*lEEE
`
`Handbook
`A Designer’s Companion
`
`SMM TM OM remitWVm sltnt4
`
`LN
`
`10,079,707
`
`Standards Information Network ause)
`
`IEEE Press the world”
`
`Published by
`
`DELL-1040
`
`DELL-1040
`10,079,707
`
`

`

`

`

`
`
`The IEEE 802.11 Handbook
`
`
`
`A Designer's Companion
`
`
`
`Authored by
`
`Bob O'Hara
`
`Al Petrick
`
`
`
`Published by
`
`
`
`
`
`Standards Information Network
`
`IEEE Press
`IEEE
`
`
`
`
`
`
`
`
`The Institute of Electrical and Electronics Engineers, Inc.
`
`http://standards.ieee.org
`
`

`

`Library of Congress Cataloging-in-Publication Data
`
`O’Hara, Bob, 1956-
`The IEEE 802.11 handbook: a designer’s companion / authored by
`Bob O'Hara and AlPetrick.
`p.
`cm.
`ISBN 0-7381-1855-9 (paperback : alk. paper} —ISBN 0-7381-1857-5 (pdf}
`1. Local Area Networks (Computer networks}—Standards. 2. Wireless
`communication systems.I. Petrick, AL 1957- Il. Title.
`
`CIP
`
`TKS105.7 O37 1999
`621.382’1—dc21
`
`99-057887
`
`The Institute ofElectrical and Electronics Engineers, Inc.
`3 Park Avenue, New York, NY, 10016-5997, USA
`
`Copyright © 1999 by the Institute ofElectrical and Electronics Engineers, Inc.
`All rights reserved. Published December 1999. Printed in the United States of
`America.
`
`No part ofthis publication may be reproduced in any form, in an electronic
`retrieval system or otherwise, without the prior written permission of the
`publisher.
`
`ii
`
`The IEEE 802.11 Handbook: A Designer's Companion
`
`

`

`IEEE PRESS/Standards Information Network (SIN) publications are not
`consensus documents. Information contained in this and other works has
`been obtained from sources believed to be reliable, and reviewed by credi-
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`Likewise, while the author and publisher believe that the infor-
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`

`pany that specializes in strategic, technology, and network consulting. He is
`actively involved in the development of networking, telecommunications, and
`computing standards and products. His areas of expertise are: network and com-
`munication protocols and their implementation, operating systems, system spec-
`ification and integration,
`standards development,
`cryptography and its
`application, strategy development, and product definition. Mr. O’Hara has been
`involved with the developmentof the IEEE 802.11 WLANstandard since 1992.
`Heis the technical editor of that standard and chairman ofthe revisions and reg-
`ulatory extensionstask groups.
`
`Prior to starting Informed Technology, Mr. O’Hara worked for Advanced Micro
`Devices in both senior engineering and management positions for the I/O and
`Network Products Division and in the Advanced Development Lab, as well as
`engineering positions at Fairchild Space and Communications and TRW
`Defense and Space Systems Group. He Graduated with a BSEE from the Uni-
`versity of Maryland in 1978.
`
`AlPetrick is Director of Marketing and Business Developmentat ParkerVision
`for the wireless product line. Mr. Petrick’s experience includes over 20 years of
`combined marketing and systems engineering in wireless communications with
`emphasis on semiconductor technology. Prior to ParkerVision, Mr. Petrick held
`senior management marketing and business development positions at Intersil
`Semiconductor. He successfully pioneered semiconductor technology for the
`WLAN market from inception through announcement. Mr. Petrick serves as
`Vice-Chair of the IEEE 802.11 WLAN standards committee. Mr. Petrick pub-
`lished various marketing and technical papers on wireless communications and
`is a distinguished writer with leading wireless trade journals and market and
`financial analysts. Mr. Petrick holds a BSET from RochesterInstitute of Tech-
`nology, Rochester, New York and an MBAfrom Rollins College, Winter Park,
`Florida. He also studied business-strategies at Northwestern University Kellogg
`Graduate School of Management.
`
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`

