`
`CHAPTER 9
`
`Physical Layer Overview
`
`Anygirl can be glamorous. All you have
`to do is stand still and look stupid.
`—Hedy Lamarr
`Protocollayering allows for research, experimentation, and improvement on differ-
`ent parts of the protocol stack. The second major component of the 802.11 architec-
`ture is the physical layer, which is often abbreviated PHY. This chapter introduces
`the common themes and techniques that appear in each of the radio-based physical
`layers and describes the problems commontoall radio-based physical layers;it is fol-
`lowed by more detailed explanations of each of the physical layers that are standard-
`ized for 802.11.
`
`Physical-Layer Architecture
`The physical layeris divided into two sublayers: the Physical Layer Convergence Pro-
`cedure (PLCP) sublayer and the Physical Medium Dependent (PMD) sublayer. The
`PLCP (Figure 9-1) is the glue between the frames of the MAC andtheradio transmis-
`sions in the air. It adds its own header. Normally, frames include a preamble to help
`synchronize incoming transmissions. The requirements of the preamble may depend
`on the modulation method, however, so the PLCP addsits own headerto any trans-
`mitted frames. The PMDis responsible for transmitting any bits it receives from the
`PLCPinto the air using the antenna. The physical layer also incorporates a clear
`channel assessment (CCA) functionto indicate to the MAC whena signalis detected.
`
`
`OS! Layer 2: Data Link
`
`OSI Layer 1; Physical
`
`
`Figure 9-1. Physical layer logical architeciure
`
`
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`DELL-1031, Part 2
`10,079,707
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`DELL-1031, Part 2
`10,079,707
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`The Radio Link
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`Three physical layers were standardizedin the initial revision of 802.11, which was
`published in 1997:
`* Frequency-hopping (FH) spread-spectrum radio PHY
`* Direct-sequence (DS) spread-spectrum radio PHY
`*
`Infrared light (IR) PHY
`In 1999, two further physical layers based on radio technology were developed:
`* 802.11a: Orthogonal Frequency Division Multiplexing (OFDM) PHY
`* 802.11b: High-Rate Direct Sequence (HR/DS or HR/DSSS) PHY
`This book discusses the four physical layers based on radio waves in detail; it does
`notdiscuss the infrared physical layer, which isn’t widely used.
`
`
`
`
`The IR PHY
`
`802.11 also includes a specification for a physical layer based on infrared (IR) light.
`Using infraredlight instead of radio waves seems to have several advantages. IR ports
`are less expensive than radio transceivers—infact, the cost is low enoughthat IR ports
`are standard on practically every laptop.
`IR is extremelytolerant of radio frequency (RF) interference because radio waves oper-
`ate at a totally different frequency. This leads to a second advantage: IR is unregulated.
`Product developers do not needto investigate and comply with directives from several
`regulatory organizations throughout the world.
`Security concerns regarding 802.11 are largely based on the threat of unauthorized
`users connecting to a network, Light can be confined to a conference room oroffice by
`simply closing the door. IR-based LANscan offer some ofthe advantagesofflexibility
`and mobility but with fewer security concerns. This comes at a price. IR LANsrely on
`scattering light off the ceiling, so range is much shorter.
`This discussion is academic, however. No products have been created based on the IR
`PHY. Theinfrared ports on laptops comply with a set of standards developed by the
`Infrared Data Association (IrDA), not 802.11. Even if products were created around
`the IR PHY,the big drivers to adopt 802.11 are flexibility and mobility, which are bet-
`ter achieved by radio’s longer range and ability to penetrate solid objects.
`
`
`
`.
`
`Licensing
`The classic approach to radio communications is to confine an information-carry-
`ing signal to a narrow frequency band and pump as much poweras possible (or
`legally allowed) into the signal. Noise is simply the naturally present distortion in
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` the frequency band. Transmitting a signalin the face of noise relies on brute force—
`
`you simply ensure that the powerofthe transmitted signal is much greater than the
`noise.
`
`In theclassic transmission model, avoiding interference is a matter of law, not phys-
`ics. With high poweroutput in narrow bands,a legal authority must impose rules on
`howthe RFspectrumis used. In the United States, the Federal Communications
`Commission (FCC) is responsible for regulating the use of the RF spectrum. Many
`FCCrules are adopted by other countries throughout the Americas. Europeanalloca-
`tion is performed by CEPT’s European Radiocommunications Office (ERO) and the
`European Telecommunications StandardsInstitute (ETSI). Other allocation work is
`performed by the International Telecommunications Union (ITU).
