An Overview of Selective Mutism Essay -- Health, Disorder, Aphasia Vol

Hesselman coined the term discriminating silence in 1983 to describe the disorder previously called aphasia voluntaria, elective mutism, speech phobia, psychological mutism, and hearing mute among fourteen other historic terms (Dow, Freeman, Garcia, Leonard, & Miller, 2004 Kearney, 2010). The American psychiatrical Association, or APA, characterizes selective mutism by a persistent misery to speak in specific social situations where speech is expected, despite oratory in other situations. Selective mutism shifted in recent long time from being viewed as a response to trauma sustained too soon in life to a manifestation of an care disorder (Dow et al, 2004). This cover briefly covers all aspects of selective mutism from signs and symptoms to diagnosis and treatment. Selective mutism is a rare condition and some discrepancies exist pertaining to its incidence. Beidel and Turner (2005) domain that the incidence falls surrounded by .18-.76 per centum, while Kearney (2010 ) states that the incidence falls among .20-2.0 percent. No known cause currently exists. Researchers identified common similarities between children with selective mutism. The most prevalent similarities are gender (1.5 females have selective mutism compared to every one male), having a language or anxiety disorder, having a social phobia, and a family history of selective mutism or social phobia. Dow, Freeman, Garcia, Leonard, and Miller (2004) state that out of patients with selective mutism, 70 percent had a first pointedness relative with a social phobia or avoidant disorder and 37 percent had a first degree relative with selective mutism (p. 286). The APA classifies selective mutism under DSM-IV. A childs behavior must meet these criteria for diagnosis of sele... ...n. If a childs speech and language skills are inadequate, the speech-language pathologist impart work with the child to improve the skill that the child struggles with. They also service of process paren ts and teachers understand what the child is experiencing and how to assist with implementing the provided treatment plan. Selective mutism is a rare disorder that affects mainly children. Treating selective mutism early and cleverly leads to success. Research about treatment methods and causes of selective mutism is limited delinquent to the rarity and uniqueness of the disorder. While no known cause is know for contri just nowing to selective mutism, professionals use many indicators to assess and diagnose a child. Children with selective mutism, contrary to popular belief, do communicate through gestures and nods but fail to speak in unfamiliar and uncomfortable situations.

Assessing the Impact of Using Internet for Competitive Intelligence

Assessing the daze of using cyberspace for combative intelligence Here is the summary of an interest scientific paper dealing with the impact of using the Internet for belligerent intelligence. The Internet, as an breeding-rich resource and interorganizational communication tool, has transformed the way that firms gather, produce and fetch competitive intelligence (CI). The aim of this article is to assess the impact of the Internet on CI and the subsequent set up on the organization. What is CI?It is a process of knowing what the competition is up to and staying one step ahead(predicate) of it, by gathering teaching close to competitors and ideally, applying this tuition in short- and long-term strategic planning. It refers to actionable information about the external business milieu that could affect a fellowships competitive position. CI is non industrial espionage given that 90% of all information that a company needs to make critical decisions and to understand it s market and competitors is already familiar or can be systematically developed from public data.The issue of the Internet has led to CI professionals exploiting its information richness and hypermedia capabilities for CI activities. By using the Internet, a company can monitor the presence, posture, products, and prices of former(a) players in its industry. It can track the views of customers and seek out new ideas and expertise internationally. Model they examine the downstream effects of using the Internet (for research, natural and external use) on case of CI information and its subsequent effects on organizational performance.Concretely, the model could be summarized as followed Research + infixed Use + External Use Quality of CI information relate on organization The first three components symbolized the Internet Usage. Results and implications The results of this study contend that using the Internet has a positive impact on the quality of CI information. There is a positive relationship surrounded by exercise of CI information and organizational performance. It shows that the improved quality of CI information through Internet usage has a positive impact on organization.The results suggest that on that point is a direct positive link between usage of the Internet (for research, internal or external purpose) and the quality of CI information, and a positive downstream impact on the organizations strategic benefits. Although there may be some concerns about the reliability and timeless of information published on the Internet, there is little doubt that it is one of the roughly cost-effective means of obtaining information. However, data must be transformed into association and order has to be created from the chaos of the Internet.The information overload from the Internet overly makes focused research difficult but intelligent agents have to be developed to automate and alleviate the CI professionals information gathering workload. My op inion This paper has been written in 2001, which means a very long time ago when we are talking about the Internet and its information storage capabilities. Do you think this paper is no longer suitable for the current situation we are experiencing (empowerment of the customers through neighborly medias, increased access to the Internet, hackers and so on) ?Moreover, they say that the intelligent agents have to deal how to scan the information to find the relevant one and that its the or so cost-effective means of obtaining information, do you think this notion is still aline or must be nuanced ? Bron Assessing the impact of using Internet for competitive intelligence Thompson S. H. Teo & Wing Yee Choo Information & Management 39 (2001) 67-83 http//www. cuaed. unam. mx/puel_cursos/cursos/d_gcfe_m_tres/modulo/modulo_3/m3-4. pdf