`

`
`
`Foreword
`
`Since the publication of the IEEE 802.11 WLAN standard, many equipment
`manufacturers have entered the market with interoperable WLAN systems. In
`September 1999, the IEEE-SA Standards Board approved the 2.4 GHz, 11 Mbps
`802.11b and 5 GHz, 54 Mbps 802.11a extensions. However, standards are writ-
`ten as specifications for interoperable products and not as handbooks for obtain-
`ing a thorough understanding ofthe protocol. It is impossible to include in the
`standardsall the reasons for decisions taken to get the standard ratified.
`
`The only people who could write a handbook with the qualities I have in mind
`are those that have followed the standards process from the beginning. I applaud
`Bob O’HaraandA!Petrick for taking on the task of writing this handbook. Bob
`and Al have been very instrumental
`throughout
`the development of the
`IEEE 802.11 standard and are recognized for their contributions and technical
`leadership. This bookisa first-of-a-kind and provides a perfect balance ofinfor-
`mation for embracing the physical and MAClayersofthe standard.
`
`I expect The IEEE 802.11 Handbook: A Designer's Companion to become a
`standard reference for every WLAN systems engineer and anticipate the reader
`will find this text extremely useful.
`
`Chairman, IEEE P802.11, Standards WG for Wireless LANs
`Lucent Technologies Nederland B. V.
`Zadelstede 1-10
`The Netherlands
`
`

`

`ularity. The WLANindustry is taking off and expanding beyondits vertical
`niche market roots. Oneof the key drivers of this new market expansion for
`WLANsis the IEEE 802.11 standard. Simply having a WLANstandard was not
`enough to spark the industry. IEEE 802.11 has been aroundsince June of 1997.
`The IEEE 802.11b High-Rate Physical Layer extension enabledus to deliver 11
`Mbpsandproducts conformingto that standard have been shipping for a while.
`Wireless LANshavefinally hit the right price and performanceto appeal to a
`broader market. Breaking the 10 Mbps barrier makes IEEE 802.11 LANs
`appealing for enterprise applications. Home networking is becoming more pop-
`ular, and WLANsarean attractive option. By the time you readthis, you will be
`able to purchase an IEEE 802.11-compliant, 11 Mbps consumer WLANadapter
`for $99 or less. Wireless LANsare ready for prime time and IEEE 802.11 made
`it happen.
`
`The IEEE 802.11 standard represents many years of work from a global team of
`engineers. Oneof the challenges of developing the IEEE 802.11 standard was
`bringing together experts from twodifferent disciplines —analog radio design
`and network protocol design. We had many arguments about whetherthis is a
`radio standard or a network standard. Very clearly, IEEE 802.11 is a network
`standard. That is the whole point. Because IEEE 802.11 fits into the IEEE 802
`framework, systems conformingto the standard can be addedto existing net-
`workstransparently. IEEE 802.11 WLANswill support the network protocols
`and applications that were developed for the other IEEE 802 LAN standards
`over the past 25 years. So IEEE 802.11 is a network standard that happensto
`have a radio physical layer. This book benefits from the fact that Bob and Alare
`experts in both of these disciplines. They have a deep understandingof the
`material gained through their many years of contribution to the standard.
`
`viii
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`The IEEE 802.11 Handbook: A Designer's Companion
`
`
`
`

`

`The standard was over 400 pages wheninitially published, and recently two new
`physical layer extensions were added. Bob and Alhelp the reader navigate
`through the complexity ofthe standard and focus on the core issues. This bookis
`a great guide to the standard for anyone developing IEEE 802.11 products or
`those simply wanting to gain a better understanding of the standard.
`
`Phil Belanger
`Chairman of the Wireless Ethernet Compatibility Alliance, www.wi-fi.com
`Co-Author of the DFWMACprotocol, the proposal that was used as the basis
`for the IEEE 802.11 MAC
`
`

