Matthew S. Gast
`
`ireless
`SAYVO)8<4
`
`O’REILLY*
`
`DELL-1031, Part 1
`10,079,707
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`DELL-1031, Part 1
`10,079,707
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`

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`802.11 Wireless Networks
`The Definitive Guide
`
`

`

`802.11Wireless Networks
`The Definitive Guide
`
`
`
`
`
`
`
`
`Matthew S. Gast
`
`Beijing - Cambridge + Farnham + Kéln + Paris - Sebastopol
`
`O’REILLY*
`+ Taipei
`+ Tokyo
`
`

`

`802.11® Wireless Networks: The Definitive Guide
`by Matthew S. Gast
`
`Copyright © 2002 O’Reilly Media,Inc. All rights reserved.
`Printed in the United States of America.
`
`Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North,
`Sebastopol, CA 95472.
`
`O’Reilly Media, Inc. books may be purchased for educational, business,orsales promotional use. On-
`line editions are also available for mosttitles (safari.oreilly.com). For more information contactour cor-
`porate/institutional sales department: (800) 998-9938 or corporate@oreilly.com.
`
`Editor:
`Production Editor:
`
`Mike Loukides
`Matt Hutchinson
`
`Cover Designer:
`
`Ellie Volckhausen
`
`Printing History:
`
`April 2002:
`
`First Edition.
`
`|
`
`Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered trademarks of
`O'Reilly Media, Inc. 802.11Wireless Networks: The Definitive Guide, the image of a horseshoe bat,
`andrelated trade dress are trademarks of O'Reilly Media, Inc. Manyofthe designations used by
`manufacturers andsellers to distinguish their products are claimed as trademarks. Where those
`designations appearin this book, and O'Reilly Media, Inc. was aware of a trademark claim, the
`designations have been printed in capsorinitial caps.
`802.11® andall 802.11-based trademarks and logos are trademarksorregistered trademarks of IEEE,
`Inc. in the United States and other countries. O'Reilly Media,Inc. is independent of IEEE.
`While every precaution has been takeninthepreparationofthis book, the author and publisher assume
`no responsibility for errors or omissions, or for damages resulting from theuse of the information
`contained herein.
`
`eam This bookuses RepKover’) a durable andflexible lay-flat binding.
`Se
`P
`
`ISBN: 0-596-00183-5
`[M]
`
`[11/04]
`
`
`
`

