our focus in this chapter will be on the use of wire-
`less communication around the edges of the network, as it is here that many of
`the most exciting technical challenges, and most of the growth, are occurring.
`
`Q Base station. The base station is a key part of the wireless network infrastruc-
`ture. Unlike the wireless host and wireless link, a base station has no obvious
`
`counterpart in a wired network. A base station is responsible for sending and
`receiving data (e.g., packets) to and from a wireless host that is associated with
`
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`CHAPTER 6
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`° WIRELESS AND MOBILE NETWORKS
`
`42%ther .
`infrastructure
`
`Coverage area
`
`Figure 6.1
`
`6 Elements of a wireless network
`
`that base station. A base station will often be responsible for coordinating the
`transmission of multiple wireless hosts with which it is associated. When we say
`a wireless host is “associated” with a base station, we mean that (1) the host is
`within wireless communication distance of the base station, and (2) the host uses
`
`that base station to relay data between it (the host) and the larger network. Cell
`towers in cellular networks and access points in an 802.11 wireless LANs are
`examples of base stations.
`
`In Figure 6.1, the base station is connected to the larger network (i.e., the Inter-
`net, corporate, or home network, or telephone network), thus functioning as a
`link-layer relay between the wireless host and the rest of the world with which
`the host communicates.
`
`Hosts associated with a base station are often referred to as operating in infra-
`structure mode, since all traditional network services (e. g., address assignment
`
`
`
`
`. l...)
`g Wireless host in motion
`
`and routing) are provided by the network to which a host is connected via the
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`6.1
`
`-
`
`INTRODUCTION
`
`507
`
`base station. In ad hoc networks, wireless hosts have no such infrastructure with
`which to connect. In the absence of such infrastructure, the hosts themselves
`must provide for services such as routing, address assignment, DNS-like name
`translation, and more. In this book, we’ll focus our attention primarily on infra-
`structure-mode networks.
`
`When a mobile host moves beyond the range of one base station and into the
`range of another, it will change its point of attachment into the larger network
`(i.e., change the base station with which it is associated)—a process referred to
`as handoff. Such mobility raises many challenging questions. If a host can
`move, how does one find its current location in the network so that data can be
`
`forwarded to the mobile host? How is addressing performed, given that a host
`can be in one of many possible locations? If the host moves during a TCP con-
`nection or phone call, how is data routed so that the connection continues unin-
`terrupted? These and many (many!) other questions make wireless and mobile
`networking an area of exciting networking research.
`
`0 Network infrastructure. This is the larger network with which a wireless host
`may wish to communicate.
`
`Let’s now dig deeper into the technical challenges that arise in wireless and mobile
`networks. We’ll begin by first considering the individual wireless link: deferring our
`discussion of mobility until later in this chapter.
`
`54 Mbps
`
`gonna-,9} 1
`
`;
`
`_
`
`'Tf'j'jir;
`
`5—11Mbps
`
`1 Mbps
`
`384 Kbps
`
`56 Kbps
`
`M '
`
`'1
`
`H
`
`i
`
`
`
`Indoor
`
`10—30m
`
`Outdoor
`
`50—200m
`
`Mid range
`outdoor
`200m—4Km
`
`Long range
`outdoor
`5Km—20Km
`
`Figure 6.2 0
`
`Link characteristics of selected wireless network standards
`
`80f63
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`CHAPTER 6
`
`o WIRELESS AND MOBILE NETWORKS
`
`6.2
`
`Wireless Links and Network Characteristics
`
`Let’s begin by considering a simple wired network, say a home network, with a
`wired Ethernet switch (see Section 5.6) interconnecting the hosts. If we replace the
`wired Ethernet with a wireless 802.11 network, a wireless NIC card would replace
`the wired Ethernet cards at the hosts, and an access point would replace the Ethernet
`switch, but virtually no changes would be needed at the network layer or above.
`This suggests that we focus our attention on the link layer when looking for impor-
`tant differences between wired and wireless networks. Indeed, we can find a num-
`
`ber of important differences between a wired link and a wireless link:
`
`destination, B. This is shown in Figure 6.3(a). A second scenario that results in
`
`0 Decreasing signal strength. Electromagnetic radiation attenuates as it passes
`through matter (e.g., a radio signal passing through a wall). Even in free space,
`the signal will disperse, resulting in decreased signal strength (sometimes
`referred to as path loss) as the distance between sender and receiver increases.