`

`WLANs. Whyis it necessary to bring another one to your shelves? We believe
`that, with the advent of the IEEE 802.11 standard for WLANs,the consolidation
`of the WLAN market will commence. Therefore, it is important that WLAN
`designers, network planners and administrators, and users understand the opera-
`tion and application of IEEE 802.11. This handbook will provide the detail
`required to attain that understanding.
`
`With the advent of IEEE 802.11 WLANs,anera of multivendor product compe-
`tition and innovation has begun, similar to that begun by the adoption of the
`IEEE 802.3 standard. This era is closing the door on proprietary WLANsthat
`have seenlimited acceptance, mostly in vertical applications such as warehous-
`ing, inventory control, and retail. The goal of the IEEE 802.11 Working Group
`was to define a complete WLAN system that would allow the use of WLANsin
`all application areas, including the typical horizontal application of corporate
`LANs, where wired LANsare found today. It is our belief that the working
`group has been successfulin reaching this goal.
`
`There are two major components of the WLAN described by IEEE 802.11, the
`mobile station and the access point (AP). Going well beyond what other IEEE
`802 standards have done in the past, IEEE 802.11 defines a complete manage-
`ment protocol between the mobile station and AP. This managementprotocol
`makesit possible for a single IEEE 802.11 WLANto comprise equipment from
`many vendors, marking true multivendorinteroperability.
`
`There is a huge amount of information in the IEEE 802.11 standard andits
`extensions. Finding the information required in a short time can be challenging.
`To help meet the challenge, a mapping between the information in the standards
`and that presented in this handbook is given here. IEEE standards are divided
`into clauses and annexes. Information in the standard is referred to by the clause
`and annex in which it is found. This book is divided into chapters. Information
`in this book is referred to by the chapter in whichit is found.
`
`Clauses | through 4 of the standard contain a brief overview of the standard,
`other references that are required to implement the standard, definitions of
`terms, and the abbreviations and acronymsused in the standard. This informa-
`tion correspondsto the Introduction and abbreviations in this handbook.
`
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`The IEEE 802.11 Handbook: A Designer’s Companion
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`

`Clause 5 of the standard provides a description of the architecture and compo-
`nents of an IEEE 802.11 WLAN system. This corresponds to Chapter 2 in this
`
`Clause 6 of the IEEE 802.11 standard describes the MACservice interface. This
`is an abstract interface for the exchange of data between the MACandtheproto-
`col layer above the MAC.This is not described explicitly in this handbook.
`
`Clause 7 of the standard describes the MAC framesandtheir content. Clause 8
`of the standard describes the WEPfunctionality that may be implemented in an
`IEEE 802.11 station. Clause 9 describes the functionality and frame exchange
`protocols of the MAC.Information from these clauses is found in Chapter3.
`
`Clause 10 describes the layer managementserviceinterface primitives and their
`functionality. Clause 11 describes the MAC managementfunctionality and pro-
`tocols. This information may be found in Chapter4.
`
`Clause 12 describes the PHY service interface. This is an abstract interface for
`the exchange of data between the MAC and PHY. Clause 13 describes the PHY
`managementservice interface, which consists solely of the MIB interface. This
`is not described explicitly in this handbook.
`
`layer.
`Clause 14 describes the frequency hopping spread spectrum physical
`Clause 15 describes the direct sequence spread spectrum physical layer. Clause
`16 describes the infrared baseband physical layer. Clause 17 (IEEE 802.11a)
`describes the orthogonal frequency division multiplexed physical layer. Clause
`18 (IEEE 802.11b) describes the higher rate direct sequence spread spectrum
`physical layer. Information on all physical layers is found in Chapter 6.
`
`Annex A of the standard is the Protocol Implementation Conformance State-
`ment (PICS) pro forma. This form may be used to identify the exact options
`implemented in a device claiming conformance to IEEE 802.11. This annex is
`not discussed in this handbook.
`
`Annex B of the standard is a set of tables of the hopping patterns for the fre-
`quency hopping physicallayer. This annex is not discussed in this handbook.
`
`

`

`The figure below provides a quick, graphical map between the information in
`the JEEE 802.11 standard and this handbook.
`MAC Service Interface
`Clause 6
`
`Interface
`Clause 10
`
`Medium Access Control Sublayer
`
`MAC Framing, Clause 7, Chapter 3
`MACOperation, Clause 9, Chapter 3
`WEPOperation, Clause 8, Chapter 3
`State Machines, Annex C
`
`MAC Management
`
`Protocols and Operation, Clause 11, Chapter 4
`State Machines, Annex C
`ManagementInformation Base, Annex D,
`Chapter 5
`
`Chapter 6 MAC ManagementService
`
`PHY Service Interface
`Clause 12, Chapter 6
`
`PHY ManagementService
`Interface
`Clause 13
`
`PHYLayer
`
`PHY Management
`
`Frequency Hopping, Clause 14, Chapter 6
`Direct Sequence,Clause 15, Chapter 6
`Infrared Baseband, Clause 16, Chapter 6
`Orthogonal Frequency Division Multiplexed,
`Clause 17, Chapter 6
`Higher Rate Direct Sequence, Clause 18,
`
`ManagementInformation Base, Annex D
`
`Where to find information on IEEE 802.11
`
`Updated information about IEEE 802.11 and responses to questions by users of
`this handbookare provided by the authorsat the following Website:
`http://www.informed-technology.com/handbook_additional_material.htm.
`
`xii
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`The IEEE 802.11 Handbook: A Designer’s Companion
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`
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`

