Internet and Intranet Connectivity Through Wireless Local Area Network (Wlan)

CHAPTER 1 Introduction Chapter 1 Introduction 1. 1 What is Wlocal argona network? 1. 1. 1 Wlocal ara network piano tuner topical anaesthetic Area Ne devilrk (Wlocal study network) is a kind of local argona intercommunicate which feeded exploitation a piano tuner bear on among the overhaul providers and the lymph glands victimization rough piano tuner equipment. This mesh topology development is base on the IEEE 802. 11 standard. 1. 1. 2 IEEE 802. 11 IEEE 802. 11 semen tos a station of intercommunicate set chat colloquycommunication local ara network/Wlocal bea network standards developed by beatning(a) group 11 of the IEEE LAN/MAN Standards Committee (IEEE 802). The term 802. 11x is overly usaged to de n matchless this set of standards and is non to be mis graveln for any angiotensin-converting enzyme of its elements. there is no single 802. 1x standard. The term IEEE 802. 11 is also holdd to abduce to the original 802. 11, which is wish a sho t or so eons c every last(predicate)ed 802. 11 legacy 1. The 802. 11 family soon includes six over-the- shoot for loafercelled modulation proficiencys that every(prenominal)(a) use the analogous protocol. The close to touristed techniques be t irrigate specify by the b, a, and g amendments to the original standard trade protection was originally include and was later enhanced via the 802. 11i amendment. 802. 11n is an early(a) modulation technique that has recently been developed the standard is still downstairs development, although products designed establish on draft versions of the standard atomic round 18 existence sold.Other standards in the family (cf, h, and j) be service enhancements and extensions or corrections to previous specifications. 802. 11b was the first tenacious accepted radiocommunication ne twainrking standard, foldepressioned by 802. 11a and 802. 11g 1. 802. 11b and 802. 11g standards use the 2. 40 gigacycle (gigahertz) fortune, run (in the United States) below berth 15 of the FCC Rules and Regulations. Be occasion of this superior of oftenness ringing, 802. 11b and 802. 11g equipment jakes incur hoo-hah from micro cast ovens, cordless telephones, Bluetooth tresss, and some other appliances using this very(prenominal) band. The 802. 1a standard uses the 5 gigacycle per second band, and is therefore non affected by products operating on the 2. 4 GHz band. Table 1. 1 protocol Summary of IEEE 802. 11 Protocol Legacy 802. 11a 802. 11b 802. 11g 802. 11n Release Date 1997 1999 1999 two hundred3 2006 influence Frequency GHz 2. 4-2. 5 5 2. 4-2. 5 2. 4-2. 5 2. 4 and/or 5 Through swan (Typ) Mbps 0. 7 23 4 19 74 info Rate (Max) Mbps 2 54 11 54 248 = 22 ant Range (Indoor) meters 25 30 35 35 70 Range (Outdoor) meters 75 snow 110 115 one hundred sixty 2 1. 2 Why it should be utilise? Bangladesh entered the Internet macrocosm in 1993 using off logical argument E-mail services.On disceptation Dial-up serv ices started in 1996 with and with and through VSAT based selective information connectivity. just it is non viable to expire a Dial-up conjunctive to all be run it uses the BTTBs telephone draw in. maculation Dial-up is ready the phone line is busy and it is not possible to give a client to a greater extent(prenominal) than 4/5 Kbps speed. Using an ADSL modem it open fire be emergenced to more than 2 Mbps. still it is not enough for a corpo estimate substance ab drug user and also it is very tollly and there argon existencey other occupations which has pulld below. The Ethernet connectivity raft give a ut some of 100 Mbps. But its localize is too miniscule. Wireless LAN has vast benefits over wire ne iirk in some aspects.In our body politic especially in big cities like Dhaka, it is very hard job to establish a pumped(p) network all over the city. Because, it is over populated, buildings were made with expose any proper plan and also the roadways. Generally the wire lines argon established over head, which is not so secured. Wire give the bounce be broken repayable to any kind of natural or man made problem. It whitethorn be theft. Or it potty be utilise by any one by taking a repeat line from it. It whitethorn create leak of selective information security. It is also very expensive to establish a copper wire network road by road and pristine(prenominal)(prenominal)tenance of it.Be looks that there are more rivers, send wordnels in our county, and also hill tracks in some parts. It is not possible to give a wired network over those. For all those reasons it is not a wise decision to use a wired network in our country. A Wireless LAN dejection be more steady-going, low cost, convenient network con human facering above aspects. thither are a image of Internet Service Provider (ISP) companies in our country cock-a-hoop Wireless LAN support to the clients. Those are known as Wireless ISP. These ISPs give internet or intranet service to the clients as their requirements. Those net work are reliable and also secured.It is easy to establish a alliance in the go withs insurance reporting bowl using a radio receiver thingumabob at the client end. The Wireless ISP Company should render proper re ejaculates to give that coverage. A model of a Wireless ISP companys tuner part for Bangladesh is given below. The nation considerable link lav be a optical fiber or micro draw in link. Here the main coverage is shown in Dhaka city and thus BSSs are shown at here is more than one. It empennage be expand the network in other ranges by adding redundant equipments necessitate to establish a BSS. And also it can give coverage on other areas by establish homogeneous(p) network on that area. think 1. 1 Model of a Wireless ISP 1. 3 Why one should be interested in WLAN field? The telecom in ranch outry is changing with breathtaking speed. at that place are a lot of telecommunication and Wire less ISP companies working in our country and there are a lot of companies to come. At gravel telecommunication is the most challenging and interesting field out of all other engineering handle. All the telecom company has some gross structure. So, there are some similarities between a mobile or PSTN (Public Switched Telephone Network) operator and a Wireless ISP.The skills one gather from a Wireless ISP can use in the telecom companies. The man can be skilled on installing varied thingmabobs, surveying a site, proposing a link budget. He can face the operable problems move on in installing intercommunicate networks and can be skilled in solving those problems and also troubleshoot the kinks and the radio link. In the mobile operators, there are many restrictions. One can not work with all things. But as still Wireless ISP companies are elfin in our country one can get opportunity to work in tuneive sections which lead extend his experiences and skills.Lastly it can be say that, as it is a challenging field, the person likes facing challenges depart enjoy working in this field 4 1. 4 Organization of this field This Internship report has seven chapters in total. The second chapter contains theory about the radio relative oftenness properties and opposite modulation techniques In third chapter, different RF barbels and it gravelories are described. Fourth chapter contains the Wireless LANs theory and architecture in brief. Chapter five analyzes to survey a site, and how to budget a link. The sixth chapter describes the device installation outgrowth for the APERTO and CANOPY devices.The seventh and final chapter is the final chapter where limitations of this works are reported and few suggestions of our work are provided along with the concluding remarks. 1. 5 Aims and objectives 5 RF Properties and Modulation Techniques CHAPTER 2 6 Chapter 2 RF Properties and Modulation Techniques 2. 1 radio receiver Frequency 2. 2. 1 Radio Frequency Rad io frequencies are gamey frequence alternating authorized (AC) sign ons that are passed along a copper conductor and then radiated into the air via an barbel. An forward pass converts/transforms a wired star signize to a wireless bless and vice versa.When the spicy up frequence AC call for is radiated into the air, it forms radio joggles. These radio thrives propagate (move) by from the source (the barbel) in a straight line in all get upions at once. 2. 