`

`
`
`CHAPTER 1
`
`Introduction to Wireless Networks
`
`Over the past five years, the world has becomeincreasingly mobile. Asa result, tradi-
`tional ways of networking the world have proven inadequate to meet the challenges
`posed by our new collective lifestyle. If users must be connected to a network by
`physical cables, their movementis dramatically reduced. Wireless connectivity, how-
`ever, poses no suchrestriction and allows a great deal more free movement on the
`part of the network user. As a result, wireless technologies are encroaching on the
`traditional realm of“fixed” or “wired” networks. This change is obvious to anybody
`whodrives on a regular basis. One of the “life and death” challenges to those of us
`whodrive on a regularbasis is the daily gauntletoferratically driven cars containing
`mobile phoneusersin thedriver’s seat.
`Weare on the cusp of an equally profound change in computer networking. Wire-
`less telephony has been successful because it enables people to connect with each
`otherregardless of location. New technologies targeted at computer networks prom-
`ise to do the samefor Internet connectivity. The most successful wireless network-
`ing technology this far has been 802.11.
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`WhyWireless?
`To dive into a specific technology at this point is getting a bit ahead ofthestory,
`though. Wireless networks share several important advantages, no matter how the
`protocols are designed, or even whattypeof data they carry.”
`The most obvious advantage of wireless networking is mobility. Wireless network
`users can connect to existing networks and are then allowed to roam freely. A mobile
`telephone user can drive miles in the course of a single conversation because the
`phone connects the user throughcell towers. Initially, mobile telephony was expen-
`sive. Costs restricted its use to highly mobile professionals such as sales managers
`and important executive decision makers who might need to be reached at a
`moment’s notice regardless of their location. Mobile telephony has proven to be a
`
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`
`
` useful service, however, and nowit is relatively common in the United States and
`
`extremely common among Europeans.’
`Likewise, wireless data networks free software developers from the tethers of an
`Ethernet cable at a desk. Developers can workin thelibrary, in a conference room,in
`the parking lot, or even in the coffee house across the street. As long as the wireless
`users remain within the range of the base station, they can take advantageof the net-
`work. Commonly available equipment can easily cover a corporate campus; with
`some work, more exotic equipment, and favorable terrain, you can extend the range
`of an 802.11 network up to a few miles.
`Wireless networks typically have a great deal of flexibility, which can translate into
`rapid deployment. Wireless networks use a numberof basestations to connectusers
`to an existing network. The infrastructure side of a wireless network, however,
`is
`qualitatively the same whether you are connecting one user or a million users. To
`offer service in a given area, you need basestations and antennas in place. Once that
`infrastructure is built, however, addinga user to a wireless network is mostly a mat-
`ter of authorization. With the infrastructure built, it must be configured to recognize
`and offer services to the new users, but authorization does not require more infra-
`structure. Adding a user to a wireless network is a matter of configuring the infra-
`structure, but it does not involve running cables, punching down terminals, and
`patching in a new jack.t
`Flexibility is an importantattribute for service providers. One of the markets that
`many 802.11 equipment vendors have been chasingis the so-called “hot spot” con-
`nectivity market. Airports andtrainstationsarelikely to have itinerant business trav-
`elers interested in network access during connection delays. Coffeehouses and other
`public gathering spots are social venues in which networkaccess is desirable. Many
`cafes already offer Internet access; offering Internet access over a wireless networkis a
`natural extension of the existing Internet connectivity. While it is possible to serve a
`fluid group of users with Ethernet jacks, supplying access over a wired networkis
`problematic for several reasons. Running cablesis time-consuming and expensive and
`may also require construction. Properly guessing the correct number of cable drops is
`more an art than a science. With a wireless network, though,there is no need to suf-
`fer through construction or make educated (or wild) guesses about demand. A sim-
`ple wired infrastructure connects to the Internet, and then the wireless network can
`
`* While most of mycolleagues, acquaintances, andfamily in the U.S. have mobile telephones,itis still possible
`to be a holdout. In Europe,it seemsasif everybody has a mobile phone—onecabdriver in FinlandI spoke
`with while writing this book took great pride inthe fact that his family of four hadsix mobile telephones!
`t This simple example ignores the challenges of scale. Naturally, if the new users will overloadthe existing
`infrastructure, the infrastructureitself will need to be beefed up. Infrastructure expansion can be expensive
`and time-consuming, especially if it involves legal and regulatory approval. However, mybasic pointholds:
`adding a user to a wireless network canoften be reducedtoa matter of configuration (moving or changing
`bits) while addinga usertoa fixed network requires making physical connections (moving atoms), and mov-
`ing bits is easier than moving atoms.
`
`2
`
`| Chapter 1:
`
`
`Introduction to Wireless Networks
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`

`

`
`
`
`accommodate as manyusers as needed. Although wireless LANs have somewhatlim-
`
`ited bandwidth, the limiting factor in networking a small hotspotis likely to be the
`
`cost of WAN bandwidth to the supporting infrastructure.
`
`
`Flexibility may be particularly important in older buildings because it reduces the
`need for constructions. Once a building is declared historical, remodeling can be par-
`
`ticularly difficult. In addition to meeting owner requirements, historical preservation
`
`agencies mustbesatisfied that new construction is not desecrating the past. Wire-
`
`less networks can be deployed extremely rapidly in such environments because there
`
`is only a small wired network to install.
`
`
`Flexibility has also led to the developmentof grassroots community networks. With
`the rapid price erosion of 802.11 equipment, bands of volunteers are setting up
`
`shared wireless networks open to visitors. Community networks are also extending
`
`the range of Internet access past the limitations for DSL into communities where
`
`high-speed Internet access has been only a dream. Community networks have been
`
`particularly successful in out-of-the way places that are too rugged for traditional
`
`wireline approaches.
`
`
`Like all networks, wireless networks transmit data over a network medium. The
`medium is a form of electromagnetic radiation.” To be well-suited for use on mobile
`
`networks,
`the medium must be able to cover a wide area so clients can move
`
`throughout a coverage area. The two media that have seen the widest use in local-
`
`area applications are infrared light and radio waves. Most portable PCs sold now
`
`have infrared ports that can make quick connections to printers and other peripher-
`
`als. However, infrared light has limitations; it is easily blocked by walls, partitions,
`
`and other office construction. Radio waves can penetrate most office obstructions
`
`and offer a wider coverage range. It is no surprise that most,if notall, 802.11 prod-
`
`ucts on the market use the radio wave physicallayer.
`
`
`Radio Spectrum: The Key Resource
`Wireless devices are constrained to operate in a certain frequency band. Each band
`has an associated bandwidth, which is simply the amountof frequency space in the
`band. Bandwidth has acquired a connotation of being a measureof the data capacity
`of a link. A great deal of mathematics, information theory, and signal processing can
`be used to show that higher-bandwidthslices can be used to transmit more informa-
`tion. As an example, an analog mobile telephony channel requires a 20-kHz band-
`width. TV signals are vastly more complex and have a correspondingly larger
`bandwidth of 6 MHz.
`
`* Laserlight is also used by some wireless networking applications, but the extremefocus of a laser beam
`makesit suited only for applications in which the endsarestationary. “Fixed wireless” applications, in which
`lasers replace other access technology such as leased telephonecircuits, are a commonapplication.
`
`
`
`Why Wireless?
`|
`
`3
`
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`