All i Want For my Birthday

Mars Is a delightful and enlightening al-Quran that reveals the unparalleled complexity of the human brain. Sacks, an accomplished neurologist and author, presents septette compositors movement studies that highlight diametric neurologic phenomena. In his shell studies, Sacks follows a newly colliding painter, a man who can create no new memories, a surgeon with Trustees syndrome, a blind man who regains his sight, a painter haunt with images from his childhood, an autistic boy artist, and a high-functioning autistic roofless.Sacks does not treat his case studies as dry medical oddities al whizz preferably discusses their neurological experiences indoors their broader human existence. Un similar other authors who sock their patients only distantly, Sacks works easily with his case studies and develops meaningful relationships that translate into a deeper, more(prenominal) Insightful apprehension of his patients and their experiences. While Sacks Is clearly a brilliant neurologist, what makes this book so powerful Is his ability to weave In medicine, science, history, and philosophy Into a coherent narrative.Every case study illuminates a series of important and thought-provoking questions that challenge the perfunctory assumptions of percept, reality, intelligence, and what it means to be human. In the end, the reader emerges with a better perceptivity of the complexity of the human mind. Sacks does not look at simply the diseased and physiological way that the disease affects the individual but how the individual reacts to the ail and how, in each of these cases, they retain their own sense of self in spite of what the disease/doodler does to them.Sacks does not Just throw a barrage of patients with neurological scissors at the reader, but rather goes through the lives of seven patients and observes them In their natural life. He presents not only their disorder, but how It affects their daily life, how their perception of the world is diff erent, and the creative ways that they have come up to lie with with their disorder. According to his case studies and brief synopsis there are seven cases he presented in the book.One is The Case of the Colliding Painter this case his case negotiation about the predicament of a painter who after sixty quintet years had an accident which robbed him entirely of his color vision. A man, who had had a luxurious career as an artist with numerous vividly colored paintings and abstractions In his studio, could no longer even Imagine color. The painter eventually trustworthy his predicament and started to paint black-and-white representations Instead of dwelling on the handout of his ability to paint In color.As Sacks explains, A revision was occurring, so that as his former color world and even the memory of it became fainter and died inside besides involves an artist who loses his color perception ability after an accident. Would it be normal from the moment vision was restored? Was not experience necessary to see? Did atomic number 53 have to learn to see? (Sacks 109). The author details the patient cases and uses it as one of the ways in giving an account of how the modern rationality of vision works.From this, there are lessons learnt from the inability of the artist to also reckon the colors. The diseases focused on in the essays affect the ways in which individuals last and understand themselves.. In this case they call this illness is Cerebral ebon is a type of color-blindness caused by damage to the noetic cortex of the brain, rather than abnormalities in the cells of he eyes retina. It is often confused with congenital achromatic but underlying physiological deficits of the disorders are completely distinct.It is shows the signs and symptoms of Patients with cerebral achromatic deny having any experience of color when asked and fail standard clinical assessments like the Farnsworth- Mussels 100-hue test (a test of color ordering with no nam ing requirements). Patients whitethorn often not notice their loss of color vision and plainly describe the world they see as being drab. just about describe seeing the world in shades of gray. This reflexion totes a key difference between cerebral and congenital achromatic, as those born with achromatic have never had an experience of color or gray.It can diagnosis he most common tests perform to analyze cerebral achromatic are the Farnsworth-Mussels 100-hue test, the Ashier plate test, and the color-naming test. Testing and diagnosis for cerebral achromatic is often incomplete and misdiagnosed in doctors offices. 2 Remarkably, almost 50% of tested patients diagnosed with cerebral achromatic are able to perform ordinarily on the color-naming test. However, these results are Mathew in question because of the sources from which many of these reports come.Only 29% of cerebral achromatic patients successfully pass the Ashier plate test, which is a more recognized and more standar dized test for color blindness. In order for one to be in a position to understand their subjects appropriately, the own(prenominal)ity method acting of investigation is vital. Therefore, spending ample sentence with your subjects is very crucial in this business line. I rule An anthropologist on Mars fascinating since it gives man luck to view peoples brains conditions as well as study them to the letter. The fascinating neurological stories explore some of the unique experiences and perceptions of oneself.The saddest thing about the study on disorders of the nervous system and the brain is that the condition of most of the patients is beyond repair. This is regardless of the diverse scope of knowledge in the book. The passion in me to know more about science related cases especially on starting time hand authors method of finding ways to help patients to be find out again is fantastic. I arrive to this conclusion after reading how he has tackled cases in certain disorders f acing the neuron system and the brain. These are Kormas syndrome and Trustees syndrome.Patients in these unusual disorders should be given information on how to cope to the conditions they find themselves in. This should be done without necessarily considering whether the patients outcome. All the professionals involved in this orbital cavity should incorporate this idea into their profession to spur them to enviable success. In addition, utilizing different neurological techniques to learn each of the subjects in a respectful and personal manner is also important. 3 Most of those operating in this field tend to go by the results given by the clinic.However, this is not forever and a day advisable since you maybe condemning someone to a their death whereas a lot can be done to improve his condition. Having the curiosity to interpret the beauty in the minds of the affected people will help you carry out this goal far much easier. All this should be done in environments that make the affected feel comfortable rather than undermined. This is through creating time for private outings with every patient you are in contact with as well making arrangements to bond with them through their activities. This enables one to learn more and figure out their problems.Being a step ahead and having better ideas on how to treat the individual under medical interrogation is also important. all(prenominal) of the chapters in An anthropologist on Mars has a cast of significant characters, setting, and plot. The elements envisioned in the book weave together creating a fascinating story. The individuals undergoing examination are astonishing and how the author manages to counter the sterile account of the comparative neurological functioning found in psychiatric Journals is brilliant. I am amazed by how the author describes interactions, setting and personal feelings of the subjects.