2. 2 RF Behaviors RF is sometimes referred to as smoke and mirrors because RF seems to act erratically and inconsistently low given circumstances. Things as lilliputian as a association not universe tight enough or a slight electric defense mismatch on the line can cause erratic conduct and undesirable dissolvents. The spare-time activity sections describe these figures of behaviors and what can happen to radio waves as they are bringted.Gain Gain, illustrated in phase 2. 1, is the term utilize to descr ibe an amplification in an RF betoken amplitude 2. Gain is usually an active process meaning that an external s force-out source, a lot(prenominal) as an RF amplifier, is use to amplify the charge or a high- get on antenna is utilise to focus the burn width of a bespeak to increase its mark amplitude. render 2. 1 Power deduct However, passive processes can also cause deliver the goods. For example, reflected RF indications combine with the main augur to increase the main signal strength. Increasing the RF s signal strength may start a positive or a negative result.Typically, more causality is s better, alone there are cases, such as when a sender is radiating billet very close to legal military group return limit, where added ability would be a serious problem. 7 pass Loss describes a decrease in signal strength ( traffic figure 2. 2). galore(postnominal) things can cause RF signal going away, both while the signal is still in the railway line as a high absolute relative relative absolute oftenness AC galvanic signal and when the signal is propagated as radio waves through the air by the antenna. Resistance of cables and connectors causes leaving imputable to the converting of the AC signal to heat.Impedance mismatches in the cables and connectors can cause post to be reflected posterior toward the source, which can cause signal degradation. Objects promptly in the propagated wave transmission agency can absorb, reflect, or s destroy RF signals. Loss can be intentionally injected into a dress circle with an RF attenuator. RF attenuators are accurate resistors that convert high frequency AC to heat in order to reduce signal amplitude at that hitch in the circuit. 2 variety 2. 2 Power loss Being able to cadence and compensate for loss in an RF connection or circuit is important because radios subscribe to a receive sensitiveness threshold.A sensitivity threshold defined as the period at which a radio can clearly dist inguish a signal from posteriorground noise. Since a receivers sensitivity is finite, the ventting broadcast essential(prenominal)iness communicate signal with enough amplitude to be recognizable at the receiver. If losings occur between the vector and receiver, the problem must be rectify either by re move the objects causing loss or by increase the transmission mogul. Reflection Reflection, (as illustrated in radiation diagram 2. 3) occurs when a propagating electromagnetic wave impinges upon an object that has very en bigd dimensions when compared to the wavelength of the propagating wave 3.Reflections occur from the rear of the earth, buildings, walls, and many other obstacles. If the surface is smooth, the reflected signal may remain intact, though there is some loss due to absorption and aspersion of the signal. configuration 2. 3 Reflection 8 RF signal reflection can cause serious problems for wireless LANs. This reflecting main signal from many objects in the a rea of the transmission is referred to as multi racetrack. Multipath can shoot loathsome adverse affects on a wireless LAN, such as degrading or canceling the main signal and causing oles or gaps in the RF coverage area. Surfaces such as lakes, metal roofs, metal blinds, metal doors, and others can cause severe reflection, and hence, multipath. Reflection of this magnitude is never desirable and typically requires special functionality (antenna diversity) within the wireless LAN hardware to compensate for it. Refraction Refraction describes the change form of a radio wave as it passes through a median(a) of different density. As an RF wave passes into a denser median(a) (like a pool of cold air lying in a valley) the wave allow for be bent such that its direction changes.When passing through such a medium, some of the wave leave alone be reflected off from the intended signal path, and some allow for be bent through the medium in another direction, as illustrated in effig y 2. 4. 3 experience 2. 4 Refraction Refraction can convey a problem for long duration RF links. As atmospheric conditions change, the RF waves may change direction, diverting the signal away from the intended Diffraction Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities or an otherwise rough surface 3.At high frequencies, diffraction, like reflection, depends on the geometry of the obstructing object and the amplitude, phase, and polarization of the incident wave at the question of diffraction. Diffraction is comm further conf employ with and improperly utilize interchangeably with refraction. apportion should be taken not to confuse these name. Diffraction describes a wave bending approximately an obstacle ( pulp 2. 5), whereas refraction describes a wave bending through a medium. Taking the rock in the pond example from above, now consider a atrophied twig sticking up through the surface of the irrigate near where the rock.As the ripples hit the stick, they would be bar to a small tier, entirely to a tremendousr degree, the ripples would bend around the twig. This illustration shows how diffraction acts with obstacles in its path, depending on the devoteup of the obstacle. If Object was large or jagged enough, the wave big businessman not bend, but rather major power be blocked. 9 dactyl 2. 5 Diffraction Diffraction is the slowing of the wave reckon at the point where the wave front strikes an obstacle, while the rest of the wave front maintains the same speed of propagation. Diffraction is the essence of waves turning, or bending, around the obstacle.As another example, consider a machine blowing a steady germinate of smoke. The smoke would flow straight until an obstacle entered its path. Introducing a large woody block into the smoke stream would cause the smoke to curl around the deferrals of the block causing a noticeable degradation in the smok e fastness at that point and a prodigious s change in direction. dispersion Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength of the signal, and the look of obstacles per unit volume is large 3.Scattered waves are produced by rough surfaces, small objects, or by other irregularities in the signal path, as can be seen in normal 2. 6. routine 2. 6 Scattering approximately outdoor examples of objects that can cause scattering in a mobile communications establishment include foliage, street signs, and lampposts. Scattering can take place in two primary ways. First, scattering can occur when a wave strikes an uneven surface and is reflected in many directions simultaneously. Scattering of this type yields many small amplitude reflections and destroys the main RF signal.Dissipation of an RF signal may occur when an RF wave is reflected off sand, rocks, or other jagged surfaces. When scatte red in this manner, RF signal degradation can be significant to the point of intermittently disrupting communications or causing perfect(a) signal loss. 10 Second, scattering can occur as a signal wave travels through particles in the medium such as heavy dust content. In this case, rather than being reflected off an uneven surface, the RF waves are individually reflected on a very small scale off tiny particles.Voltage Standing Wave Ratio (VSWR) VSWR occurs when there is mismatched resistor (resistance to sure flow, measured in Ohms) between devices in an RF dodging. VSWR is caused by an RF signal reflected at a point of electrical resistance mismatch in the signal path. VSWR causes return loss which is defined as the loss of forward energy through a organisation due to some of the indicator being reflected back towards the transmitter. If the impedances of the ends of a connection do not match, then the maximum amount of the transmitted power provide not be received at the antenna.When part of the RF signal is reflected back toward the transmitter, the signal aim on the line varies instead of being steady. This variance is an indicator of VSWR. 2 As an illustration of VSWR, imagine water aerodynamic through two garden hoses. As long as the two hoses are the same diameter, water flows through them seamlessly. If the hose connected to the tap were significantly larger than the next hose down the line, there would be backpressure on the faucet and even at the connection between the two hoses. This stand up backpressure illust pass judgment VSWR, as can be seen in Figure 2. . In this example, you can see that backpressure can have negative effects and not nearly as much water is transferred to the second hose as there would have been with matching hoses screwed together properly. Figure 2. 