802.11“ Wireless Networks
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`The Definitive Guide
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`Matthew 5. Cast
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`Beijing - Cambridge - Famham - K6In . Paris - Sebastopol
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`- Ialpei - Tokyo
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`O’REILLY'
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`REMBRANDT EXHIBIT 201 3
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`REMBRANDT EXHIBIT 2013
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`The Definitive Guide
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`Matthew 5. Cast
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`Beijing - Cambridge - Famham - K6In . Paris - Sebastopol
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`- Ialpei - Tokyo
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`O’REILLY'
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`REMBRANDT EXHIBIT 201 3
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`REMBRANDT EXHIBIT 2013
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`802.11“ Wireless Networks: The Definitive Guide
`by Matthew S. Gast
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`Copyright © 2002 O’Reilly 6! Assocmtes. Inc. All rights reserved.
`Printed in the United States of America.
`
`Published by O'Reilly 6! Assocmtes. lnc.. IOOS Gravenstein Highway North,
`Sebastopol. CA 95472.
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`O'Reilly 6! Associates books may be purchased for educational. busmess. or sales promotional
`use. Online editions are also available for most titles (safanpreillycom). For more information
`contact our corporate/institutional sales department: (800) 9989938 or corporate@oreilly.com.
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`Editor:
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`Mike Loukides
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`Production Editor:
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`Matt Hutchinson
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`(over Designer:
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`Ellie Volckhausen
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`Printing History:
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`April 2002:
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`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 desngnations appear
`in this book. and O'Reilly 6: Assoaates. Inc. was aware of a trademark claim. the deSIgnations
`have been printed in caps or initial caps. The assoctation between the image of a horseshoe bat
`and 802.11 Wireless networks is a trademark of O'Reilly 6! Assocmtes. Inc.
`
`802.111 and all 802.11-based trademarks and logos are trademarks or registered trademarks of
`lEEE. Inc. in the United States and other countries. O'Reilly 6: Assoaates. 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.
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`ISBN: 0-596-00183-5
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`Figure 10-17. Throughput response to Interference in DSSS systems
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`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)
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`Differential phase shift keying (DPSK) is the baSlS 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 waveform 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.
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`
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`Figure 10-18. Phase difference between two stne waves
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`Differential binary phase shift keying (DBPSK)
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`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
`wave.
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`in | mm mnsumm,os,mims
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`Figure 10-19. DBPSK encoding
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`Table 10-6. DBPSK phase shifts
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`w M“
`0
`0
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`1
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`m‘ (n Mans)
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`To stick With the same example. encoding the letter M (1001101 in binary) is a mat-
`ter of dmding 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 15 divided into a series of symbol periods. each of which is several times the
`penod 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 1 is transmitted and a smooth
`transition across the symbol time boundary for 0.
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`Differential quadrature phase shift keying (DQPSK)
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`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 10-7 summa-
`rizes the phase shifts.
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`M=270°
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`1
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`thure 10-21. DQPSK encoding
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`Table 10- 7. DQPSK phase shifts
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`9]“
`on
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`II
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`90' (1V) rains)
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`180' (8 WIS)
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`270'(31t/20t-1t/2radms)
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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]-
`00-11-01. 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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`I“ I M10: ThelSMHlefflJSUIdHMJS
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`Figure 10-22. The letter M encoded in DQPSK
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`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.
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`DS Physical-Layer Convergence (PLCP)
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`As in the PH PHY. frames must be processed by the PLCP before being transmitted
`into the air.
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`Framing and scrambling
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`The PLCP for the DS PHY adds a six-field header to the frames it receives from the
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`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.
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`Figure 10-23. DS PLCP/ranting
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`The FH PHY uses a data whitener to randomize the data before transmission. but the
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`data whitener applies only to the MAC frame trailing the PLCP header. The DS PHY
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