`
`0 Interference from other sources. Radio sources transmitting in the same fre-
`quency band will interfere with each other. For example, 2.4 GHz wireless
`phones and 802. 1 lb wireless LANs transmit in the same frequency band. Thus,
`the 802.11b wireless LAN user talking on a 2.4GHz wireless phone can expect
`that neither the network nor the phone will perform particularly well. In addition
`to interference from transmitting sources, electromagnetic noise within the envi-
`ronment (e.g., a nearby motor, a microwave) can result in interference.
`
`o Multipath propagation. Multipath propagation occurs when portions of the
`electromagnetic wave reflect off objects and the ground, taking paths of different
`lengths between a sender and receiver. This results in the blurring of the received
`Signal at the receiver. Moving objects between the sender and receiver can cause
`multipath propagation to change over time.
`
`The discussion above suggests that bit errors will be more common in wireless links
`than in wired links. For this reason, it is perhaps not surprising that wireless link
`protocols (such as the 802.11 protocol we’ll examine in the following section)
`employ not only powerful CRC error detection codes, but also link-level ARQ pro-
`tocols that retransmit corrupted frames.
`A higher and time-varying bit error rate are not the only differences between a
`wired and wireless link. Recall that in the case of wired broadcast links, all nodes
`receive the transmissions from all other nodes. In the case of wireless links, the situ-
`
`ation is not as simple, as shown in Figure 6.3. Suppose that Station A is transmitting
`to Station B. Suppose also that Station C is transmitting to Station B. With the so-
`called hidden terminal problem, physical obstructions in the environment (for
`example, a mountain or a building) may prevent A and C from hearing each other’s
`transmissions, even though A’s and C’s transmissions are indeed interfering at the
`
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`
`
`6.2
`
`. WIRELESS LINKS AND NETWORK CHARACTERISTICS
`
`509
`
`undetectable collisions at the receiver results from the fading of a signal’s strength
`as it propagates through the wireless medium. Figure 6.3(b) illustrates the case
`where A and C are placed such that their signals are not strong enough to detect each
`other’s transmissions, yet their transmissions are strong enough to interfere with
`each other at station B. As we’ll see in Section 6.3, the hidden terminal problem and
`fading make multiple access in a wireless network considerably more complex than
`in a wired network.
`
`6.2.1 CDMA
`
`Recall from Chapter 5 that when hosts communicate over a shared medium, a pro-
`tocol is needed so that the signals sent by multiple senders do not interfere at the
`receivers. In Chapter 5 we described three classes of medium access protocols:
`channel partitioning, random access, and taking turns. Code division multiple access
`(CDMA) is yet a fourth type of a shared-medium access protocol, one that is preva-
`lent in wireless LAN and cellular technologies. Because CDMA is so important in
`the wireless world, we’ll take a quick look at CDMA now, before getting into spe—
`cific wireless access technologies in the subsequent sections.
`In a CDMA protocol, each bit being sent is encoded by multiplying the bit by a
`signal (the code) that changes at a much faster rate (known as the chipping rate)
`than the original sequence of data bits. Figure 6.4 shows a simple, idealized CDMA
`encoding/decoding scenario. Suppose that the rate at which original data bits reach
`the CDMA encoder defines the unit of time; that is, each original data bit to be
`
`transmitted requires a one-bit slot time. Let dl. be the value of the data bit for the ith
`
`0
`
`
`
`Signalstrength
`
`
`
`3)\EE;
`
`ifl
`
`Location
`
`Figure 6.3 0 Hidden ierminoI problem (o) and Fading (b)
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`CHAPTER 6
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`. WIRELESS AND MOBILE NETWORKS
`
`bit slot. For mathematical convenience, we represent a data bit with a 0 value as —1.
`Each bit slot is further subdivided into M mini-slots; in Figure 6.4, M = 8, although
`in practice M is much larger. The CDMA code used by the sender consists of a
`sequence of M values, cm, m = 1, .
`.
`.
`, M, each taking a +1 or —1 value. In the exam-
`ple in Figure 6.4, the M—bit CDMA code being used by the sender is (1, l, l, —1, l,
`—1, —l, —1).
`To illustrate how CDMA works, let us focus on the ith data bit, di. For the mth
`mini—slot of the bit-transmission time of dv the output of the CDMA encoder, Zm, is
`the value of dr multiplied by the mth bit in the assigned CDMA code, cm:
`
`Sender
`
`Channel output 2,)”,
`
`
`
`Time slot 1
`.-
`i channel output
`
`!