`

` The use of a radio spectrum is rigorously controlled by regulatory authorities
`
`through licensing processes. In the U.S., regulation is done by the Federal Communi-
`cations Commission (FCC). Many FCC rules are adopted by other countries
`throughout the Americas. European allocation is performed by CEPT’s European
`Radiocommunications Office (ERO). Otherallocation work is done by the Interna-
`tional Telecommunications Union (ITU). To prevent overlapping uses of the radio
`waves,frequencyis allocated in bands, which are simply ranges of frequenciesavail-
`able to specified applications. Table 1-1 lists some common frequency bands used in
`the US.
`
`
`
`Table 1-1. Common U.S. frequency bands
`
`Band
`UHFISM
`
`5-Band
`
`S-Band ISM
`
`C-Band
`
`C-Bandsatellite downlink
`
`C-BandRadar(weather)
`
`C-Band ISM
`
`C-Bandsatellite uplink
`X-Band
`
`X-Band Radar(police/weather)
`Ku-Band
`
`Ku-Band Radar(police)
`
`The ISM bands
`
`Frequency range
`902-928 MHz
`
`2-4 GHz
`
`2.4-2.5 GHz
`
`4-8 GHz
`
`3.7-4.2 GHz
`
`5.25-—5.925 GHz
`
`5.725-5.875 GHz
`
`5.925-6.425 GHz
`8-12 GHz
`
`8.5-10.55 GHz
`12-18 GHz
`
`13.4-14 GHz
`15.7-17.7 GHz
`
`In Table 1-1, there are three bands labeled ISM, which is an abbreviation for indus-
`trial, scientific, and medical. ISM bandsareset aside for equipmentthat, broadly
`speaking,is related to industrial or scientific processes or is used by medical equip-
`ment. Perhaps the most familiar ISM-band device is the microwave oven, which
`operates in the 2.4-GHz ISM band because electromagnetic radiation at that fre-
`quencyis particularly effective for heating water.
`I pay special attention to the ISM bands becausethat’s where 802.11 devices oper-
`ate. The more common 802.11b devices operate in S-band ISM. The ISM bandsare
`generally license-free, provided that devices are low-power. How much sense doesit
`make to require a license for microwave ovens,after all? Likewise, you don’t need a
`license to set up and operate a wireless network.
`
`
`|
`Chapter 1:
`Introduction to Wireless Networks
`
`4
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`
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`