`For the most part, an institution must have a license to transmit at a given fre-
`quency.Licensescan restrict the frequencies and transmission powerused,as well as
`the area over whichradio signals can be transmitted. For example, radio broadcast
`stations must havealicense from the FCC. Likewise, mobile telephone networks
`mustobtain licenses to use the radio spectrum in a given market. Licensing guaran-
`tees the exclusive use of a particular set of frequencies. When licensed signals are
`interfered with, the license holder can demandthata regulatory authority step in and
`resolve the problem,usually by shutting down thesourceofinterference.
`
`Frequencyallocation and unlicensed frequency bands
`Radio spectrum is allocated in bands dedicated to a particular purpose. A band
`defines the frequencies thata particular application mayuse.It often includes guard
`bands, which are unused portionsof the overall allocation that prevent extraneous
`leakage from thelicensed transmission from affecting anotherallocated band.
`Several bands have beenreserved for unlicensed use. For example, microwave ovens
`operate at 2.45 GHz, butthereislittle sense in requiring homeownersto obtain per-
`mission from the FCC to operate microwave ovens in the home. To allow consumer
`markets to develop around devices built for home use, the FCC (and its counter-
`Parts in other countries) designated certain bands for the use of “industrial, scien-
`tific, and medical” equipment. These frequency bands are commonlyreferred to as
`the ISM bands. The 2.4-GHzbandis available worldwidefor unlicensed use.’ Unli-
`censed use, however, is not the same as unlicensedsale. Building, manufacturing,
`and designing 802.11 equipment does requirealicense; every 802.11 card legally
`sold in the U.S. carries an FCC identification number. The licensing process requires
`the manufacturertofile a fair amountof information with the FCC.All this informa-
`tion is a matter of public record and can be looked up online by using the FCC iden-
`tification number.
`
`* The 2.4-GHz ISM bandis reserved by the FCCrules (Title 47 of the Code ofFederal Regulations), part 15.247,
`ETSIreserved the same spectrumin ETSI Technical Specifications (ETS) 300-328.
`
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`The Nonexistent Microwave Absorption Peak of Water
`It is often said that microwave ovens operate at 2.45 GHz becauseit correspondsto a
`particular excitation modeof water molecules. This is sometimes even offeredas a rea-
`son why 802.11 cannotbe used over long distances. Atmospheric water vapor would
`severely attenuate any microwavesignals in rain or in humidclimates.
`The existence of a water excitation mode in the microwave rangeis a myth.If there was
`an excitation mode, water would absorba significant amountofthe microwave energy.
`Andif that energy was absorbed effectively by water, microwave ovens would be
`unable to heat anything other than the water near the surface of food, which would
`absorball the energy, leaving the center cold and raw. An absorption peak would also
`mean that atmospheric water vapor would disruptsatellite communications, whichis
`not an observed phenomenon. NASA Reference Publication 1108(02), Propagation
`Effects onSatellite Systems at Frequencies Below 10 GHz, discusses the expectedsignal
`loss due to atmospheric effects, and the loss is much more pronouncedat frequencies
`above 10 GHz. The microwaveabsorptionpeakfor water, for example, is at 22.2 GHz,
`Microwave ovens do not work by moving water molecules into an excited state.
`Instead, they exploit the unusually strong dipole moment of water. Althoughelectri-
`cally neutral, the dipole momentallows a water molecule to behaveasif it were com-
`posed of small positive and negative charges ateither end of a rod. In the cavity of a
`microwave oven, the changing electric and magnetic fields twist the water molecules
`back andforth. Twisting excites the water molecules by adding kinetic energy to the
`entire molecule but does not changethe excitation state of the molecule or anyofits
`components,
`
`:
`
`
`
`Use of equipmentin the ISM bandsis generally license-free, provided that devices
`operating in them do not emit significant amountsof radiation. Microwave ovens are
`high-powered devices, but they have extensive shielding to restrict radio emissions.
`Unlicensed bands have seen a great deal of activity in the past three years as new
`communications technologies have been developed to exploit the unlicensed band.