`
`Time slot 0
`channel output
`
`
`
`
`
`
`decoding
`
`Time slot 1
`l received input
`
` |
`1
`Time slot 0
`l
`received input
`
`Figure 6.4 o A simple CDMA example: sender encoding, receiver
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`

`6.2
`
`- WIRELESS LINKS AND NETWORK CHARACTERISTICS
`
`5'I'I
`
`
`
`(6.1)
`
`In a simple world, with no interfering senders, the receiver would receive the
`encoded bits, Z, m, and recover the original data bit, (1,, by computing:
`
`1 M
`d,- = M 2 z,_,,,
`m=1
`
`- cm
`
`(6.2)
`
`The reader might want to work through the details of the example in Figure 6.4 to
`see that the original data bits are indeed correctly recovered at the receiver using
`Equation 6.2.
`The world is far from ideal, however, and as noted above, CDMA must work in
`the presence of interfering senders that are encoding and transmitting their data
`using a different assigned code. But how can a CDMA receiver recover a sender’s
`original data bits when those data bits are being tangled with bits being transmitted
`by other senders? CDMA works under the assumption that the interfering transmit-
`ted bit signals are additive. This means, for example, that if three senders send a 1
`value, and a fourth sender sends a —1 value during the same mini-slot, then the
`received signal at all receivers during that mini-slot is a 2 (since 1 + 1 + l — 1 = 2).
`In the presence of multiple senders, sender s computes its encoded transmissions,
`Zim, in exactly the same manner as in Equation 6.1. The value received at a receiver
`during the mth mini-slot of the ith bit slot, however, is now the sum of the transmit-
`ted bits from all N senders during that mini-slot:
`N
`* _
`.r
`Zi,m " ZZi,mr=l
`
`Amazingly, if the senders’ codes are chosen carefully, each receiver can recover the
`data sent by a given sender out of the aggregate signal simply by using the sender’s
`code in exactly the same manner as in Equation 6.2:
`
`.
`1 M
`d- =— z.
`-
`l M Z z,m C
`m=1
`
`6.3
`
`(
`
`)
`
`Figure 6.5 illustrates a two-sender CDMA example. The M—bit CDMA code being
`used by the upper sender is (1, 1, 1, —1, 1,—1,—1,—1), while the CDMA code being
`used by the lower sender is (1, —1, 1, 1, 1,—1, 1, 1). Figure 6.5 illustrates a receiver
`recovering the original data bits from the upper sender. Note that the receiver is able
`to extract the data from sender 1 in spite of the interfering transmission from sender 2.
`Recall our cocktail analogy from Chapter 5. A CDMA protocol is similar to
`having partygoers speaking in multiple languages; in such circumstances humans
`are actually quite good at looking into the conversation in the language they
`understand, while filtering ou_t the remaining conversations. We see here that
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`0 WIRELESS AND MOBILE NETWORKS
`
`Senders
`
`
`
`Channel, 25‘,"
`
`
`
`Time slot 0
`
`Time slot 1
`
`
`i
`received input
`received input
`
`
`
`
`
`Figure 6.5 O A two-sender CDMA example
`
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`

`6.3
`
`- WI-Fl: 802.11 WIRELESS LANS
`
`5'l3
`
`CDMA is a partitioning protocol in that it partitions the codespace (as opposed to
`time or frequency) and assigns each node a dedicated piece of the codespace.
`Our discussion here of CDMA is necessarily brief; in practice a number of dif-
`ficult issues must be addressed. First, in order for the CDMA receivers to be able to
`
`extract a particular sender’s signal, the CDMA codes must be carefully chosen. Sec-
`ond, our discussion has assumed that the received signal strengths from various
`senders are the same; in reality this can be difficult to achieve. There is a consider-
`able body of literature addressing these and other issues related to CDMA; see
`[Pickholtz 1982; Viterbi 1995] for details.
`
`6.3 Wi-Fi: 802.11 Wireless LANs
`
`Pervasive in the workplace, the home, educational institutions, cafe’s, airports, and
`street corners, wireless LANs are now one of the most important access network
`technologies in the Internet today. Although many technologies and standards for
`wireless LANs were developed in the 1990s, one particular class of standards has
`clearly emerged as the winner: the IEEE 802.11 wireless LAN, also known as Wi-
`Fi. In this section, we’ll take a close look at 802.11 wireless LANs, examining the
`802.11 frame structure, the 802.11 medium access protocol, and the internetworking
`of 802.11 LANs with wired Ethernet LANs.