`

` The Limits of Wireless Networking
`
`
`
`Wireless networks do notreplacefixed networks. The main advantage of mobility is
`
`that the network user is moving. Servers and other data center equipment must
`access data, but the physical location of the serveris irrelevant. As long as the serv-
`
`
`ers do not move, they may as well be connected to wires that do not move.
`
`vorks is constrained by the available bandwidth. Informa-
`
`The speed of wireless netw
`tion theory can be used to deduce the upperlimit on the speed of a network. Unless
`the regulatory authorities are willing to make the unlicensed spectrum bandsbigger,
`
`
`there is an upperlimit on the speed ofwireless networks. Wireless-network hard-
`
`ware tends to be slower than wired hardware. Unlike the 10-GB Ethernet standard,
`
`
`wireless-network standards must carefully validate received frames to guard against
`
`loss due to the unreliability of the wireless medium.
`
`Using radio waves as the network mediumposes several challenges. Specifications
`for wired networks are designed so that a network will work as long as it respects the
`
`specifications. Radio waves can suffer from a numberof propagation problems that
`
`
`mayinterruptthe radio link, such as multipath interference and shadows.
`
`Security on any networkis a prime concern. On wireless networks,it is often a criti-
`
`cal concern because the network transmissionsare available to anyone within range
`
`of the transmitter with the appropriate antenna. On a wired network,the signals stay
`
`in the wires and can be protected bystrong physical-access control (locks on the
`
`doors of wiring closets, and so on). On a wireless network, sniffing is much easier
`because the radio transmissions are designed to be processed by anyreceiver within
`
`
`range. Furthermore, wireless networks tend to have fuzzy boundaries. A corporate
`
`wireless network may extend outside the building. It is quite possible that a parked
`car across the street couldbe receiving the signals from your network. As an experi-
`
`ment onone of mytrips to San Francisco, I turned on mylaptopto countthe num-
`ber of wireless networks near a major highway outsidethecity. I found eight without
`
`expending any significant effort. A significantly more motivated investigator would
`
`undoubtedly have discovered many more networks by using a much moresensitive
`
`
`antenna mounted outsidethesteel shell of the car.
`
`
` ANetwork by Any OtherName... | 5
`
`
`
`A Networkby Any Other Name...
`Wireless networking is a hot industry segment. Several wireless technologies have
`been targeted primarily for data transmission. Bluetoothis a standard used to build
`small networks between peripherals: a form of “wireless wires,” if you will. Most
`people in the industry are familiar with the hype surrounding Bluetooth.
`I haven't
`met many people who have used devices based on the Bluetoothspecitication.
`Third-generation (3G) mobile telephony networksare also a familiar source of hype.
`They promise data rates of megabits percell, as well as the “always on” connections
`
`
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`

`

`
`
` that have proven to be quite valuable to DSL and cable modem customers. In spite of
`
`the hype and press from 3G equipmentvendors, the rollout of commercial 3G ser-
`vices has been continually pushed back.
`In contrast to Bluetooth and 3G, equipment based on the IEEE 802.11 standard has
`been an astounding success. While Bluetooth and 3G may be successful in the
`future, 802.11 is a success now. Apple initiated the pricing moves that caused the
`market for 802.11 equipmentto explode in 1999. Price erosion made the equipment
`affordable and started the growth that continues today.
`This is a book about 802.11 networks. 802.11 goes by a variety of names, depending
`on whois talking aboutit. Some people call 802.11 wireless Ethernet, to emphasize
`its shared lineage with the traditional wired Ethernet (802.3). More recently, the
`Wireless Ethernet Compatibility Alliance (WECA)has been pushingits Wi-Fi (“wire-
`less fidelity”) certification program.’ Any 802.11 vendorcan haveits products tested
`for interoperability. Equipment thatpassesthe test suite can use the Wi-Fi mark. For
`newer products based on the 802.11a standard, WECA will allow use of the Wi-Fi5
`mark. The “5” reflects the fact that 802.11a products use a different frequency band
`of around 5 GHz.
`
`Table 1-2 is a basic comparisonof the different 802.11 standards. Products based on
`802.11 wereinitially released in 1997. 802.11 included an infrared (IR) layer that was
`never widely deployed, as well as two spread-spectrum radio layers: frequency hop-
`ping (FH) and direct sequence (DS). (The differences between these two radio layers
`is described in Chapter 10.) Initial 802.11 products were limited to 2 Mbps, whichis
`quite slow by modern network standards. The IEEE 802.11 working group quickly
`began working on faster radio layers and standardized both 802.11a and 802.11b in
`1999. Products based on 802.11b werereleased in 1999 and can operate at speeds of
`up to 11 Mbps. 802.11a uses a third radio technique called orthogonal frequency
`division multiplexing (OFDM). 802.1la operates in a different frequency band
`entirely and currently has regulatory approval only in the United States. As you can
`see from the table, 802.11 already provides speeds faster than 1OBASE-T Ethernet
`and is reasonably competitive with Fast Ethernet.
`
`Table 1-2. Comparison of 802.11 standards
`
`
`IEEE standard=Speed Frequencyband Notes
`
`802.11
`1 Mbps
`2.4GHz
`First standard (1997). Featured both frequency-hopping and
`2 Mbps
`direct-sequence modulation techniques.
`upto54Mbps
`Second standard (1999), but products not releaseduntillate 2000.
`5.5 Mbps
`Third standard, but second waveof products. The most common
`11 Mbps
`802.11 equipmentasthis book was written.
`upto54Mbps
`Notyet standardized.
`
`5 GHz
`2.4 GHz
`
`2.4GHz
`
`802.11a
`802.11b
`
`802.119
`
`™ More details on WECAand the Wi-Ficertification can be foundat htip://www.wi-fi.org/.
`
`6| Chapter 1:
`Introduction to Wireless Networks
`
`
`
`