`Users can deploy new devices that operate in the ISM bands without going through
`any licensing procedure, and manufacturers do not need to be familiar with the
`licensing procedures and requirements. At the time this book was written, a number
`of new communications systems were being developed for the 2.4-GHz ISM band:
`* The variants of 802.11 that operate in the band (the frequency-hopping layer
`and both spread spectrum layers)
`* Bluetooth, a short-range wireless communications protocol developed by an
`industry consortium led by Ericsson
`* Spread-spectrum cordless phones introduced by several cordless phone manu-
`facturers
`
`* X10, a protocol used in home automation equipmentthat can use the ISM band
`for video transmission
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` Unfortunately, “unlicensed” does not necessarily mean “plays well with others.” All
`
`that unlicensed devices must dois obey limitations on transmitted power. No regula-
`tions specify coding or modulation,soit is not difficult for different vendors to use
`the spectrum in incompatible ways. As a user, the only wayto resolve this problemis
`to stop using one of the devices; because the devices are unlicensed, regulatory
`authorities will notstep in.
`
`Other unlicensed bands
`Additional spectrumis available in the 5-GHz range. In the UnitedStates,the follow-
`ing three bandsare called the Unlicensed National Information Infrastructure (UNID
`bands:
`
`© 5.15-5.25 GHz
`
`° 5.25-5.35 GHz
`
`® 5.725—5.825 GHz
`
`Devices operating in the UNII bands mustobey limitations on radiated power, but
`there are no further constraints imposed on them. European regulatory authorities
`have set aside the same frequency bands, but thefirst two bands are dedicated to
`HiperLANtechnology; the third bandis the only one potentially available for 802.11
`networks.
`
`Spread Spectrum
`Spread-spectrum technology is the foundation used to reclaim the ISM bands for
`data use. Traditional radio communications focus on cramming as much signal as
`possible into as narrow a band aspossible. Spread spectrum works by using mathe-
`matical functionsto diffuse signal powerovera large range of frequencies. When the
`receiver performs the inverse operation, the smeared-outsignal is reconstituted as a
`narrow-bandsignal, and, more importantly, any narrow-band noise is smeared out
`so the signal shines throughclearly.
`Use of spread-spectrum technologies is a requirement for unlicensed devices. In
`some cases, it is a requirement imposedbytheregulatory authorities; in other cases,
`it is the only practical way to meet regulatory requirements. As an example, the FCC
`requires that devices in the ISM band use spread-spectrum transmission and impose
`acceptable ranges on several parameters.
`Spreading the transmission over a wide band makestransmissions looklike noise to
`a traditional narrowbandreceiver. Some veridors of spread-spectrum devices claim
`that the spreading adds security because narrowbandreceivers cannot be used to
`Pick up thefull signal. Any standardized spread-spectrum receivercaneasily be used,
`though,so additional security measures are mandatory in nearly all environments.
`
`
`
` * The UNII bandsare defined by FCC part 15.407,
`
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`This does not mean that spread spectrum is a “magic bullet” that eliminates interfer-
`ence problems. Spread-spectrum devices can interfere with other communications
`systems, as well as with each other; and traditional narrow-spectrum RF devices can
`interfere with spread spectrum. Although spread spectrum doesa better job of deal-
`ing with interference within other modulation techniques, it doesn’t make the prob-
`lem go away. As more RF devices (spread spectrum or otherwise) occupy the area
`that your wireless network covers, you'll see the noise level go up, the signal-to-noise
`ratio decrease, and the range over which you canreliably communicate drop.
`
`To minimize interference between unlicenced devices, the FCC imposes limitations
`on the power of spread-spectrum transmissions. The legal limits are one watt of
`transmitter output power and four watts of effective radiated power (ERP). Four
`watts of ERP are equivalent to 1 watt with an antenna system that has 6-dB gain,or
`500 milliwatts with an antenna of 9-dB gain, etc.” The transmitters and antennas in
`PC Cards are obviously well within those limits—and you’re notgetting close even if
`you use a commercial antenna. But it is possible to cover larger areas by using an
`external amplifier and a higher-gain antenna. There’s no fundamental problem with
`doing this, but you must makesure that you stay within the FCC’s powerregulations.