`
`There are several 802.11 standards for wireless LAN technology, including
`802.11b, 802.11a, and 802.11g. Table 6.1 summarizes the main characteristics of
`‘these standards. As of this writing (spring 2004), the 802.llb wireless LANs are by
`far the most prevalent. However, 802.11a and 802.11g products are also widely
`available, and these higher-speed wireless LANs should enjoy significant deploy—
`ment in the coming years.
`The three 802.11 standards share many characteristics. They all use the same
`medium access protocol, CSMA/CA, which we’ll discuss shortly. All three use the
`same frame structure for their link-layer frames as well. All three standards have the
`
`
`
`
`Standard
`Frequency Range
`Data Rate
`
`
`802.] lb
`2.4-2.485 GHz
`up to H Mbps
`
`
`802.] la
`5.1-5.8 GHz
`up to 54 Mbps
`
`
`
`802.] lg Up to 54 Mbps 2.4-2.485 GHz
`
`
`
`Table 6.1
`
`O Summary of IEEE 802.11 Standards
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`' WIRELESS AND MOBILE NETWORKS
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`ability to reduce their transmission rate in order to reach out over greater distances.
`And all three standards allow for both “infrastructure mode and “ad hoc mode,” as
`
`we’ll also shortly discuss. However, as shown in Table 6.1, the three standards have
`some major differences at the physical layer.
`The 802.11b wireless LAN has a data rate of 11 Mbps, which is more than suf-
`ficient for most home networks with broadband cable or DSL Internet access.
`
`802.11b LAN5 operate in the unlicensed frequency band of 2.4—2.485 GHZ, compet-
`ing for frequency spectrum with 2.4 GHz phones and microwave ovens. 802.11a
`wireless LANs can run at significantly higher bit rates, but do so at higher frequen-
`cies. By operating at a higher frequency, however, 802.11a LANs have a shorter
`transmission distance for a given power level and suffer more from multipath prop-
`agation. 802.11g LANs, operating in the same lower frequency band as 802.11b yet
`with the higher-speed transmission rates of 802.11a, should allow users to eat their
`cake and have it too.
`
`6.3.1 The 802.11 Architecture
`
`Figure 6.6 illustrates the principal components of the 802.11 wireless LAN architec-
`ture. The fundamental building block of the 802.11 architecture is the basic service
`set (BSS). A BSS contains one or more wireless stations and a central base station,
`known as an access point (AP) in 802.11 parlance. Figure 6.6 shows the AP in each
`of two BSSs connecting to an interconnection device (such as a hub, switch or
`router), which in turn leads to the Internet. In a typical home network, there is one
`AP and one router (often packaged with a cable or ADSL modem, all in the same
`box) that connects the BSS to the Internet.
`As with Ethernet devices, each 802.11 wireless station has a 6-byte MAC
`address that is stored in the firmware of the station’s adaptor (that is, 802.11 network
`interface card). Each AP also has a MAC address for its wireless interface. As with
`Ethernet, these MAC addresses are administered by IEEE and are (in theory) glob-
`ally unique.
`As noted in Section 6.1, wireless LANs that deploy APs are often referred to as
`infrastructure wireless LANs, with the “infrastructure” being the APs along with
`the wired Ethernet infrastructure that interconnect the APs and a router. Figure 6.7
`shows that IEEE 802.11 stations can also group themselves together to form an ad
`hoc network—a network with no central control and with no connections to the
`
`“outside world.” Here, the network is formed “on the fly,” by mobile devices that
`have found themselves in proximity to each other, that have a need to communicate,
`and that find no preexisting network infrastructure in their location. An ad hoc net-
`work might be formed when people with laptops get together (for example, in a con-
`ference room, a train, or a car) and want to exchange data in the absence of a
`centralized AP. There has been tremendous interest in ad hoc networking, as com—
`municating portable devices continue to proliferate. In this section, though, we’ll
`focus our attention on infrastructure wireless LANs.
`
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`6.3
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`- Wl-FI: 802.11 WIRELESS LANS
`
`515
`
`
`
`j. .5)
`
`.r
`
`-.j
`
`Hub, switch
`or router
`
`Internet
`
`
`
`B55 2
`
`Figure 6.6 6
`
`IEEE 802.] 1 LAN architecture
`
`Channels and Association
`
`In 802.11, each wireless station needs to associate with an AP before it can send or
`
`receive 802.11 frames containing network-layer data. Although all of the 802.11
`standards use association, we’ll discuss this topic specifically in the context of IEEE
`802.11b.