`

`
`;
`CHAPTER 2
`
`Overview of 802.11 Networks
`
`
`
`
`
`Before studyingthe details of anything,it often helps to get a general“lay ofthe land.”
`A basic introduction is often necessary when studying networking topics because the
`numberof acronymscan be overwhelming. Unfortunately, 802.11 takes acronyms to
`new heights, which makes the introduction that much more important. To under-
`stand 802.11 on anything more than a superficial basis, you must get comfortable with
`some esoteric terminology anda herd of three-letter acronyms. This chapteris the glue
`that binds the entire book together. Readit for a basic understanding of 802.11, the
`concepts thatwill likely be importantto users, and how the protocolis designed to
`provide an experience as muchlike Ethernet as possible. After that, move onto the
`low-level protocol details or deployment, depending on yourinterests and needs.
`Part of the reason whythis introduction is important is because it introduces the
`hroughout the book. With 802.11, the introduction serves another
`acronyms used t
`02.11 is superficially similar to Ethernet. Understanding the
`important purpose. 8
`background of Ethernet helpsslightly with 802.11, but there is a host of additional
`background needed to appreciate how 802.11 adapts traditional Ethernet technol-
`ogy to a wireless world. To accountfor the differences between wired networks and
`the wireless media used by 802.11, a number of additional management features
`were added. At the heart of 802.11 is a white lie about the meaning of media access
`control (MAC). Wireless network interface cardsare assigned 48-bit MAC addresses,
`and, for all practical purposes, they look like Ethernet network interface cards. In
`idress pool so that 802,11
`fact, the MAC address assignment is done from the same ac
`cards have unique addresses even when deployed into a network with wired Ether-
`To outside network devices, these MACaddresses appearto be fixed,just as in other
`IEEE 802 networks; 802.11 MAC addresses go into ARP tables alongside Ethernet
`addresses, use the same set of vendor prefixes, and are otherwise indistinguishable
`from Ethernet addresses. The devices that comprise an 802.11 network (access
`points and other 802.11 devices) know better. There are many differences between
`an 802.11 device and an Ethernet device, but the most obviousis that 802.11 devices
`
`
`net stations.
`
`
`
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`

`

` are mobile; they can easily move from one part of the network to another. The 802.
`
`
`
`11 devices on your network understand this and deliver frames to the current loca-
`tion of the mobilestation.
`
`IEEE 802 Network Technology Family Tree
`802.11 is a member of the IEEE 802 family, which is a series of specifications for
`local area network (LAN) technologies. Figure 2-1 shows the relationship between
`the various componentsof the 802 family and their place in the OSI model.
`
`
`
`
`
`
`802 | [ “802.1
`]
`| Data linklayer
`.
`k
`
`Overview||Management eda euregeenCU | LLCsublayer
`
`
`
`and
`|
`=
`-
`~
`802,11
`802.5
`802.3
`| architecture
`pra Di —
`
`
`
`802.3 802.5|| |
`MAC
`|
`MAC
`862.11 MAC
`MACsublayer
`802.3
`|
`802.5
`[ 802.11
`662.11
`jf 802.114
`802.11b | aa
`
`
`
`
`|__ pay||Pay rasspay || ossspuy leer ss| Physicallayer
`
`
`
`
`Figure 2-1. The IEEE 802 family andits relation to the OSI model
`
`IEEE 802 specifications are focused on the two lowest layers of the OSI model
`because they incorporate both physical and data link components. All 802 networks
`have both a MAC and a Physical (PHY) component. The MACisa set ofrules to
`determine how to access the medium and send data, but the details of transmission
`and receptionare left to the PHY.
`
`Individual specifications in the 802 series are identified by a second number. For
`example, 802.3 is the specification for a Carrier Sense Multiple Access network with
`Collision Detection (CSMA/CD), which is related to (and often mistakenly called)
`Ethernet, and 802.5 is the Token Ring specification. Other specifications describe
`other parts of the 802 protocol stack. 802.2 specifies a commonlink layer, the Logi-
`cal Link Control (LLC), which can be used by any lower-layer LAN technology.
`Managementfeatures for 802 networks are specified in 802.1. Among 802.1’s many
`provisionsare bridging (802.1d) and virtual LANs, or VLANs (802.1q).
`
`802.11 is just another link layer that can use the 802.2/LLC encapsulation. The base
`802.11 specification includes the 802.11 MAC andtwophysical layers: a frequency-
`hopping spread-spectrum (FHSS) physical layer and a direct-sequence spread-spec-
`trum (DSSS) physicallayer. Later revisions to 802.11 added additional physical layers.
`802.11b specifies a high-rate direct-sequence layer (HR/DSSS); products based on
`802.11b hit the marketplace in 1999 and make up the bulk of the installed base. 802.
`1la describes a physical layer based on orthogonal frequency division multiplexing
`(OFDM); products based on 802.11a were released as this book was completed.
`
`
`| Chapter2: Overview of802.11 Networks
`
`8
`
`
`
`