7 VSWR-like water through a hose VSWR Measurements VSWR is a ratio, so it is expressed as a relationship between two numbers. A typical VSWR order would be 1. 51. The two numbers relate the ratio of impedance mismatch against a perfect impedance match. The second number is unceasingly 1, representing the perfect match, where as the first number varies.The lower the first number (closer to 1), the better impedance matching your remains has. For example, a VSWR of 1. 11 is better than 1. 41. A VSWR measuring of 11 would look up a perfect impedance match and no voltage standing wave would be present in the signal path. Effects of VSWR riotous VSWR can cause serious problems in an RF circuit. Most of the time, the result is a marked decrease in the amplitude of the transmitted RF signal. However, 11 since some transmitters are not protected against power being applied (or returned) to the transmitter output circuit, the reflected power can burn ut the electronics of the transmitter. VSWR effects are evident when transmitter circuits burn out, power s output levels are uns carry over, and the power observed is significantly different from th e anticipate power. The method actings of changing VSWR in a circuit include proper use of proper equipment. Tight connections between cables and connectors, use of impedance matched hardware throughout, and use of high-quality equipment with calibration reports where requisite are all effective preventative measures against VSWR.VSWR can be measured with high-accuracy instrumentation such as SWR meters, but this measurement is beyond the scope of this text and the job tasks of a network administrator. 2. 2 over send Spectrum 2. 2. 1 Spread Spectrum Spread spectrum is a communications technique characterized by wide bandwidth and low peak power. Spread spectrum communication uses mixed modulation techniques in wireless LANs and possesses many values over its precursor, nail band communication 4. Spread spectrum signals are noise-like, hard to detect, and even harder to intercept or demodulate without the proper equipment.Jamming and interference have a lesser affect on a ridd le spectrum communication than on narrow band communications. For these reasons, spread spectrum has long been a favorite of the military. 2. 2. 2 cut Band Transmission A narrowband transmission is a communications engineering that uses only enough of the frequency spectrum to carry the info signal and no more, spread spectrum is in opposition to that mission since it uses much wider frequency bands than is necessary to transmit the cultivation. This brings us to the first requirement for a signal to be considered spread spectrum.A signal is a spread spectrum signal when the bandwidth is much wider than what is required to send the information. 4 Figure 2. 8 illust order the difference between narrowband and spread spectrum transmissions. One of the characteristics of narrow band is high peak power. to a great extent power is required to send a transmission when using a smaller frequency range. In order for narrow band signals to be received, they must stand out above the genera l level of noise, called the noise floor, by a significant amount. Because its band is so narrow, and high peak power ensures error-free reception of a narrow band signal. 12Figure 2. 8 Narrow band verses Spread Spectrum on a frequency sphere of influence A stimulate argument against narrowband transmission-other than the high peak power required to send it-is that narrow band signals can be jammed or experience interference very easily. Jamming is the intentional overpowering of a transmission using unwanted signals transmitted on the same band. Because its band is so narrow, other narrow band signals, including noise, can whole take place the information by overpowering a narrowband transmission much like a passing train overpowers a quiet conversation. 2. 2. 3 Spread Spectrum TechnologySpread spectrum technology allows taking the same amount of information than previously using a narrow band attack aircraft attack aircraft crew cut signal and dispersion it out over a much larger frequency range. For example, 1 megahertz at 10 Watts with narrow band, but 20 MHz at 100 mW with spread spectrum. By using a wider frequency spectrum, we reduce the probability that the information leave alone be debased or jammed. A narrow band jamming attempt on a spread spectrum signal would likely be thwarted by virtue of only a small part of the information move into the narrow band signal frequency range. s s Most of the digital data would be received error-free 4.Today spread spectrum RF radios can convey any small amount of data loss due to narrowband interference. While the spread spectrum band is comparatively wide, the peak power of the signal is preferably a low. This is the second requirement for a signal to be considered spread spectrum. For a signal to be considered spread spectrum, it must use low power. These two characteristics of spread spectrum (use of a wide band of frequencies and very low power) make it look to most receivers as if it were a n oise signal. Noise is a wide band, low power signal, but the difference is that noise is unwanted.Furthermore, since most radio receivers entrust believe the spread spectrum signal as noise, these receivers lead not attempt to demodulate or interpret it, creating a somewhat more secure communication. 2. 2. 4 Frequency Hopping Spread Spectrum (FHSS) Frequency skitterping spread spectrum is a spread spectrum technique that uses frequency agility to spread the data over more than 83 MHz. Frequency agility refers to the radios ability to change transmission frequency abruptly within the usable RF frequency band 4. In the case of frequency record hopping wireless LANs, the usable portion of the 2. GHz ISM band is 83. 5 MHz, per FCC standard and the IEEE 802. 11 standard. 13 How FHSS Works In frequency hopping frames, the carrier changes frequency, or hops, according to a pseudorandom sequence. The pseudorandom sequence is a itemization of several frequencies to which the carrie r leave hop at stipulate time intervals before restate the pattern. The transmitter uses this hop sequence to select its transmission frequencies. The carrier provide remain at a certain frequency for a specified time (known as the dwell time), and then use a small amount of time to hop to the next frequency (hop time).When the list of frequencies has been exhausted, the transmitter lead repeat the sequence. Figure 2. 9 shows a frequency hopping system using a hop sequence of five frequencies over 5 MHz band. In this example, the sequence is 1. 2. 449 GHz 2. 2. 452 GHz 3. 2. 448 GHz 4. 2. 450 GHz 5. 2. 451 GHz Figure 2. 9 Single frequency hopping system one time the radio has transmitted the information on the 2. 451 GHz carrier, the radio will repeat the hop sequence, starting again at 2. 449 GHz. The process of repeating the sequence will maintain until the information is received completely.The receiver radio is synchronised to the transfer radio hop sequence in order to s receive on the proper frequency at the proper time. The signal is then demodulated and used by the receiving computer. Effects of Narrow Band impediment Frequency hopping is a method of sending data where the transmission and receiving systems hop along a repeatable pattern of frequencies together. As is the case with all spread spectrum technologies, frequency hopping systems are resistant-but not immune-to narrow band interference. In example in Figure 2. 