`
`Types of spread spectrum
`The radio-based physical layers in 802.11 use three different spread-spectrum
`techniques:
`
`Frequency hopping (FH or FHSS)
`Frequency-hopping systems jump from one frequency to another in a random
`pattern, transmitting a short burst at each subchannel. The 2-Mbps FH PHYis
`specified in clause 14.
`Direct sequence (DS or DSSS)
`Direct-sequence systems spread the powerout over a wider frequency band using
`mathematical coding functions. Two direct-sequence layers were specified. The
`initial specification in clause 15 standardized a 2-Mbps PHY,and 802.11b added
`clause 18 for the HR/DSSS PHY.
`
`Orthogonal Frequency Division Multiplexing (OFDM)
`OFDMdivides an available channel into several subchannels and encodes a por-
`tion of the signal across each subchannelin parallel. The techniqueis similar to
`the Discrete Multi-Tone (DMT) technique used by some DSL modems. Clause
`17, added with 802.11a, specifies the OFDM PHY.
`
`* Remember that the transmissionline is part of the antenna system, andthe system gain includes transmis-
`sion line losses. So an antenna with 7.5-dB gain and a transmissionline with 1.5-dB loss has an overal system
`gain of 6 dB. It’s worth noting that transmission line losses at UHF freqenciesare often very high; asaresult,
`you should keep your amplifier as close to the antenna aspossible.
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`The Unlikely Invention of Spread Spectrum
`Spread spectrum was patented in the early 1940s by Austrian-born actress Hedy
`Lamarr. She wascertainly better knownfor other reasons: appearingin thefirst nude
`scene onfilm in the Czech film Ecstasy, herlater billing as “the most beautiful woman
`in the world” by Hollywood magnate Louis Mayer, and as the model for Catwoman in
`the Batman comics.
`Beforefleeing the advance of Nazi Germany, she was married to an Austrian arms mer-
`chant. While occupyingthe only socially acceptable role available to her as a hostess
`and entertainer of her husband’s businessclients, she learned that radio remote control
`of torpedoes was a majorarea of research for armaments vendors. Unfortunately, nar-
`rowband radio communications were subject to jamming, which neutralized the
`advantage of radio-guided weapons. From thesediscussions,shefirst hit on the idea of
`using a complex butpredetermined hoppingpattern to move the frequencyofthe con-
`trol signal around. Even if short bursts on a single frequency could be jammed, they
`would move around quickly enough to prevent total blockage. Lamarr worked out
`everything except howto precisely control the frequency hops.
`After arriving in the United States, she met George Antheil, an avant-garde American
`composer known as the “bad boy of music”for his dissonantstyle. His famous Ballet
`mécanique used (among many outrageous noisemakers) 16 player pianos controlled
`from a single location. Performing the piece required precisely controlled timing
`between distributed elements, which was Lamarr’s only remaining challenge in con-
`trolling the hopping pattern. Together,
`they were granted U.S. patent number
`2,292,387 in 1942. The patent expired in 1959 without earninga cent foreither of
`them, and Lamarr’s contributions went unacknowledged for many years because the
`name on the patent was Hedy Kiesler Markey, her married nameatthe time. The
`emerging wireless LAN marketin the late 1990sled to the rediscovery of her invention
`and widespreadrecognition for the pioneering work that laid the foundation for mod-
`ern telecommunications.
`Frequency-hopping techniques werefirst used by U.S. ships blockading Cuba during
`the Cuban Missile Crisis. It took many years for the electronics underpinning spread-
`spectrum technology to become commercially viable. Now that they have, spread-
`spectrum technologies are used in cordless and mobile phones, high-bandwidth wire-
`less LAN equipment, and every device that operates in the unlicensed ISM bands.
`Unfortunately, Hedy Lamarr died in early 2000, just as the wireless LAN market was
`gaining mainstream attention.
`
`Frequency-hopping systems are the cheapest to make. Precise timing is needed to
`control the frequency hops, but sophisticated signal processing is not required to
`extract the bit stream from the radio signal. Direct-sequence systems require more
`sophisticated signal processing, whichtranslates into more specialized hardware and
`higherelectrical power consumption. Direct-sequence techniquesalso allow a higher
`data rate than frequency-hopping systems.