`

`
`To say that 802.11 is “just anotherlink layer for 802.2” is to omit the details in the
`rest of this book, but 802.11 is exciting precisely because of these details. 802.11
`
`allows for mobile network access; in accomplishingthis goal, a numberof additional
`features were incorporated into the MAC.Asaresult, the 802.11 MAC may seem
`
`baroquely complex compared to other IEEE 802 MAC specifications.
`
`The use of radio waves as a physicallayer requires a relatively complex PHY,as well.
`
`802.11 splits the PHY into two generic components: the Physical Layer Convergence
`
`Procedure (PLCP),
`to map the MAC frames onto the medium, and a Physical
`Medium Dependent (PMD)system to transmit those frames. The PLCP straddles the
`
`boundary of the MAC and physical layers, as shown in Figure 2-2. In 802.11, the
`
`PLCP adds a numberoffields to the frameasit is transmitted “in theair.”
`
`
`
`
`
`
`
`
`
`
`
`
`Physical
`
`Data link
`
`Figure 2-2. PHY components
`All this complexity begs the question of how much you actually need to know. As
`with any technology, the more you know,the better off you will be. The 802.11 pro-
`tocols have many knobsanddials that you can tweak, but most 802.11 implementa-
`tions hide this complexity. Many ofthe features of the standard come into their own
`only whenthe network is congested, either with a lot of traffic or with a large num-
`ber of wireless stations. Today’s networks tend not to push the limits in either
`respect. At any rate, I can’t blame you for wanting to skip the chapters about the
`protocols and jump aheadto the chapters about planning andinstalling an 802.11
`network. After you’ve read this chapter, you can skip ahead to Chapters 12-17 and
`return to the chapters on the protocol’s inner workings when you need (or want) to
`know more.
`
`802.11 Nomenclature and Design
`802.11 networksconsist of four major physical components, which are summarized
`in Figure 2-3. The componentsare:
`
`Distribution system
`When several access points are connected to form a large coverage area, they
`must communicate with each other to track the movements of mobile stations.
`The distribution system is the logical component of 802.11 used to forward
`frames to their destination. 802.11 does not specify any particular technology for
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`802.11 Nomenclature and Design
`|
`9
`
`
`
`