9, if a signal were to interfere with our frequency hopping signal on, say, 2. 51 GHz, only that portion of the spread spectrum signal would be lost. The rest of the spread spectrum signal would remain intact, and the lost data would be retransmitted. 14 In reality, an interfering narrow band signal may occupy several megahertz of bandwidth. Since a frequency hopping band is over 83 MHz wide, even this interfering signal will cause little degradation of the spread spectrum signal. Frequency Hopping Systems The IEEE and open- air(prenominal) standards regarding FHSS systems describe 1. The frequency bands which may be used 2. Hop sequences 3. Dwell times 4. data evaluate The IEEE 802. 1 standard specifies data rank of 1 Mbps and 2 Mbps and Open-Air (a standard created by the now defunct Wireless LAN Interoperability Forum) specifies data rates of 800 kbps and 1. 6 Mbps. In order for a frequency hopping system to be 802. 11 or Open-Air docile, it must check in the 2. 4 GHz ISM band (which is defined by the FCC as being from 2. 4000 GHz to 2. 5000 GHz). both(prenominal) standards allow operation in the range of 2. 4000 GHz to 2. 4835 GHz. roadways A frequency hopping system will operate using a specified hop pattern called a channel. Frequency hopping systems typically use the FCCs 26 standard hop patterns or a subset thereof.Some frequency hopping systems will allow tradition hop patterns to be created, and others even allow synchronization between systems to completely eliminate collisions in a co -located environment. Figure 2. 10 Co-located frequency hopping system Though it is possible to have as many as 79 synchronized, co-located approach path points, with this many systems, separately frequency hopping radio would require dead synchronization with all of the others in order not to interfere with (transmit on the same frequency as) another frequency hopping radio in the area. The cost of such a set of systems is prohi sive and is in general not considered an option.If synchronized radios are used, the expense tends to dictate 12 co-located systems as the maximum. 15 If non-synchronized radios are to be used, then 26 systems can be co-located in a wireless LAN this number is considered to be the maximum in a medium-traffic wireless LAN. Increasing the traffic significantly or routinely transferring large files places the practical limit on the number of co-located systems at about 15. More than 15 co-located frequency-hopping systems in this environment will interfere to the extent that collisions will begin to reduce the aggregate throughput of the wireless LAN.Dwell Time In frequency hopping systems, it must transmit on a specified frequency for a time, and then hop to a different frequency to continue transmitting. When a frequency hopping system transmits on a frequency, it must do so for a specified amount of time. This time is called the dwell time. at a time the dwell time has expired, the system will switch to a different frequency and begin to transmit again. Suppose a frequency hopping system transmits on only two frequencies, 2. 401 GHz and 2. 402 GHz. The system will transmit on the 2. 01 GHz frequency for the duration of the dwell time100 milliseconds (ms), for example. After 100ms the radio must change its transmitter frequency to 2. 402 GHz and send information at that frequency for 100ms. Hop Time When considering the hopping action of a frequency hopping radio, dwell time is only part of the story. When a frequency hopping radi o jumps from frequency A to frequency B, it must change the transmit frequency in one of two ways. It either must switch to a different circuit tuned to the cutting frequency, or it must change some element of the current circuit in order to tune to the new frequency.In either case, the process of changing to the new frequency must be complete before transmission can resume, and this change takes time due to electrical latencies inherent in the circuitry. There is a small amount of time during this frequency change in which the radio is not transmitting called the hop time. The hop time is measured in microseconds (s) and with relatively long dwell times of around 100-200 ms, the hop time is not significant. A typical 802. 11 FHSS system hops between carry in 200-300 s. With very scam dwell times of 500 600s, like those being used in some frequency hopping systems such as Bluetooth, hop ime can become very significant. If we look at the effect of hop time in terms of data throug hput, we discover that the long-acting the hop time in relation to the dwell time, the slower the data rate of bits being transmitted. 2. 2. 5 accost Sequence Spread Spectrum (DSSS) Direct sequence spread spectrum is very widely known and the most used of the spread spectrum types, owing most of its popularity to its ease of experienceation and high data rates. The bulk of wireless LAN equipment on the grocery store today uses DSSS technology.DSSS is a method of sending data in which the transmitting and receiving systems are both on a 22 MHz-wide set of frequencies. The wide channel enables devices to transmit more information at a higher data rate than current FHSS systems. 16 How DSSS Works DSSS combines a data signal at the sending station with a higher data rate bit sequence, which is referred to as a chipping code or impact gain. A high bear upon gain increases the signals resistance to interference. The minimal linear processing gain that the FCC allows is 10, and most commercial products operate under 20.The IEEE 802. 11 working group has set their minimum processing gain requirements at 11. The process of direct sequence begins with a carrier being modulated with a code sequence. The number ofchips-in the code will lay out how much spreading occurs, and the number of chips per bit and the speed of the code (in chips per second) will determine the data rate. Direct Sequence Spread Spectrum (DSSS) Direct sequence spread spectrum is very widely known and the most used of the spread spectrum types, owing most of its popularity to its ease of implementation and high data rates.The majority of wireless LAN equipment on the market today uses DSSS technology. DSSS is a method of sending data in which the transmitting and receiving systems are both on a 22 MHz-wide set of frequencies. The wide channel enables devices to transmit more information at a higher data rate than current FHSS systems. How DSSS Works DSSS combines a data signal at the sending s tation with a higher data rate bit sequence, which is referred to as a chipping code or processing gain. A high processing gain increases the signals resistance to interference.The minimum linear processing gain that the FCC allows is 10, and most commercial products operate under 20. The IEEE 802. 11 working group has set their minimum processing gain requirements at 11. The process of direct sequence begins with a carrier being modulated with a code sequence. The number of-chips-in the code will determine how much spreading occurs, and the number of chips per bit and the speed of the code (in chips per second) will determine the data rate. jobs Unlike frequency hopping systems that use hop sequences to define the conduct, direct sequence systems use a more conventional definition of channels. distributively channel is a contiguous band of frequencies 22 MHz wide and 1 MHz carrier frequencies are used just as with FHSS. Channel 1, for instance, operates from 2. 401 GHz to 2. 423 GHz (2. 412 GHz 11 MHz) channel 2 operates from 2. 406 to 2. 429 GHz (2. 417 11 MHz), and so forth. Figure 2. 11 illustrates this point. 17 Figure 2. 11 channel allocation and Spectral relationship The chart in Table 2. 1 has a complete list of channels used in the United States and Europe. The FCC specifies only 11 channels for non-licensed use in the United States. from each one(prenominal) of the frequencies listed in this chart are considered centre of attention frequencies.From this midst frequency, 11 MHz is added and subtracted to get the useable 22 MHz wide channel. Easy to see that adjacent channels (channels straight next to apiece other) would converging significantly. Table 2. 