`

`Contents
`
`Chapter 1 Similarities and Differences between Wireless
`and Wired LANS.........ccccccsccsssssesecsecssssecsecsevaccensenessenecsessasensenseneenss 1
`
`Similarities between WLANs and Wired LANS..............cccscseees 1
`
`Differences between WLANSand Wired LANG..............ccceeceees 2
`
`Chapter 2 JEEE Standard 802.11: The First International Standard for
`WLANSuu. eeccsssscseeseesectsssecsesesscsssesessasseacnessseacaessssanscesssavsessssacaase 7
`
`TEEE 802.11 Architecture .....0.....cccccccccsesesceecseecceceseeneeceseeceseeseasenes 7
`
`SUMMALY 00... ee eeeccsscesscssessesccssesseesscssecssesasssseneetscseencsseseeseesereesees 18
`
`Chapter 3 Medium Access Control 0.0... eeecesssesctsescerseeessnenecsenseneeranersensees 19
`
`MAC Functionality 20... cssscssesensenterserenseetecseneeetscseenensesenens 19
`MACFrame Exchange Protocol...........:scssesssscsssesetseessessessseeee 20
`Frame Formats «00.00.00... seseseeesetsesseseeneaeeeesevessceaeeeseeeneenseeeeeeaes 31
`
`Data Frame Subtypes....0.....ccsesceseseeetenrenters
`
`Control Frame Subtypes... ccccccsecesssssssessssseseesseseeseeneseesenees 44
`
`Management Frame Subtypes... essessscseceseteeseceersetesseeseenees 54
`Components of the Management Frame Body............c.cccccseseeees 58
`Other MAC Operations..0........cccccccscesscsscssesessecessseeeeesessesesescseenses 72
`
`Chapter 4 MAC Management... eccccecseceseseeessesseeeeeeseeseseatseseeseeceeneseanses 81
`
`Tools Available to Meet the Challenges ...........cccccssccscsesetssesenens 82
`Combining Management Tools .0..........cccssssesseetsseeeeteeersssseseess 98
`
`Chapter 5 MAC ManagementInformation Base.............:.::scscsesseeeeteaee 101
`
`

`

`Direct Sequence Spread Spectrum (DSSS) PHY........0..cceue 114
`
`' The Frequency Hopping Spread Spectrum (FHSS) PHY.......... 124
`
`Infrared (IR) PHY oc. ceecceseessessessesesnesesecseeseevsscessesstesssneaeevenss 131
`
`IR PHY Modulation Method .0.0......cccscscssecsssssssesssestsersesenees 134
`
`Geographic Regulatory Bodies .......c.c.cccsescsssessessssessssstsecersesesees 136
`
`Chapter 7 Physical Layer Extensions to IEEE 802.11 .....cecscccceeeeseeeee 139
`
`IEEE 802.1 1a —The OFDM Physical Layer........0.0000 cece 139
`
`Geographic Regulatory Bodies .........ccccecessssscsssssessesssesrsersesenses 147
`
`IEEE 802.1 1b~2.4 High Rate DSSS PHY 0... eeseee 148
`
`Chapter 8 System Design Considerations for IEEE 802.11 WLANS......... 161
`
`The Medium... ieee eeessessseceseeeesessseseseseseneessssessesssesseseessses 161
`
`Multipath ....0.0... cece cccseceeeeseeseseseessseseseseesssesesescseesesseesesseses 162
`
`Multipath Channel Model... eesssessssesesssssessesseteessssesseces 164
`
`Path Loss in a WLAN System ..........ccecccccesseessecsstscscsessessesesnes 166
`
`Multipath Fading... cc ececcsessssssseesssssssnenesssssensesssssssscessenes 168
`
`Es/No vs BER Performance.........c.ccccscssssessssssssescessesssessseseaveres 168
`
`Data Rate vs Aggregate Throughput............ccccccssseeseeeeneeeees 170
`
`WLANInstallation and Site Survey ..........ccccsescssessesesesees 170
`
`Interference in the 2.4 GHz Frequency Band..........ceceeee 171
`
`Antenna Diversity 00.0... cccccccccescscescscsescssesvenssesssssveceseveusensess 172
`
`Acronymsand Abbreviations 00... cecessssssssssssssseecssessnessesessecssessnsssssesanes 174
`
`xiv
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`The IEEE 802.11 Handbook: A Designer's Companion
`
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`
`