Sticking With Arnis

Somehow this variation is ignored and perhaps most Filipinos run short to recall it, and even the layers of Earns have been quite unnoticed. Take for example fictile castling. Despite all the accomplishments and contributions Palace has made for the indoctrinate and the body politic through and through playing Earns, she unfortunately still does not seem to get the management and appreciation she deserves. A passionate and graceful Earns varsity player born(p) on April 4, 1 996, Hazels Gristliness R. Palace has given pride to the school and her country through her achievements in the sport.Truly, junior BBS Psychology student plastic Palace is not comely your ordinary Eaten. During one interview, Tensile hares that she just heard about Earns from her uncle when she was in fifth variety. He taught her Earns basically for egotism defense, but that was to a fault the time when Tensile became engrossed to earns. However, it wasnt until college that she learned to a greater ext ent about the sport and actually got to practice it. She started learning earns under Sir Richard congius in his PEE class.Soon and luckily enough, she was invited to become part of the Atone College Earns varsity team the semester after. Everything escalated from then on. Of all the mainstream sports, Palace chose to play a snubbed sport. And so e ask, M/why Earns? Why not volleyball or basketball or any former(a) mainstream sport? Unexposed and out of sight, the mackintosh (Martial Arts Center) located near the college covered courts of ADAM is where Palace has regularly been practicing Earns for 2 years and 2 months now. Undeniably, she has grown in love with the sport.To her, what got her provoke in Earns is because of its history. She was also inspired by the stories of Sir Gallon on how Earns was employ in the past. She says Earns could say a lot about our cultural desktop as Filipinos. l guess I just really mat up that I could help reserve that cultural heritage and t hat part of our personal identity as Filipinos through learning Earns and most probably momentary it on to the younger generations in the future. So its not really about the sport and martial art being mainstream but the cultural heritage and moxie of identity as Filipino, says Tensile.Earns indeed has a huge part on our individuality as Filipinos and it contributes immensely to our cultural growth. This ignored martial art, Earns, is what completes us as countrymen. And because we are armed with one of the many Filipino prides, it is our certificate of indebtedness to uphold this art. Holding on strongly to that obligation is none other than Tensile. As proof, besides being part of the Atone College Earns Varsity team, she is also a part of KAMALA (Kali Earns Martial Arts Organization) and the Philippine Demonstration Team.Moreover, she teaches Earns to grade 7 and 8 students from ASH every Wednesday to fulfill her duty of passing it on to the younger generations. Since Tensil e bountifully gives her time and efforts to preserve Earns, she inescapably to manage her time well. The building where she just had her consultation with one of her professors was hundreds of travel away from he MAC, so she bustled through the buildings and roads in campus as she was late for her training on one Monday afternoon. She beamed, though catching her breath, when she dictum me waiting for her at Managing, then she apologized for being late.Then and there, we started walking to their teams training ground. As I watched her train until the late-night hour, I contemplated on how she brings about her time. Admittedly, Tensile express that she has difficulty with time management with piled-up school tasks, training and family time, she tries her best to oddment and sacrifice time for each. Neglected school responsibilities is a big no-no because academics is her prototypal priority, and to help herself out, she decided not to be part of any school organizations this sch ool year.On the other hand, amidst all the busyness, keeping in touch with her family was neer a failure for her. She says she is very open to them. When her parents call her on the peal daily, she often talks about what happened during her training. My friends also get updated on whats misadventure in my Earns career, she laughs shyly. Although she is still a fresh competitor in the world of Earns, Tensile Palace has won numerous awards and only her university, family members and a couple friends recognized her success.Listed below are some of the awards she received for only the past few months. In spite being nervous and pressured, Palace did not make these an excuse not to excel in her number one kickoff in the world stage. She was one of the hand-picked members of KAMALA to represent the Philippines in the 2014 ground Martial Arts Festival in South Korea last August. This formerly in a lifetime experience has encouraged her to pursue Earns more, and she is emphatically determined to make Earns now as a sport and as a martial art.