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`RF and 802.11
`
`
`
`802.11 has been adoptedat a stunning rate. Many network engineers accustomed to
`signals flowing along well-defined cable paths are now faced with a LAN that runs
`over a noisy, error-prone, quirky radio link. In data networking, the success of 802.11
`has inexorably linked it with RF engineering. A true introduction to RF engineering
`requires at least one book, and probably several. For the limited purposes I have in
`mind, the massive topic of RF engineering can be divided into two parts: how to
`make radio waves and how radio waves move.
`
`RF Components
`RF systems complement wired networks by extending them. Different components
`may be used depending on the frequency andthedistancethatsignals are required to
`reach, but all systems are fundamentally the same and made from a relatively small
`numberof distinct pieces. Two RF components are of particularinterest to 802.11
`users: antennas and amplifiers. Antennas are of general interest since they are the
`most tangible feature of an RF system. Amplifiers complement antennas by allowing
`them to pump out more power, which maybeofinterest depending on the type of
`802.11 network you are building.
`
`Antennas
`
`Antennas are the most critical component of any RF system because they convert
`electrical signals on wires into radio waves andvice versa. In block diagrams, anten-
`nas are usually represented bya triangular shape, as shownin Figure 9-2.
`
`
`Figure 9-2, Antenna representationsin diagrams
`
`_Y
`
`To function at all, an antenna must be made of conducting material. Radio waves
`hitting an antenna cause electrons to flow in the conductor and create a current.
`Likewise, applying a current to an antenna creates an electric field around the
`antenna. As the current to the antenna changes, so does theelectric field. A chang-
`ing electric field causes a magnetic field, and the waveis off.
`the higher the fre-
`The size of the antenna you need depends on the frequency:
`quency, the smaller the antenna. The shortest simple antenna you can make at any
`frequency is 1/2 wavelength long (though antenna engineers can playtricks to reduce
`antenna size further). This rule of thumb accounts for the huge size of radio broad-
`cast antennas andthe small size of mobile phones. An AMstation broadcasting at 830
`kHz has a wavelength of about 360 meters and a correspondingly large antenna, but
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`an 802.11b network interface operating in the 2.4-GHz bandhas a wavelength of just
`12.5 centimeters. With some engineering tricks, an antenna can be incorporated into
`a PC Card, and a moreeffective external antenna can easily be carried in a backpack.
`Antennas can also be designed with directional preference. Many antennas are omni-
`directional, which means they send and receive signals from any direction. Some
`applications may benefit from directional antennas, which radiate and receive on a
`narrowerportionof the field. Figure 9-3 comparesthe radiated power of omnidirec-
`tionaland directional antennas.
`
`
`Omnidirectional
`
`Directional
`
`eS a
`
`Figure 9-3. Radiated power for omnidirectional anddirectional antennas
`
`
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`—w
`|
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`For a given amount of input power, a directional antenna can reach farther with a
`clearer signal. They also have much highersensitivity to radio signals in the domi-
`nant direction. When wireless links are used to replace wireline networks, direc-
`tional antennas are often used. Mobile telephone network operators also use
`directional antennas when cells are subdivided. 802.11 networkstypically use omni-
`directional antennas for both ends of the connection, thoughthere are exceptions—
`particularly if you want the network to span a longer distance. Also, keep in mind
`that there is no such thing as a truly omnidirectional antenna. We’re accustomedto
`thinking of vertically mounted antennas as omnidirectional because the signal
`doesn’t vary significantly as you travel around the antennain a horizontal plane. But
`if you lookat the signal radiated vertically (i.e., up or down) from the antenna,you'll
`find thatit’s a different story. And that part of the story can become importantif
`you’re building a network for a college or corporate campus and wantto locate
`antennas on thetop floors of your buildings.
`Ofall the componentspresented in this section, antennas are the mostlikely to be
`separated from therest of the electronics. In this case, you need a transmissionline
`(some kind of cable) between the antenna and the transceiver. Transmission lines
`usually have an impedanceof 50 ohms.