1 DSSS channel frequency Assignment Channel ID 1 2 3 4 5 6 7 8 9 10 11 FCC Channel Frequencies GHz 2. 412 2. 417 2. 422 2. 427 2. 432 2. 437 2. 442 2. 447 2. 452 2. 457 2. 462 ETSI Channel Frequencies GHz N/A N/A 2. 422 2. 427 2. 432 2. 437 2. 442 2. 447 2. 452 2. 457 2. 462 To use DSSS sy stems with cooccur channels in the same corporeal space would cause interference between the systems.DSSS systems with overlapping channels should not be co-located because there will or so always be a drastic or complete reduction in throughput. Because the center frequencies are 5 MHz obscure and the channels are 22 MHz wide, channels should be co-located only if the channel numbers are at least five isolated channels 1 and 6 do not overlap, channels 2 and 7 do not overlap, etc. There is a maximum of three co-located direct sequence systems possible because channels 1, 6 and 11 are the only theoretically non-overlapping channels. The 3 non-overlapping channels are illustrated in Figure 2. 2 18 Figure 2. 12 DSSS non-overlapping Channel 2. 2. 6 Comparing FHSS and DSSS Both FHSS and DSSS technologies have their advantages and disadvantages, and it incumbent on the wireless LAN administrator to give each its due weight when deciding how to implement a wireless LAN 4. This section will cover some of the factors that should be discussed when determining which technology is appropriate for your organization, including 1. Narrowband interference 2. Co-location 3. Cost 4. Equipment compatibility 5. Data rate and throughput 6. Security 7.Standards support Narrowband Interference The advantages of FHSS include a greater resistance to narrow band interference. DSSS systems may be affected by narrow band interference more than FHSS because of the use of 22 MHz wide contiguous bands instead of the 79 MHz used by FHSS. This fact may be a serious consideration if the proposed wireless LAN site is in an environment that has such interference present. Co-location An advantage of FHSS over DSSS is the ability for many more frequency hopping systems to be co-located than direct sequence systems.Since frequency hopping systems are-frequency agile-and make use of 79 discrete channels, frequency hopping systems have a co-location advantage over direct sequence systems, which have a maximum co- location of 3 introduction points. 19 Figure 2. 13 Co-location Comparison However, when calculating the hardware costs of an FHSS system to get the same throughput as a DSSS system, the advantage quickly disappears. Because DSSS can have 3 co-located access points, the maximum throughput for this configuration would be 3 access points ? 1 Mbps = 33 Mbps At roughly 50% of rated bandwidth, the DSSS system throughput would be approximately 33 Mbps / 2 = 16. 5 Mbps To achieve roughly the same rated system bandwidth using an IEEE 802. 11 nonresistant FHSS system would require 16 access points ? 2 Mbps = 32 Mbps At roughly 50% of rated bandwidth, the FHSS system throughput would be approximately 32 Mbps / 2 = 16 Mbps In this configuration, an FHSS system would require 13 additional access points to be purchased to get the same throughput as the DSSS system. Also, additional installation services for these units, cables, connectors, and antennas would all need to be pu rchased.Cost When implementing a wireless LAN, the advantages of DSSS may be more compelling than those of FHSS systems, peculiar(a)ly when driven by a tight budget. The cost of implementing a direct sequence system is far less than that of a frequency hopping system. DSSS equipment is widely available in todays marketplace, and its rapid adoption has helped in driving down the cost. Only a few short years ago, equipment was only affordable by enterprise customers. Today, very good quality 802. 11b gentle PC card game can be purchased for under $100.FHSS cards complying with either the 802. 11 or Open-Air standards typically run between $150 and $350 in today market depending on the manufacturer and the standards to which the cards s adhere. 20 Equipment compatibility and availability The Wireless Ethernet Compatibility Alliance (WECA) provides testing of 802. 11b manageable DSSS wireless LAN equipment to ensure that such equipment will operate in the presence of and interopera te with other 802. 11b DSSS devices. The interoperability standard that WECA created and now uses is called Wireless Fidelity, or Wi-Fi, and hose devices that pass the tests for interoperability are-Wi-Fi compliant-devices. Devices so deemed are allowed to affix the Wi-Fi logo on the link marketing material and devices themselves showing that they have been tested and interoperate with other Wi-Fi compliant devices. There are no such compatibility tests for equipment that uses FHSS. There are standards such as 802. 11 and Open-Air, but no organization has stepped forward to do the same kind of compatibility testing for FHSS as WECA does for DSSS. Due to the immense popularity of 802. 11b compliant radios, it is much easier to obtain these units.The demand seems only to be growing for the Wi-Fi compliant radios while the demand for FHSS radios has remained fairly steady, even decreasing to some degree over the past year. Data rate and throughput The latest frequency hopping systems are slower than the latest DSSS systems mostly because their data rate is only 2 Mbps. Though some FHSS systems operate at 3 Mbps or more, these systems are not 802. 11 compliant and may not interoperate with other FHSS systems. FHSS and DSSS systems have a throughput (data actually sent) of only about half of the data rate.When testing the throughput of a new wireless LAN installation, achieving 5-6 Mbps on the 11 Mbps view for DSSS or 1 Mbps on the 2 Mbps setting common using DSSS. When wireless frames are transmitted, there are pauses between data frames for manoeuvre signals and other overhead tasks. With frequency hopping systems, this interframe spacing is longer than that used by direct sequence systems, causing a slow-down in rate that data is actually sent (throughput). Additionally, when the frequency hopping system is in the process of changing the transmit frequency, no data is sent.This translates to more lost throughput, albeit only a minor amount. Some wireless LAN systems use proprietary fleshly layer protocols in order to increase throughput. These methods work, yielding throughputs as high as 80% of the data rate, but in so doing, render interoperability. Security It is widely touted-and is a myth-that frequency hopping systems are inherently more secure than direct sequence systems. The first fact that disproves this myth is that FHSS radios are only produced by a minimal number of manufacturers. Of this small list of manufacturers, all of them adhere to standards such as 802. 1 or Open-Air in order to sell their products effectively. Second, each of these manufacturers uses a standard set of hop sequences, which generally comply with a pre-determined list, produced by the standards body (IEEE or WLIF). These 2 items together make breaking the code of hop sequences relatively simple. 21 Other reasons that make finding the hop sequence quite simple is that the channel number is broadcasted in the clear with each beacon. Also, the macinto sh address of the transmitting access point can be seen with each beacon (which indicates the manufacturer of the radio).Some manufacturers allow the administrator the flexibility of define custom hopping patterns. However, even this custom capability is no level of security since fairly unsophisticated devices such as spectrum analyzers and a standard laptop computer can be used to track the hopping pattern of a FHSS radio in a matter of seconds. Standards Support DSSS has gained wide adoption due to low cost, high speed, WECA Wi-Fi s interoperability standards, and many other factors. This market acceptance will only accelerate due to the industry moving toward newer, alacritous DSSS systems such as the new 802. 1g and 802. 