`
`BSS
`
`
`
`Figure 6.7 0 An IEEE 802.] l cud hoc network
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`516
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`CHAPTER 6
`
`° WIRELESS AND MOBILE NETWORKS
`
`When a network administrator installs an AP, the administrator assigns a one-
`or two—word Service Set Identifier (SSID) to the access point. (When you “view
`available networks” in Microsoft Windows XP, for example, a list is displayed
`showing the SSID of each AP in range.) The administrator must also assign a chan-
`nel number to the AP. To understand channel numbers, recall that 802.11b operates
`in the frequency range of 2.4 GHZ to 2.485 GHz. Within this 85 Mllz band, 802.11b
`defines 1| partially overlapping channels. Any two channels are non—overlapping if
`and only if they are separated by four or more channels. in particular, the set of
`channels 1, 6, and 11 is the only set of three non-overlapping channels. This means
`that an administrator could create a wireless LAN with an aggregate maximum
`transmission rate of 33 Mbps by installing three 802.11b APs at the same physical
`location, assigning channels 1, 6, and 11 to the APs, and interconnecting each of the
`APs with a switch.
`
`Now that we have a basic understanding of 802.11 channels, let’s describe an
`interesting (and not completely uncommon) situation—that of a Wi-Fi jungle. A Wi-
`Fi jungle is any physical location where a wireless station receives a sufficiently
`strong signal from two or more APs. For example, in many cafés in New York City,
`a wireless station can pick up a signal from numerous nearby APs. One 0|" the APs
`might be managed by the café, while the other APs might be in residential apart—
`ments near the café. Each of these APs would likely be located in a different subnet
`and would have been independently assigned a channel.
`Now suppose you enter such a Wi-Fi jungle with yourportable computer, seek-
`ing wireless Internet access and a blueberry muffin. Suppose there are five APs in
`the jungle. To gain Internet access, your wireless station will need to join exactly
`one of the subnets and hence need to associate with exactly one of the APs. Associ-
`ating means the wireless station creates a virtual wire between itself and the AP.
`Specifically, only the associated AP will send data frames (that is, frames containing
`data, such as a dalagram) to your wireless station, and your wireless station will
`send data frames into the Internet only through the associated AP. But how does
`your wireless station associate with a particular AP? And more fundamentally, how
`does your wireless station know which APs, if any, are out there in the jungle?
`The 802.11 standard requires that an AP periodically send beacon frames, each
`of which includes the AP’s SSID and MAC address. Your wireless station, knowing
`
`cally send a DHCP discovery message (see Section 5.4.3) into the subnet via the
`
`that APs are sending out beacon frames, scans the 11 channels, seeking beacon
`frames from any APs that may be out there (some of which may be transmitting on
`the same channel—it’s a jungle out there!). Having learned about available APs
`from the beacon frames, you (or your wireless host) select one of the APs for
`association. After selecting the AP, your wireless host and the chosen AP dialogue
`with each other using the 802.11 association protocol. If all goes well in this dia-
`logue, your wireless station becomes associated with the selected AP. Implicitly,
`during the association phase, your wireless station is joining the subnet to which the
`selected AP belongs. Just after the association phase, the wireless station will typi-
`
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`6.3
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`. WI-Fl: 802.11 WIRELESS LANS
`
`5'I7
`
`associated AP in order to obtain an IP address in the AP’s subnet. At this point, the
`rest of the Internet now views your computer simply as a host in the AP’s subnet.
`In order to create an association with a particular AP, the wireless station may
`be required to authenticate itself to the AP. 802.11 wireless LANs provide a number
`of alternatives for authentication and access. One approach, used by many compa-
`nies, is to permit access to a wireless network based on a station’s MAC address. A
`second approach, used by many Internet cafes, employs user names and passwords.
`In both cases, the AP typically communicates with an authentication server, relaying
`information between the wireless endpoint station and the authentication server
`using a protocol such as RADIUS [RFC 2138] or DIAMETER [RFC 3588]. Sepa-
`rating the authentication server from the AP allows one authentication server to
`serve many APs, centralizing the (often sensitive) decisions of authentication and
`access within the single server, and keeping AP costs and complexity low. We’ll see
`in Section 8.8.4 that the new IEEE 802.11i protocol defining security aspects of the
`802.11 protocol family takes precisely this approach.
`
`6.3.2 The 802.11 MAC Protocol
`
`Once a wireless station is associated with an AP, it can start sending and receiving
`data frames to and from the access point. But because multiple stations may want to
`transmit data frames at the same time over the same channel, a multiple access pro-
`tocol is needed to coordinate the transmissions. Here, a station is either a wireless
`
`station or an AP. As discussed in Chapter 5 and Section 6.2.1, broadly speaking there
`are four classes of