`

` Distribution
`
`system
`
`Wireless
`medium
`won 9) a
`
`point
`
`Station
`
`
`
`
`Figure 2-3. Components of 802.11 LANs
`the distribution system. In most commercial products, the distribution system is
`implemented as a combination of a bridging engine and a distribution system
`medium, which is the backbone network used to relay frames between access
`points; it is often called simply the backbone network. In nearly all commer-
`cially successful products, Ethernetis used as the backbone network technology.
`Access points
`Frames on an 802.11 network must be converted to another type of frame for
`delivery to the rest of the world. Devices called access points perform the wire-
`less-to-wired bridging function. (Access points perform a number of other func-
`tions, but bridging is by far the most important.)
`Wireless medium
`To move frames from station to station, the standard uses a wireless medium.
`Several different physical layers are defined;
`the architecture allows multiple
`physical layers to be developed to support the 802.11 MAC.Initially, two radio
`frequency (RF) physical layers and one infrared physical layer were standard-
`ized, though the RF layers have proven far more popular.
`Stations
`Networks are built to transfer data between stations. Stations are computing
`devices with wireless networkinterfaces. Typically, stations are battery-operated
`laptop or handheld computers. There is no reason whystations must be porta-
`ble computing devices, though. In some environments, wireless networking is
`used to avoid pulling new cable, and desktops are connected by wireless LANs.
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`Types of Networks
`The basic building block of an 802.11 network is the basic service set (BSS), whichis
`simplya groupof stations that communicate with each other. Communicationstake
`place within a somewhat fuzzy area, called the basic service area, defined by the
`propagation characteristics of the wireless medium.’ Whena station is in the basic
`
`* All of the wireless media used will propagate in three dimensions. From that perspective, the service area
`should perhaps becalled the service volume. However, the term area is widely used andaccepted,
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`Chapter 2: Overview of 802.11 Networks
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`service area, it can communicate with the other members of the BSS. BSSs come in
`twoflavors, both of which areillustrated in Figure 2-4.
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`IndependentBSS
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`infrastructureBSS
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`Figure 2-4. Independent and infrastructure BSSs
`Independent networks
`On theleft is an independent BSS (IBSS). Stations in an IBSS communicate directly
`with each other and thus must be within direct communication range. The smallest
`possible 802.11 network is an IBSS with two stations. Typically, IBSSs are composed
`of a small numberof stations set up for a specific purpose and for a short period of
`time, One commonuseis to create a short-lived network to support a single meeting
`in a conference room. As the meeting begins, the participants create an IBSS to share
`data. When the meeting ends, the IBSS is dissolved. Due to their short duration,
`small size, and focused purpose, IBSSsare sometimesreferred to as ad hoc BSSs or ad
`
`hoc networks.
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`Infrastructure networks
`Ontheright side of Figure 2-4 is an infrastructure BSS (never called an IBSS). Infra-
`structure networksare distinguished by the use of an access point. Access points are
`used for all communications in infrastructure networks, including communication
`between mobile nodesin the same service area. If one mobile station in an infrastruc-
`ture BSS needs to communicate with a second mobile station, the communication
`must take two hops.First, the originating mobile station transfers the frame to the
`ointtransfers the frame to the destination station.
`access point. Second, the access p
`With all communications relayed through an access point, the basic service area COr-
`responding to an infrastructure BSSis defined by the points in which transmissions
`from the access point can be received. Although the multihop transmission takes
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`* }BS5s have founda similar use at LAN parties throughout the world.
`302.11 Nomenclature and Design |
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`more transmission capacity than a directed frame from the senderto thereceiver, it
`has two major advantages:
`* An infrastructure BSS is defined by the distance from the access point. All mobile
`stations are required to be within reach of the access point, but no restriction is
`placed on the distance between mobile stations themselves. Allowing direct
`communication between mobile stations would save transmission capacity but
`at the cost of increased physical layer complexity because mobile stations would
`need to maintain neighbor relationships with all other mobile stations within the
`service area.
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`* Access points in infrastructure networksare in a position to assist with stations
`attempting to save power. Access points can note whena station enters a power-
`saving mode and buffer frames for it. Battery-operated stations can turn the
`wireless transceiver off and power it up only to transmit and retrieve buffered
`frames from the access point.
`In an infrastructure network, stations must associate with an access point to obtain
`networkservices. Association is the process by which mobile station joins an 802.11
`network;it is logically equivalent to plugging in the network cable on an Ethernet. It
`is not a symmetric process. Mobile stations alwaysinitiate the association process,
`and access points may choose to grant or deny access based on the contents of an
`association request. Associations are also exclusive on the part of the mobile station:
`a mobile station can be associated with only oneaccess point.’ The 802.11 standard
`places no limit on the number of mobile stations that an access point may serve.
`Implementation considerations may, of course, limit the number of mobile stations
`an access point may serve. In practice, however, the relatively low throughputof
`wireless networksis far morelikely to limit the numberofstations placed on a wire-
`less network.
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`Extended service areas
`
`BSSs can create coverage in small offices and homes, but they cannot provide net-
`work coverage to larger areas. 802.11 allows wireless networks of arbitrarily large
`size to be created by linking BSSs into an extended service set (ESS). An ESSis cre-
`ated by chaining BSSs together with a backbone network. 802.11 does not specify a
`particular backbone technology; it requires only that the backbone provide a speci-
`fied set of services. In Figure 2-5, the ESS is the union ofthe four BSSs (provided that
`all the access points are configured to be part of the same ESS). In r