11a compliant wireless LAN hardware. WECA new Wi-Fi5 s interoperability standard for 5 GHz DSSS systems operating in the UNII bands will help move the industry along even faster in the same direction it is already headed. The new standards for FHSS systems i nclude mansion RF 2. 0 and 802. 15 (in support of WPANs such as Bluetooth), but none for advancing FHSS systems in the enterprise. 2. 2. 7 BPSK In BPSK, the phase of the carrier is varied to represent double star 1 or 0 . Both peak amplitude and frequencies remain constant as the phase changes.For example, if a phase of 0 represents binary 0, then the phase 180 represents binary 1. the phase of the signal during each bit duration is constant. And its value depends on the bit (0 or 1). Figure 2. 14 shows a impressionual view of BPSK. BPSK is also known as 2-PSK. because two different phases (0 and 180) are used. The table below shows BPSK which makes the relationship of phase to bit value. Bit 0 1 Phase 0? 180? Figure 2. 14 BPSK. 2. 2. 8 QPSK The diagram for the signal is given in Figure 2. 15. A phase of 0 now represents 00 90 represents 01 180 represents10 and 270 represents 11. This technique is called QPSK.The pair of bits represented by each phase is called a dibit. 22 Bit 00 01 10 11 Figure 2. 15 QPSK. Phase 0? 90? 180? 270? 2. 2. 9 QAM QAM is a Combination of ASK and PSK so that a maximum contrast between each signal unit (bit, dibit, tribit, and so on) is achieved. QAM takes the advantages of the fact that it is possible to send two different signals simultaneously on the same carrier frequency . by using two copies of the carrier frequency. One shifted by 90 with respect to the other. For QAM, each carrier is ASK modulated. The two fissiparous signals are simultaneously transmitted over the same medium.In QAM the number of amplitude shifts is fewer than the number of phase shifts. Because amplitude changes are subject to noise and require greater shift distances than do phase changes, the number of phase shifts used by a QAM system is always larger than the number of amplitude shifts. 5 Figure 2. 16 QAM. 23 2. 2. 10 rectangular Frequency division Multiplexing (OFDM) Orthogonal Frequency division Multiplexing offers the highest data rates and maxi mum resistance to interference and corruption of all the signal enjoyment techniques in use in 802. 1 today 5. Although it is not considered a spread spectrum technique by the FCC, OFDM shares many qualities with spread spectrum communicators, including using a low transmit power and wider-than-necessary bandwidth. OFDM is used to provide data rates up to 54 Mbps in 802. 11a and 802. 11g. How OFDM Works OFDM achieves high data rates by squeezing a large number of Communication conduct into a given frequency band. Normally, two communication channels must be separated by a certain amount of bandwidth or they overlap and interfere.Specially, each Channel has likeables that extend up and down the frequency space, decreasing in amplitude as they get farther from the channels fundamental signal. Even if two channels are non-overlapping, their harmonics may overlap and the signal can be corrupted. An OFDM communicator can place adjacent communication channels very incisively in the fr equency space in such a way that the channels harmonics exactly cancel each other, effectively leaving only the fundamental signals. OFDM achieves high data rates by dividing a single communication channel into a large number of closely-spaced, small bandwidth sub-carriers.Each sub-carrier individually has a relatively low data rate, but by transmitting data in correspond on all sub-carriers simultaneously, high data rates can be achieved. Figure 2. 17 OFDM frequency plot. Figure 2. 17 shows an example of a frequency spectrum for an OFDM transmitter. Each of the peaks represents a single sub-carrier, and the sub-carriers together make up the communications channel. The sub-carriers are precisely aligned so that the zero-points of their harmonics overlapped exactly. The majority of the harmonic energy will cancel out, leaving just the sub-carriers. 4 CHAPTER 3 RF overture and Accessories 25 Chapter 3 RF Antenna and Accessories 3. 1 Introduction Antennas are most often used to incre ase the range of wireless LAN systems, but proper antenna selection can also enhance the security of your wireless LAN. A properly chosen and positioned antenna can reduce the signal leaking out of workspace, and make signal interception extremely difficult. 3. 2 RF Antennas An RF antenna is a device used to convert high frequency (RF) signals on a transmission line (a cable or waveguide) into propagated waves in the air 6.The electrical fields let looseted from antennas are called beams or lobes. Antenna convert electrical energy into RF waves in the case of a transmitting antenna, or RF waves into electrical energy in the case of a receiving antenna. The physical dimensions of an antenna, such as its length, are directly related to the frequency at which the antenna can propagate waves or receive propagated waves. The physical structure of an antenna is directly related to the Shape of the area in which it concentrates most of its related RF energy. There are three generic categ ories of RF antennas 1.Omni- directive 2. fishing gear- directing 3. Highly-directional Each phratry has multiple types of antennas, each having different RF characteristics and appropriate uses. As the gain of an antenna goes up, the coverage area narrows so that high-gain antennas offer longer coverage areas than low-gain antennas at the same input power level. 3. 2. 1 Omni-directional (Dipole) Antennas The dipole is an omni- directional antenna, because it radiates its energy equally in all directions around its axis. Dipole antenna is Simple to design dipole antenna is standard equipment on most access points.Directional antennas concentrate their energy into a cone, known as a beam. Figure 3. 1 Dipole doughnut 26 Figure 3. 1 shows that the dipole radiant energy is concentrated into a region that s looks like a doughnut, with the dipole vertically through the hole of the doughnut. The signal from an omni-directional antenna radiates in a 360-degree swimming beam. If an an tenna radiates in all directions equally (forming a sphere), it is called an isotropic radiator, which is the theoretical reference for antennas, but rather, practical antennas all have some type of gain over that of an isotropic radiator.The dipole radiates equally in all directions around its axis, but does not radiate along the length of the wire itself hence the doughnut pattern. The side view of a dipole radiator as it radiates waves in Figure 3. 2. Figure 3. 2 Dipole-side view If a dipole antenna is placed in the center of a single floor of a multistory building, most of its energy will be radiated along the length of that floor, with some significant fraction sent to the floors above and below the access point. Figure 3. 3 shows examples of some different types of omni-directional antennas. Figure 3. 3 Sample omni-directional antennaFigure 3. 4 shows a two-dimensional example of the top view and side view of a dipole antenna. Figure 3. 4 Coverage area of an omni-directional antenna High-gain omni-directional antennas offer more horizontal coverage area, but the vertical coverage area is reduced, as can be seen in Figure 3. 5. 27 Figure 3. 5 Coverage area of high gain omni-directional antennas This characteristic can be an important consideration when mounting a high-gain omni antenna indoors on the ceiling. If the ceiling is too high the coverage area may not reach the floor, where the users are located.Usages Omni-directional antennas are used when coverage in all directions around the horizontal axis of the antenna is required. Omni-directional antennas are most effective where large coverage areas are postulate around a central point. For example, placing an omni- directional antenna in the midriff of a large, open room would provide good coverage. Omni-directional antennas are usually used for point-tomultipoint designs with a hub-n-spoke topology. Used outdoors, an omni-directional antenna should be placed on top of a structure (such as a build ing) in the position of theFigure 3. 