`In terms of practical antennas for 802.11 devices in the 2.4-GHz band,the typical
`wireless PC Card has an antenna built in. As you can probably guess, that antenna
`will do the job, but it’s mediocre. Most vendors,if notall, sell an optional external
`
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`antenna thatplugsinto the card. These antennas are decentbut not great, and they
`will significantly increase the range of a roaming laptop. You can usually buy some
`cable to separate the antenna from the PC Card, which can be useful for a basesta-
`tion. However, be careful—coaxial cable (especially small coaxial cable) is very lossy
`at these frequencies, so it’s easy to imagine that anything you gain by better antenna
`placementwill be lost in the cable. People have experimented with building high-
`gain antennas, some for portable use, some for base station use. And commercial
`antennas are available—some designed for 802.11 service, some adaptable if you
`know what you’re doing.
`
`Amplifiers
`Amplifiers make signals bigger. Signal boost, or gain, is measured in decibels (dB).
`Amplifiers can be broadly classified into three categories: low-noise, high-power, and
`everything else. Low-noise amplifiers (LNAs) are usually connected to an antenna to
`boost the received signalto a level that is recognizable by the electronics the RF sys-
`tem is connected to. LNAsare also rated for noise factor, which is the measure of
`how much extraneous information the amplifier introduces. Smaller noise factors
`allow the receiver to hear smaller signals and thusallow for a greater range.
`High-power amplifiers (HPAs) are used to boost a signal to the maximum power
`possible before transmission. Output power is measured in dBm, whichare related
`to watts (see the sidebar). Amplifiers are subject to the laws of thermodynamics, so
`they give off heat in addition to amplifying the signal. The transmitter in an 802.11
`PC Cardis necessarily low-power because it needs to run offa battery ifit’s installed
`in a laptop, butit’s possible to install an external amplifier at fixed access points,
`which can be connected to the powergrid where poweris more plentiful.
`This is where things can get tricky with respect to compliance with regulations. 802.11
`devices are limited to one watt of power output and four watts effective radiated
`power (ERP). ERP multiplies the transmitter’s power output by the gain of the
`antenna minustheloss in the transmission line. So if you have a 1-watt amplifier, an
`antenna that gives you 8 dB of gain, and 2 dB of transmissionline loss, you have an
`ERP of 4 watts; the total system gain is 6 dB, which multiplies the transmitter’s power
`by a factorof 4.
`
`RF Propagation
`In fixed networks,signals are confined to wire pathways, so network engineers do
`not need to know anything about the physics ofelectrical signal propagation.
`Instead, there are a few rules used to calculate maximum segmentlength, and as long
`as the rules are obeyed, problems are rare. RF propagation is not anywhere nearas
`simple.
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`Decibels and SignalStrength
`Amplifiers may boost signals by orders of magnitude. Rather than keeptrack ofall
`those zeroes, amplifier power is measured in decibels (dB).
`dB = 10 x logi0 (powerout/powerin)
`Decibelratingsare positive when the outputis larger than the input and negative when
`the outputis smallerthan the input. Each 10-dB changecorrespondsto a factor of 10,
`and 3-dB changes are a factor of 2. Thus, a 33-dB change correspondsto a factor of
`2000:
`
`33 dB = 10 dB + 10dB+10dB+3dB=10x 10x 10x 2 = 2000
`Power is sometimes measured in dBm, whichstands for dB above one milliwatt. To
`find the dBm ratio, simply use 1 mW asthe input powerin thefirst equation.
`It’s helpful to rememberthat doubling the poweris a 3-dB increase. A 1-dB increase is
`roughly equivalent to a powerincrease of 1.25. With these numbers in mind, you can
`quickly perform mostgain calculations in your head.
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`Multipath interference
`Oneof the major problemsthat plague radio networksis multipath fading. Waves
`are added by superposition. When multiple waves converge on a point, the total
`wave is simply the sum of any componentwaves. Figure 9-4 shows a few examples of
`superposition.
`In Figure 9-4c, the two waves are almost exactly the opposite of each other, so the
`net result is almost nothing. Unfortunately, this result is more common than you
`might expect in wireless networks. Most 802.11 equipment uses omnidirectional
`antennas, so RF energyis radiated in every direction. Waves spread outward from
`the transmitting antennain all directions and are reflected by surfaces in the area.
`Figure 9-5 showsahighly simplified example of twostations in a rectangular area
`with no obstructions.
`This figure shows three paths from the transmitter to the receiver. The waveat the
`receiver is the sum ofall the different components.It is certainly possible that the
`paths shownin Figure 9-5 will all combineto give a net waveof0,in which case the
`receiver will not understand the transmission because there is no transmission to be
`received.
