802.11‘ Wireless Networks
`
`The Definitive Guide
`
`Matthew S. Gast
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`Beijing - Cambridge - Famham - Kéln - Paris - Sebastopol
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`- Ialpei - Tokyo
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`O’RE|LLY'
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`REMBRANDT EXHIBIT 2204
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`REMBRANDT EXHIBIT 2204
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`
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`The Definitive Guide
`
`Matthew S. Gast
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`Beijing - Cambridge - Famham - Kéln - Paris - Sebastopol
`
`- Ialpei - Tokyo
`
`O’RE|LLY'
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`REMBRANDT EXHIBIT 2204
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`REMBRANDT EXHIBIT 2204
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`
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`802.11‘ Wireless Networks: flie Definitive Guide
`by Matthew S. Gast
`
`Copyright © 2002 O'Reilly 51 Associates. Inc. All rights reserved.
`Printed in the United States of America.
`
`Published by O'Reilly 6! Associates. Inc.. I005 Gravenstein Highway North,
`Sebastopol. CA 95472.
`
`O'Reilly 6! Associates books may be purchased for educational. business. or sales promotional
`use. Online editions are also available for most titles (safan.oreilly.com). For more information
`contact our corporate/institutional sales department: (800) 9989938 or corporate@oreilly.com.
`
`Editor:
`
`Mike Loukides
`
`Production Editor:
`
`Matt Hutchinson
`
`(over
`
`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 6! Associates. Inc. Many of the designations used by manufacturers and
`sellers to distinguish their products are claimed as trademarks. Where those designations appear
`in this book. and O'Reilly 6: Associates. Inc. was aware of a trademark claim. the designations
`have been printed in caps or initial caps. The association between the image of a horseshoe bat
`and 802.11 wireless networks is a trademark of 0'Reilly 6! Associates. Inc.
`
`802.111 and all 802.11-based trademarks and logos are trademarks or registered trademarks of
`IEEE. Inc. in the United States and other countries. O'Reilly 6: Associates. Inc. is independent of
`IEEE.
`
`While every precaution has been taken in the preparation of this book. the author and publisher
`assume no responsibility for errors or omissions, or for damages resulting from the use of the
`information contained herein.
`
`ISBN: 0-596-00183-5
`
`[Ml
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`REMBRAN DT EXHIBIT 2204
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`Figure 10-17. Throughput response to interference in DSSS systems
`
`interference between two
`separated by an adequate amount. Generally speaking,
`direct-sequence devices is a problem long before a primary band user notices anything.
`
`Differential Phase Shift Keying (DPSK)
`
`Differential phase shift keying (DPSK) is the basis for all 802.11 direct-sequence sys-
`tems. As the name implies. phase shift keying (PSK) encodes data in phase changes
`of the transmitted signal. The absolute phase of a wavefonn is not relevant in PSK;
`only changes in the phase encode data. Like frequency shift keying, PSK resists inter-
`ference because most interference causes changes in amplitude. Figure 10-18 shows
`two identical sine waves shifted by a small amount along the time axis. The offset
`between the same point on two waves is the phase difference.
`
`
`
`Figure 10-18. Phase difference between two sine waves
`
`Differential binary phase shift nil"! (DBPSK)
`
`The simplest form of PSK uses two carrier waves, shifted by a half cycle relative to
`each other. One wave, the reference wave,
`is used to encode a 0; the half-cycle
`shifted wave is used to encode a 1. Table 10-6 summarizes the phase shifts. and
`Figure 10-19 illustrates the encoding as a phase difference from a preceding sine
`WZVC.
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`in |
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`ciapmm Thel$MPIfYs:Fil,DS,ai'idMl/D5
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`A
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`l
`l
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`Figure 10-I9. DBPSK encoding
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`Table 10-6. DBPSK phase shifts
`
`Syfid
`0
`
`1
`
`ibefll
`0
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`100' (1: radians)
`
`To stick with the same example, encoding the letter M (1001 101 in binary) is a mat-
`ter of dividing up the time into seven symbol times then transmitting the wave with
`appropriate phase shift at each symbol boundary. Figure 10-20 illustrates the encod-
`ing. Time IS divided into a series of symbol periods. each of which is several times the
`period of the carrier wave. When the symbol is a 0, there is no change from the phase
`of the previous symbol. and when the symbol is a 1. there is a change of half a cycle.
`These changes result in “pinches” of the carrier when l is transmitted and a smooth
`transition across the symbol time boundary for 0.
`
`Differential quadrature phase shift keying (DQPSK)
`
`Like ZGFSK, DBPSK is limited to one bit per symbol. More advanced receivers and
`transmitters can encode multiple bits per symbol using a technique called differen-
`tial quadrature phase shift yeying (DQPSK). Rather than a fundamental wave and a
`half—cycle shifted wave. DQPSK uses a fundamental wave and three additional
`waves, each shifted by a quarter cycle, as shown in Figure 10-21. Table l0-7 summa-
`rizes the phase shifts.
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`1
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`l
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`10
`Phas¢=270°
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`l
`Z
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`Figure I0-21. DQPSK encoding
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`Table 10-7. DQPSK phase shi/ts
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`Syfld
`N
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`Phlfl
`0
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`01
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`II
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`10
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`M’ (iv) radians)
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`180' (it radians)
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`270' (31!/Zot-it/2 rations)
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`Now encode M in DQPSK (Figure 10-22). In the UTF-8 character set, M is repre-
`sented by the binary string 01001101 or, as the sequence of four two-bit symbols, 0]-
`00-1 1-01. In the first symbol period, there is a phase shift of 90 degrees; for clarity.
`the figure shows the phase shift from a pure sine wave. The second symbol results in
`no phase shift. so the wave continues without a change. The third symbol causes a
`phase Shift of 180 degrees, as shown by the sharp change from the highest amplitude
`to the lowest amplitude. The final symbol causes a phase shift of 90 degrees.
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`I84
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`I
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`(hQt!t10:l'helSMP|lYs:F||,0S,aM|lNDS
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`Finn-+!0°
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`lockup hue-+1fl“hnu-+fl°
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`Figure 10-22. The letter M encoded in DQPSK
`
`The obvious advantage of DQPSK relative to DBPSK is that the four-level encoding
`mechanism can have a higher throughput. The cost of using DQPSK is that it cannot
`be used in some environments because of severe multipath interference. Multipath
`interference occurs when the signal takes several paths from the transmitter to the
`receiver. Each path has a different length; therefore, the received signal from each
`path has a different delay relative to the other paths. This delay is the enemy of an
`encoding scheme based on phase shifts. Wavefronts are not labeled or painted differ-
`ent colors, so a wavefront could arrive later than expected because of a long path or
`it could simply have been transmitted late and phase shifted. In environments where
`multipath interference is severe. DQPSK will break down much quicker than DBPSK.
`
`DS Physical-Layer Convergence (PLCP)
`
`As in the FH PHY. frames must be processed by the PLCP before being transmitted
`into the air.
`
`Framing and scrambling
`
`The PLCP for the DS PHY adds a six-field header to the frames it receives from the
`
`MAC. In keeping with ISO reference model terminology, frames passed from the
`MAC are PLCP service data units (PSDUS). The PLCP framing is shown in
`Figure 10-23.
`
`r.‘'&
`
`vamue
`I6
`a mulls“
`3
`ll
`16
`12:0
`
`
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`I
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`l
`l
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`Figure 10-23. D5 PLCPframmg
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`The FH PHY uses a data whitener to randomize the data before transmission. but the
`
`data whitener applies only to the MAC frame trailing the PLCP header. The DS PHY
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`REMBRANDT EXHIBIT 2204
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