`

`The IEEE 802.11 Handbook
`
`A Designer’s Companion
`
`Chapter 1
`Similarities and Differences between
`Wireless and Wired LANs
`
`There are many similarities and differences of wired LANs and the
`IEEE 802.11 wireless LAN (WLAN). This chapter will describe them.
`
`Similarities between WLANs and Wired
`
`From the beginning, the IEEE 802.11 WLANwasdesigned to look and
`feel like any IEEE 802 wired LAN. This meantthat it must appear to be
`the same as the wired networks to which a user may be accustomed.It
`must support all of the protocols and all of the LAN managementtools
`that operate on a wired network.
`
`To accomplish the task of similarity to wired LANs, IEEE 802.11 is
`
`

`

`LLC need not be aware of the network that is actually transporting
`their data.
`
`Differences between WLANs and Wired
`LANs
`
`There are also a number of differences between wired LANs and
`WLANs. The two most importantdifferences are that there are no wires
`(the air link) and the mobility thus conferred by the lack of a wired
`tether. These differences lead to both the tremendous benefits of a
`WLAN,as well as the perceived drawbacksto them.
`
`The air link is the radio or infrared link between WLAN transmitters
`and receivers. Because WLANtransmissionsare not confined to a wire,
`there may be concernsthat the data carried by a WLANisnotprivate,
`not protected. This concern is certainly valid; the data on a WLANis
`broadcast for all to hear. Many proprietary WLANsdonot provide any
`protection for the data carried. The designers of IEEE 802.11 realized
`that this concern could bea significant problem for users wishing to use
`a WLANanddesigned strong cryptographic mechanismsinto the proto-
`col to provide protection for the data thatis at least as strong as sending
`the data over a wire. Details of this protection are described in
`Chapter 3.
`
`Theair link also exposes the transmissions of a WLANto the vagaries
`of electromagnetic propagation. For both radio- and infrared-based
`WLANs, everything in the environment is either a reflector or an
`attenuator of the signal carrying the LAN data. This can cause signifi-
`cant changes in the strength of a signal received by a WLANstation,
`sometimes severing the station from the LAN entirely. At the wave-
`lengths used in the IEEE 802.11 WLAN,small changesin position can
`
`The [EEE 802.11 Handbook: A Designer’s Companion
`
`2
`
`
`
`

`

`Chapter 1: Similarities and Differences between Wireless and Wired LANs
`
`cause large changes in the received signal strength. This is due to the
`signal traveling many paths ofdiffering length to arrive at the receiver.
`Each individual arriving signalis of a slightly different phase from all
`of the others. Adding these different phases together results in the
`composite signalthat is received. Since these individual signals some-
`times add up in phase and sometimesout of phase, the overall received
`signal strength is sometimes large and sometimes small. Objects mov-
`ing in the environment, such as people, aluminized Mylar balloons,
`doors, and other objects, can also affect the strength of a signal at a
`receiver by changingthe attenuation orreflection of the many individ-
`
`Figure 1-1 is taken from the IEEE Std 802.11-1997 standard and
`showsthe result of a ray tracing simulation in a closed office environ-
`ment. The various shades of gray depict the different signal strengths
`at each location in the room. Dealing with the variability of the air link
`is also designed into the IEEE 802.11 WLAN. For more onthis, see
`
`The secondsignificant difference a WLAN has from a wired LANis
`mobility. The user of a WLANis nottethered to the network outlet in
`the wall. This is both the source of the benefits of a WLAN and the
`cause of muchof the internal complexity.
`
`The benefit of mobility is that the LAN goes whereveryou are, instantly
`and withoutthe need to search out outlets or to arrange in advance with
`the network administrators. In a laptop equipped with an IEEE 802.11
`WLANconnection, the connection to the network is available in a
`coworker’s office, down the hall in the conference room, downstairs in
`the lobby, across the parking lot in another building, even across the
`country on another campus. This meansthatall of the information avail-
`able over the network, while sitting in youroffice,is still available in all
`these locations: email, file servers, the company-internal websites, and
`
`