`Because theinterference is a delayed copy of the same transmission ona different
`path, the phenomenonis called multipath fading or multipath interference. In many
`Cases, multipath interference can be resolved by changingtheorientation orposition
`of the receiver.
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`Figure 9-5. Multiple paths
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`Inter-symbolinterference(1S!)
`Multipath fading is a special case of inter-symbol interference. Waves that take dif-
`ferent paths from the transmitter to the receiver will travel different distances and be
`delayed with respect to each other, as in Figure 9-6. Once again, the two waves com-
`bine by superposition, but the effect is that the total waveform is garbled. In real-
`world situations, wavefronts from multiple paths may be added. Thetime between
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`the arrival of the first wavefront and the last multipath echo is called the delay
`spread. Longer delay spreads require more conservative coding mechanisms. 802.11b
`networks can handle delay spreads of up to 500 ns, but performanceis muchbetter
`when the delay spread is lower. When the delay spread is large, many cards will
`reduce the transmission rate; several vendors claim that a 65-ns delay spread is
`required for full-speed 11-Mbps performanceat a reasonable frame error rate. A few
`wireless LAN analysis tools candirectly measure delay spread.
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`Figure 9-6. Inter-symbolinterference
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`CHAPTER 10
`The ISM PHYs:FH, DS, and HR/DS
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`This chapter goes into detail about the physical layers specified by the 802.11 stan-
`dards for use in the microwave ISM bandat 2.4 GHz.Assuch,it is one of the most
`difficult, most interesting, and least useful chapters in the book.Feelfree to skip it—
`everything you're likely to need to know about the physical layer is covered in
`Chapter 9. But if you wanta challenge, or if you find the internals of wireless net-
`worksfascinating, read on.
`The current version of the 802.11 standardspecifies three physicallayers in the 2.4-GHz
`ISM band:
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`FH PHY
`A low-rate, frequency-hopping layer
`DS PHY
`A low-rate, direct-sequencelayer
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`HR/DSSS PHY
`A high-rate, direct-sequence layer added by 802.11b
`The last of these is probably the only one you'll see in use, particularly if you don’t
`have a lot of older equipment. But if you’re taking the trouble to dive through this
`chapter, you might as well see how the technology developed. There is one additional
`physical layer, a very high-speed layer standardized in 802.11a;
`it
`is discussed in
`Chapter 11. Standardization work has begun on a fourth physical layer for the 2.4-GHz
`ISM bandthat promises speedsof up to 54 Mbps.
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`802.11 FH PHY
`Ofall the physical layers standardized in thefirst draft of 802.11 in 1997, the fre-
`quency-hopping, spread-spectrum (FH or FHSS) layer was the first layer to see wide-
`spread deployment. The electronics used to support frequency-hopping modulation
`are relatively cheap and do not have high power requirements. Initially, the main
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`advantage to using frequency-hopping networks was that a greater numberof net-
`works could coexist, and the aggregate throughputofall the networksin a given area
`was high. With the advent of higher-rate, direct-sequence systems, the aggregate
`throughput advantage of frequency-hopping has been demolished.
`This chapter describes the basic concepts and modulation techniques used by the FH
`PHY.It also shows how the physical layer convergence procedure prepares frames
`for transmission on the radio link and touches briefly on a few details about the
`physical medium itself. At this point, the FH PHYis largely a footnote in the history
`of 802.11, so you may wantto skip this section and move ahead to the next section
`on the DS PHY. However, understanding how 802.11 technology developed will give
`you a better feeling for how all the piecesfit together.
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`Frequency-Hopping Transmission
`Frequency hopping depends on rapidly changing the transmission frequency in a
`predetermined, pseudorandompattern,asillustrated in Figure 10-1. The vertical axis
`of the graph divides the available frequency into a numberofslots. Likewise, timeis
`divided into a series of slots. A hopping pattern controls how theslots are used. In
`the figure, the hopping pattern is {2,8,4,7}. Timing the hopsaccurately is the key to
`success; both the transmitter and receiver must be synchronized so the receiveris
`alwayslistening on the transmitter’s frequency.
`
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`opeNwPUMDA“
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`Frequency
`slot
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`Timeslot
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`Figure 10-1. Frequency hopping
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`Frequency