Important Publication of Official Statistics

Important Publication of Official Statistics A few serious publications containing official statistics for general use are given below statistical abstract of India This annual publication is brought bulge out by the CSO and contains the statistics of motley sectors of the Indian economy at least for the last five years. This also gives state-wise statistics at least for the last year. India- Pocketbook economic information This publication is brought out every year by the Ministry of Finance. It deals with the various aspects of economy particularly financial, foreign caution and intertheme economic comparisons.Basic statistics relating to Indian economy This is brought out by the statistics and surveys form of planning commission on an annual basis. It contains basic indicators on various aspects of economy for a number of back years on condemnation series basis. India- a reference manual This is published annu all toldy by the Ministry of Information and Broadcasting and it contains information on various aspect of Indian economy. Estimation of National Product, savings and capital formation- This is generally known as a white paper on national income and is published annually by the CSO.It ontains estimates of national income, savings, capital formation and consumption, expenditure together with national and public sector accounts. Agriculture situation in India This is published by the Ministry of Agriculture and Cooperation (Directorate of Economics and Statistics) on a monthly basis. This publication contains all available current statistics together with notes and articles on the assessment of current gardening situation in the country. Available district- wise selective information is also describe from time to time. Monthly statistics on production of selected industries in India This publication ofCSO contains data on output, capacity and index numbers and several other variables relating to much than 90 industries. Reserve Bank of India Bu lletin This is published by the run batted in on a monthly basis and it contains elaborated data on various aspects of Indian economy including a detailed data on currency and finance situation. Economic survey This is published annually by the Department of Economics Affair and Ministry of Finance on the eve of the insertion of budget. The document contains an elaborated review of all aspects of Indian economy. Banking statistics- Basic statistical returns This is published twice in a year by the

Compare the way a news story is presented in three different newspaper extracts Essay

In this leaven I will compare the way a red-hots invoice is presented in three different newspaper extracts. conjure A is from The quantify on Tuesday 4th September, extract B is withal from The Times on Tuesday 4th September extract b is an editorial, extract c is from The shielder G-2 on Friday 7th September extract c is a bowel movement page graphic. All three extracts cover the event that happened on Monday third September between the Catholics and the Protestants. The incidents occurred when Catholics trail children from holy Cross Primary school began there new school year.The children had to walk in between barriers of panoplied police, just to get to school. The incident started when republicans was accused of knocking over both men on ladders who were putting loyalists flags on lampposts out side Holy Cross Primary School before the marching season in June. In all three extracts there is a powerful opening night statement to catch the indorsers attention, extr act A uses a pun the progeny girls of the Holy Cross Primary School in Belfast began their new school year yesterday with a horrifying lesson in sectarian hatred.This is trying to say that in schools children are learning roughly hatred instead of being taught other lessons much(prenominal) as get along with everyone regardless of whether they are Protestant or Catholic. Extract B also uses a powerful opening statement Northern Ireland has only again shown its Gorgon-face of bigotry. This states that once again Northern Ireland has shown its monstrous side, whereas extract C states Why it takes the image of a child to wake the world this makes the commentator to feel sorry for the children and is an rhetorical question.Extract A has a big, rank(a) headline it states Children walk a gauntlet of Belfast hatred which makes the reader to want to read on. Extract B states Children in the middle this is also trying to get the attention of the reader, whereas extract C doesnt arri ve at a headline because it is a front-page graphic. Extract A uses a accredited amount of emotive dustup to make the audience feel trustworthy emotions.Extract A states screamed at not shouted at merely screamed at, spat at use of abuse on children, sobbing uncontrollably this states that the children werent crying they were uncontrollably sobbing due to the riots. The emotive expression in extract A has a certain effect on the audience, it makes the reader feel sympathetic towards the children. Extract B also uses a certain amount of emotive language using words such as Gorgon-face of bigotry and hideous spectacle. These words are usually associated with beasts, therefrom this statement is trying to show that what is happening, should be considered as monstrous and that the victims of this colossus are innocent children. Surrounded and overwhelming are adjectives meaning that there is no way out for the victims, showing that the whole situation cannot be resolved. Extract B is an editorial and is the editors point of view not the facts. Extract C doesnt use a lot of emotive language mainly because it is a front-page graphic from a supplement.

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