`

`
`
`Figure 1-1— Ray Tracing Simulation Results
`
`Of course,thereisaflip side to the benefits ofmobility. Mostof the net-
`work protocols and equipment in use today were not designed to cope
`with mobility. They were designed with an assumption that
`addresses assigned to a network node would remain in a fixed location
`on the network. For example, early WLANsrequired that a mobile sta-
`tion could only roam within an area where the WLANwasconnected to
`the wired LAN, with only layer-2 bridges between the parts of the
`WLAN.This requirement existed because there was no simple way to
`deal with the changeofa layer-3 network address should the mobile sta-
`tion cross from onepart of the network to anotherthat is connected bya
`
`
`The IEEE 802.11 Handbook: A Designer's Companion
`
`‘4
`
`
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`

`

`Chapter 1: Similarities and Differences between Wireless and Wired LANs
`
`router. Today, there are ways to deal with this problem using new proto-
`cols, including DHCP and Mobile-IP.
`
`Another problem introduced by mobility is that location-based services
`lose their “hook” to a user’s location, when network addresses are not
`locked to a physical location. Thus, notions such as the nearest network
`printer must be defined in a different way, when the physicallocation of
`a network user may be constantly changing. This may increase the com-
`plexity of the service location provider, but meets the needs of the
`
`

`

`

`

`Chapter 2
`IEEE Standard 802.11: The First
`International Standard for WLANs
`
`In 1997 the IEEE adopted the first standard for WLANs, IEEE Std
`802.11-1997. This standard was revised in 1999. IEEE Std 802.11-
`1997 defines a medium access control (MAC) sublayer, MAC manage-
`ment protocols and services, and three physical (PHY) layers. The
`three PHY layers are an infrared (IR) baseband PHY, a frequency hop-
`ping spread spectrum (FHSS)radio in the 2.4 GHz band, and a direct
`sequence spread spectrum (DSSS)radio in the 2.4 GHz band.All three
`physical layers describe both 1 and 2 Mbps operation. This chapter
`will introduce the standard andits concepts.
`
`Asthis book is being written, the IEEE 802.11 Working Groupis devel-
`oping two new PHYlayers. The first, IEEE Std 802.11a, is an orthogo-
`nal frequency domain multiplexing (OFDM)radio in the UNII bands,
`delivering up to 54 Mbpsdata rates. The second, IEEE Std 802.11b,is
`an extension to the DSSS PHYin the 2.4 GHz band,delivering up to 11
`Mbpsdatarates.
`
`The goals of the IEEE 802.11 standard is to describe a WLAN that
`delivers services previously found only in wired networks, e.g., high
`throughput, highly reliable data delivery, and continuous network con-
`nections. In addition, IEEE 802.11 describes a WLANthat allowstrans-
`parent mobility and built-in power saving operations to the network
`user. The remainder of this chapter will describe the architecture of the
`IEEE 802.11 network and the concepts that support that architecture.
`
`IEFE 802.11 Architecture
`
`The architecture of the IEEE 802.11 WLANis designed to support a
`
`