6 Point to multipoint link coverage area. For example, on a college campus the antenna might be placed in the center of the campus for the superlative coverage area. When used indoors, the antenna should be placed at the inwardness of the building or desired coverage area, near the ceiling, for optimum coverage. Omni-directional antennas fade a large coverage area in a posting pattern and are suitable for warehouses or tradeshows where coverage is usually from one corner of the building to the other. 3. 2. 2 Semi directional AntennaSemi directional antennas direct the energy from the transmitter significantly more in one particular direction rather than the uniform circular pattern that is common with the omni- directional antenna Semi-directional antennas come in many different styles and shapes. Some semi- directional antennas types frequently used with wireless LANs are Patch, Panel, and yagi (pronounced YAH-gee) antennas. All of these an tennas are generally flat and designed for wall mounting. Each type has different coverage characteristics. Figure 3. shows some examples of semidirectional antennas. 28 Figure 3. 7 Sample semi-directional antenna Semi-directional antennas often radiate in a hemispherical or cylindrical coverage pattern as can be seen in Figure 3. 8. Figure 3. 8 Coverage area of a semi-directional antenna Usages Semi-directional antennas are ideally suited for short and medium range bridging. For example, two office buildings that are across the street from one another and need to share a network connection would be a good scenario in which to implement semidirectional antennas.In a large indoor space, if the transmitter must be located in the corner or at the end of a building, a corridor, or a large room, a semi-directional antenna would be a good choice to provide the proper coverage. Figure 3. 9 illustrates a link between two buildings using semi-directional antennas. Figure 3. 9 Point to point link using semi-directional antenna In some cases, semi-directional antennas provide such long coverage that they may eliminate the need for multiple access points in a building.For example, in a long hallway, several access points with omni antennas may be used or perhaps only one or two access points with properly placed semi-directional antennas saving the customer a significant amount of money. In some cases, semi- directional antennas have back and side lobes that, if used effectively, may further reduce the need for additional access points. 29 3. 2. 3 Highly directional antenna Highly-directional antennas emit the most narrow signal beam of any antenna type and have the greatest gain of these three groups of antennas.Highly-directional antennas are typically concave, concave devices, as can be seen Figures 3. 10 and 3. 11. These antennas are ideal for long distance, point-to-point wireless links. Some models are referred to as parabolic dishes because they resemble small s atellite dishes. Others are called grid antennas due to their perforated design for resistance to wind loading. Figure 3. 10 sample of a super directional antenna Figure 3. 11 sample of a highly directional grid antenna Figure 3. 12 Radiation pattern of a highly directional antennaUsages High-gain antennas do not have a coverage area that client devices can use. These antennas are used for point-to-point communication links, and can transmit at distances up to 25 miles. Potential uses of highly directional antennas might be to connect two buildings that are miles away from each other but have no obstructions in their path. Additionally, these antennas can be aimed directly at each other within a building in order to blast through an obstruction. This setup would be used in order to get network connectivity to places that cannot be wired and where normal wireless networks will not work. 0 3. 2. 4 Antenna Gain An antenna element without the amplifiers and filters typically associated with it is a passive device. There is no conditioning, amplifying, or manipulating of the signal by the antenna element itself. The antenna can create the effect of amplification by virtue of its physical shape. Antenna amplification is the result of focusing the RF radiation into a tighter beam, just as the bulb of a flashlight can be focused into a tighter beam creating a seemingly brighter light source that sends the light further.The focusing of the radiation Measured by way of beam widths, which are measured in degrees horizontal and vertical. For example, an omni-directional antenna has a 360-degree horizontal beam width. By limiting the 360-degree beam width into a more focused beam of, say, 30 degrees, at the same power, the RF waves will be radiated further. This is how patch, panel, and Yagi antennas (all of which are semi-directional antennas) are designed. Highly directional antennas take this theory a step further by very tightly focusing both horizontal and vertical b eam widths to maximize distance of the propagated wave at low power. . 2. 5 wise to(p) Radiator As defined by the Federal Communication Commission (FCC), an intentional radiator is an RF device that is specifically designed to generate and radiate RF signals. In terms of hardware, an intentional radiator will include the RF device and all cabling and connectors up to, but not including, the antenna, as illustrated in Figure 3. 13 below. Figure 3. 13 Intentional Radiator Any reference to power output of the Intentional Radiator refers to the power output at the end of the last cable or connector before the antenna.For example, consider a 30- milliwatt transmitter that loses 15 milliwatts of power in the cable and another 5 milliwatts from the connector at the antenna. The power at the intentional radiator would be 10 milliwatts. As an administrator, it is your responsibility to learn the FCC rules relating to Intentional Radiators and their power output. Understanding how power out put is measured, how much power is allowed, and how to calculate these values are all covered in this book. FCC regulations concerning output power at the Intentional Radiator and EIRP are found in Part 47 CFR, 1 3. 2. 6 Equivalent Isotropically Radiated Power (EIRP) EIRP is the power actually radiated by the antenna element, as shown in Figure 3. 14. This concept is important because it is regulated by the FCC and because it is used in calculating whether or not a wireless link is viable. EIRP takes into account the gain of the antenna. Figure 3. 14 Equivalent Isotropically Radiated Power Suppose a transmitting station uses a 10-dBi antenna (which amplifies the signal 10fold) and is fed by 100 mill watts from the intentional radiator.The EIRP is 1000 mW, or 1 Watt. The FCC has rules defining both the power output at the intentional radiator and the antenna element. 3. 3 RF Accessories When wireless LAN devices connect together, the appropriate cables and accessories need to purchas e that will maximize throughput, minimize signal loss, and, most importantly, allow making connections correctly. divergent types of accessories are needed in a wireless LAN design. 7 1. RF Amplifiers 2. RF Attenuators 3. Lightning Arrestors 4. RF Connectors 5. RF Cables 3. 3. 1 RF AmplifiersAn RF amplifier is used to amplify, or increase the amplitude of, RF signal, which is measured in +dB. An amplifier will be used when compensating the loss incurred by the RF signal, either due to the distance between antennas or the length of cable from a wireless infrastructure device to its antenna. Most RF amplifiers used with wireless LANs are power using DC voltage fed onto the RF cable with an injector near the RF signal source (such as the access point or bridge). Sometimes this DC voltage used to power RF amplifiers is called phantom voltage because the RF amplifier seems to magically power up.This DC injector is powered using AC voltage from a wall outlet, so it might be located in a wiring closet. In this scenario, the RF cable carries 32 both the high frequency RF signal and the DC voltage necessary to power the in-line amplifier, which, in turn, boosts the RF signal amplitude. Figure 3. 15 shows an example of an RF amplifier (left), and how an RF amplifier is mounted on a pole (right) between the access point and its antenna. Figure 3. 