`

`deep power-saving modes of operation are built into the architecture
`and protocols to prolong the battery life of mobile equipment without
`losing network connectivity. The IEEE 802.11 architecture comprises
`several components:
`the station,
`the AP,
`the wireless medium,
`basic service set, the DS, and the Extended Service Set. The architec-
`ture also includes station services and distribution services.
`
`The IEEE 802.11 architecture may appear to be overly complex. How-
`ever,this apparent complexity is what provides the IEEE 802.11 WLAN
`with its robustness andflexibility. The architecture also embedsa level
`ofindirection that has not been present in previous LANs. It is this level
`of indirection, handled entirely with the IEEE 802.11 architecture and
`transparentto protocol users of the IEEE 802.11 WLAN,that provides
`the ability of a mobile station to roam throughout a WLANandappear
`to be stationary to the protocols above the MACthat have no concept of
`mobility. This “sleight of hand” performed by IEEE 802.11 allowsall of
`the existing network protocols to run over a WLANwithoutanyspecial
`considerations.
`
`Station
`
`The station is the component that connects to the wireless medium.It
`consists of a MAC and a PHY.Generally, the station may bereferred to
`as the network adapter or network interface card (NIC). These names
`may be more familiar to users of wired networks.
`
`Thestation may be mobile, portable, or stationary. Every station sup-
`ports station services. These services are authentication, deauthentica-
`tion, privacy, and delivery of the data (MACservice data unit or MSDU
`in the standard). Thestation services will be described below.
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`Chapter 2: IEEE Standard 802.11
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`Basic Service Set
`
`The IEEE 802.11 WLANarchitecture is built around a basic service set
`(BSS). A BSSisa set of stations that communicate with one another. A
`BSSdoesnotgenerally refer to a particular area, due to the uncertainties
`of electromagnetic propagation. Whenall ofthe stations in the BSS are
`mobile stations and there is no connection to a wired network, the BSS
`is called an independent BSS (IBSS). The IBSSis the entire network
`and only those stations communicating with each other in the IBSS are
`part of the LAN. An IBSSis typically a short-lived network, with a
`small numberofstations, that is created for a particular purpose, e.g., to
`exchange data with a vendorin the lobby of your company’s building or
`to collaborate on a presentation at a conference.
`
`the mobile stations all communicate directly with one
`In an IBSS,
`another. Not every mobile station may be able to communicate with
`every other mobile station, but they are all part of the same IBSS. There
`is also no relay function in an IBSS. Thus, if one mobile station must
`communicate with another, they must be in direct communication range.
`See Figure 2-1.
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`In aninfrastructure BSS,all mobile stations communicate with the AP.
`The AP provides both the connection to the wired LAN,if any, and the
`local relay function for the BSS. Thus, if one mobile station in the BSS
`must communicate with another mobile station, the communication is
`sent first to the AP and then from the AP to the other mobilestation.
`This causes communications that both originate and end in the same
`BSS to consume twice the bandwidth that the same communication
`would consume if sent directly from one mobile station to another.
`While this appearsto be a significant cost, the benefits provided by the
`APfar outweighthis cost. One of the benefits provided by the APis the
`buffering of traffic for a mobile station while that station is operating in
`a very low powerstate. The protocols and mechanismsfor the support
`of power saving by mobile stations is described in Chapter4.
`
`Extended Service Set (ESS)
`
`One of the mostdesirable benefits of a WLANis the mobility it pro-
`vides to its users. This mobility would not be of muchuseifit were con-
`fined to a single BSS. IEEE 802.11 extends the range of mobility it
`provides to any arbitrary range through the extended service set (ESS).
`An ESSis a set of infrastructure BSSs, where the APs communicate
`among themselves to forward traffic from one BSSto another and to
`facilitate the movement of mobile stations from one BSS to another.
`The APs perform this communication via an abstract medium called the
`distribution system (DS). The DS is the backbone of the WLAN and
`may be constructed of either wired or wireless networks. The DS is a
`thin layer in each AP that determines if communications received from
`the BSSare to be relayed back to a destination in the BSS, forwarded on
`the DS to another AP, or sent into the wired network infrastructure to a
`destination notin the ESS. Communications received by an AP from the
`DSare transmitted to the BSS to be received by the destination mobile
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`Chapter 2: IEEE Standard 802.11
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`station. To network equipmentoutside of the ESS, the ESSandall ofits
`mobile stations appears to be a single MAC-layer network whereall sta-
`tions are physically stationary. Thus, the ESS hides the mobility of the
`mobile stations from everything outside the ESS. This is the level of
`indirection provided by the IEEE 802.11 architecture, allowing existing
`networkprotocols that have no concept of mobility to operate correctly
`with a WLAN wherethereis lots of mobility. See Figure 2-2.
`
`
`
`Figure 2-2— Extended Service Set
`
`Onearea that is beyond the scope of the IEEE 802.11 standard is the
`communication between APs. There has been some industry coopera-
`tive work in this area to develop an inter-access point protocol (IAPP).
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`municates with another to exchange frames forstations in their BSSs,
`forward frames to follow mobile stations from one BSS to another, and
`exchange frames with wired networks, if any. As IEEE 802.11 describes
`it, the DS is not necessarily a network. The standard does not place any
`restrictions on how the DSis implemented, only on the services it must
`provide. Thus, the DS may be a wired network, such as IEEE 802.3, or
`it may be a purpose-built box that interconnects the APs and provides
`the required distribution services.
`
`Services
`
`There are nine services defined by the IEEE 802.11 architecture. These
`services are divided into two groups, station services and distribution
`services. The station services comprise authentication, deauthentication,
`privacy, and delivery of the data. The distribution services comprise
`association,disassociation, reassociation, distribution, and integration.
`
`Station Services
`
`The four station services—authentication, deauthentication, privacy,
`and data delivery—provide