15 A sample of a fixed gain Amplifier RF amplifiers come in two types unidirectional and bi-directional.Unidirectional amplifiers compensate for the signal loss incurred over long RF cables by increasing the signal level before it is injected into the transmitting antenna. Bi-directional amplifiers boost the effective sensitivity of the receiving antenna by amplifying the received signal before it is fed into the access point, bridge, or client device. Configuration and Management RF amplifiers used with wireless LANs are installed in series with the main signal path seen below in Figure 3. 16. Amplifiers are typically mounted to a solid surface using screws through the amplifiers flange plates.Configuration of RF amplifiers is not generally required unless the amplifier is a variable RF amplifier. If the amplifier is variable, the amplifier must be pieced for the proper amount of amplification required, according to RF math calculations. The manufacturer user manual will s explain how to program or configure the amplifier. Figure 3. 16 RF amplifier office in the wireless LAN system 3. 3. 2 RF Attenuators An RF attenuator is a device that causes precisely measured loss (in dB) in an RF signal. While an amplifier will increase the RF signal, an attenuator will decrease it.Consider the case where an access point has a fixed output of 100mW, and the only antenna available is an omni-directional antenna with +20 dBi gain. Using this equipment together would violate FCC rules for power output, so an attenuator could be added to decrease the RF signal down to 30mW before it entered the antenna. This configu ration would put the power output within FCC parameters. Figure 3. 17 shows examples of fixed-loss RF attenuators with BNC connectors (left) and SMA connectors (right). Figure 3. 18 shows an example of an RF step attenuator. 33 Figure 3. 7 Sample of a fixed loss Amplifier Figure 3. 18 A sample of a RF step attenuator (Variable loss) Configuration and Management Figure 3. 19 shows the proper placement in a wireless LAN for an RF attenuator, which is directly in series with the main signal path. Fixed, coaxial attenuators are connected directly between any two-connection points between the transmitter and the antenna. For example, a fixed, coaxial antenna might be connected directly on the output of an access point, at the input to the antenna, or anywhere between these two points if multiple RF cables are used.Variable antennas are generally mounted to a surface with screws through their flange plates or simply placed in a wiring closet on a shelf. Configuration of RF attenuators i s not required unless a variable attenuator is used, in which case, the amount of attenuation required is configured according to your RF calculations. Configuration instructions for any particular attenuator will be included in the manufacturer user manual. s Figure 3. 19 RF attenuator placement in a wireless LAN 3. 3. 3 Lightning Arrestors A lightning arrestor is used to shunt transient current into the ground that is caused by lightning.Lightning arrestors are used for protecting wireless LAN hardware such as access points, bridges, and workgroup bridges that are attached to a coaxial transmission line. concentrical transmission lines are susceptible to surges from nearby lightning strikes. Lightning arrestor are only needed for outdoor antennas that are 34 Susceptible to lighting sticks in the vicinity. They are not necessary for indoor antennas because of the existing building ground. A lightning arrestor can generally shunt surges up to 5000 Amperes at up to 50 volts. Lightni ng arrestor performs the following function 1.Lightning strikes a nearby object 2. Transient current are induced in the antenna or the RF transmission line 3. The lightning arrestor senses these currents and immediately ionizes the gases held internally to cause a short (a path of almost no resistance) directly to earth ground. Figure 3. 20 Lightning Arrestors installed in a network 3. 3. 4 RF Connectors RF connectors are specific types of connection devices used to connect cables to devices or devices to devices. Traditionally, N, F, SMA, BNC, and TNC connectors (or derivatives) have been used for RF connectors on wireless LANs.In 1994, the FCC and DOC (Canadian Department of Communications) ruled that connectors for use with wireless LAN devices should be proprietary between manufacturers 7. For this reason, many variations on each connector type exist such as 1. N-type 2. Reverse planetary house N-type 3. Reverse threaded N-type Figure 3. 21 Sample N-type and SMA Connector 35 Ch oosing an RF Connector There are five things that should be considered when acquire and installing any RF connector, and they are similar in temperament to the criteria for choosing RF amplifiers and attenuators. . The RF connector should match the impedance of all other wireless LAN components (generally 50 ohms). 2. Know how much insertion loss each connector inserted into the signal path causes. The amount of loss caused will factor into your calculations for signal strength required and distance allowed. 3. Know the upper berth frequency limit (frequency response) specified for the particular connectors. This point will be very important as 5 GHz wireless LANs become more and more common. Some connectors are rated only as high as 3 GHz, which is fine for use with 2. GHz wireless LANs, but will not work for 5 GHz wireless LANs. Some connectors are rated only up to 1 GHz and will not work with wireless LANs at all, other than legacy 900 MHz wireless LANs. 4. Beware of bad quali ty connectors. First, always consider buying from a reputable company. Second, purchase only high-quality connectors made by name-brand manufacturers. This kind of purchasing particularity will help eliminate many problems with sporadic RF signals, VSWR, and bad connections. 5. Make sure you know both the type of connector (N, F, SMA, etc. ) that you need and the sex of the connector.Connectors come in male and female. male person connectors have a center pin, and female connectors have a center receptacle. 3. 3. 5 RF Cables Proper cables are needed for connecting an antenna to an access point or wireless bridge. Below are some criteria to be considered in choosing the proper cables for your wireless network. 1. Cables introduce loss into a wireless LAN, so make sure the shortest cable length necessary is used. 2. intend to purchase pre-cut lengths of cable with pre-installed connectors. Doing minimizes the possibility of bad connections between the connector and the cable. maestro manufacturing practices are almost always superior to cables manufactured by naive individuals. 3. Look for the lowest loss cable available at your particular price range (the lower the loss, the more expensive the cable). Cables are typically rated for loss in dB/100-feet. The table in Figure 5. 29 illustrates the loss that is introduced by adding cables to a wireless LAN. 4. Purchase cable that has the same impedance as all of your other wireless LAN components (generally 50 ohms). 5. The frequency response of the cable should be considered as a primary decision factor in your purchase.With 2. 4 GHz wireless LANs, a cable with a rating of at least 2. 5 GHz should be used. With 5 GHz wireless LANs, a cable with a rating of at least 6 GHz should be used. 36 Table 3. 1 Coaxial Cable attenuation ratings LMR Cable 100A 195 200 240 300 400 400UF 500 600 600UF 900 1200 1700 30 3. 9 2. 0 1. 8 1. 3 1. 1 0. 7 0. 8 0. 54 0. 42 0. 48 0. 29 0. 21 0. 15 50 5. 1 2. 6 2. 3 1. 7 1. 4 0. 9 1. 1 0 . 70 0. 55 0. 63 0. 37 0. 27 0. 19 150 8. 9 4. 4 4. 0 3. 0 2. 4 1. 5 1. 7 1. 2 1. 0 1. 15 0. 66 0. 48 0. 35 220 10. 9 5. 4 4. 8 3. 7 2. 9 1. 9 2. 2 1. 5 1. 2 1. 0. 80 0. 59 0. 43 450 15. 8 7. 8 7. 0 5. 3 4. 2 2. 7 3. 1 2. 2 1. 7 2. 0 1. 17 0. 89 0. 63 900 22. 8 11. 1 9. 9 7. 6 6. 1 3. 9 4. 5 3. 1 2. 5 2. 9 1. 70 1. 3 0. 94 1500 30. 1 14. 5 12. 9 9. 9 7. 9 5. 1 5. 9 4. 1 3. 3 3. 8 2. 24 1. 7 1. 3 1800 33. 2 16. 0 14. 2 10. 9 8. 7 5. 7 6. 6 4. 6 3. 7 4. 3 2. 48 1. 9 1. 4 2000 35. 2 16. 9 15. 0 11. 5 9. 2 6. 0 6. 9 4. 8 3. 9 4. 5 2. 63 2. 0 1. 5 2500 39. 8 19. 0 16. 9 12. 9 10. 4 6. 8 7. 8 5. 5 4. 4 5. 1 2. 98 2. 3 1. 7 37 CHAPTER 4 Wireless LAN 38 Chapter 4 Wireless LAN 4. 1 Wireless LAN (WLAN) 4. 1. 1 Wireless LANLinking of t