睡仙要注意了

睡仙要注意了！新出爐的研究顯示，一天睡覺超過8小時的人，
其死亡率會比 睡 6、7小時的人高。這個發現發表在2002/2/15的
《一般精神醫學期刊》(Archives of General Psychiatry)。

過去對人類死亡率的研究，通常集中在體重、抽煙和運動的項目上，
將睡眠時 間也納入研究，可還是第一遭。常常有病人抱怨自己睡眠
不足，或是因為長時間 失眠而擔心受怕。
因為他們相信每個人一天都需要睡上8小時。於是醫師治療的方法，
便是盡量讓病人睡滿8小時，認為如此便能提高病人的存活率，而一
旦失眠便得承擔死亡的風險。他們仔細分析比較110萬個從30歲到
102歲成人的睡眠習慣，進而發現一天睡7小時的人可以活最久。
而超過8小時或少於6小時以下的其死亡率比一天7小時的人高出了
12%。更令人意外的是，一天睡5小時的人，其壽命也比一天睡8小時
以上的人還要高。不過5小時已經是基本睡眠時間的最低極限了，
因為少於4小時的人，其死亡率還是會逐漸攀升。那些睡眠超過8.5
小時或是少於3.5和4.5小時的，其死亡率則超過 5%。相對而言，患
有「失眠症」的人其死亡率並沒有超額攀升，而安眠藥卻有助於死
亡率的提高。根據Kripke的說法，由於採樣的數目夠大，足以區分出
7小時和8小時睡眠時間 所 產生後果的差異，但卻還找不出理由來
解釋為何睡愈久，死亡率就愈高。「我們無法確定是否長時間的睡
眠會導致死亡，」Kripke提到。「我們還得做進一步的研究，才能確
定是否將鬧鐘時間調早，就能增進健康。」但如果你是那種一天睡
超過6小時就 會自動驚醒的人，那你儘管放心，你不需要為睡不滿8
小時而窮緊張，因為從健康的觀 點來 看，實在沒什麼理由要睡比較
久。

月球是外星人製造的?

月球是外星人製造的?

本文作者：呂應鐘先生

月球讓科學家愈來愈迷惑月球，跟隨地球不知多少年了？也許地球上還沒有人類之前，它就在天天看著地球。以前，大家都說月裡有一座廣寒宮，住著一位古代美女 ─ 嫦娥、一隻白兔，還有一位天天在砍伐桂樹的吳剛。然而，一九六九年七月十九日，美國太陽神十一號太空船登陸月球，沒有看到廣寒宮，也沒有找到嫦娥和白兔，更沒有桂樹和吳剛，於是許多人的美麗幻想成為科學的失望。但是，時至今日，太空人登陸月球已有26年了，人類對月球的瞭解並沒有增加，反而由於從太空人留在月球上的儀器，得到更多的不解資料，讓科學家愈來愈迷惑，每當夜晚抬頭望向月球之時，產生既熟悉又陌生的複雜情緒，不禁要問：月亮呀！可不可以告訴我們，妳的真相？未有定論的月球起源目前有關月球起源的說法有三種，第一個假說是月球和地球一樣，是在46億年前由相同的宇宙塵雲和氣體凝聚而成的；第二個假說是月球係由地球拋離出去的，拋出點後來形成太平洋；第三個假說是月球為宇宙中個別形成的星體，行經地球附近時被地球重力場捕獲，而環繞地球。原本多數科學家相信第一種說法，也有少數相信第二種說法，可是自從太空人登上月球，取回不少月球土壤，經化驗分析知道月球成分和地球不同。地球是鐵多矽少，月球是鐵少矽多；地球鈦礦很少，月球卻很多，因此証明月球不是地球分出去的。第二種說法站不住腳了。同樣的原因，也使得第一個假說動搖了，因為，如果地球和月球是在46億年前經過相同過程形成的，那麼成分應該一樣才對，為何差異會那麼大呢？所以，科學家只好也放棄第一種說法。只剩第三種說法了，可是如果是其它地方飛來的星體，飛進太陽系後，太陽引力比地球引力大很多倍，照理講月球應該受到太陽的引力而飛向太陽，不是受到地球的引力而留在地球上空的。這三種「正統科學家」提出的假說，沒有一項能解答所有疑問，也沒有一項經得起嚴格的質問。事實上，時至今日，「月球來自何處」，仍是天文學未定之論。也因此任何人都可以提出自己月球起源的看法，不管多離奇，他人是不能用任何「小科學」的字眼來批評的。日、月、地球間的奇妙現在舉出一個大家都想不到的天文上的奇妙現象，讓大家用心想一想。月球離地球，平圴距離約為38萬公里。太陽離地球，平均距離約為1億5千萬公里。兩兩相除，我們得到太陽到地球的距離約為月球到地球的395倍遠。太陽直徑約為 138萬公里，月球直徑約為 3400多公里，兩兩相除，太陽直徑約為月球的 395倍大。395倍，多麼巧合的數字，它告訴我們什麼信息?大家想想看，太陽直徑是月球的395倍大，但是太陽郤離地球有395倍遠，那麼，由於距離抵銷了大小，使這兩個天體在地球上空看起來，它們的圓面就變得一樣大了 !這個現象是自然界產生的，或是人為的？宇宙中那有如此巧合的天體？從地面上看過去，兩個約略同大的天體，一個管白天，一個管夜睌，太陽系中，還沒有第二個同例。著名科學家艾西莫夫曾說過：「從各種資料和法則來衡量，月球不應該出現在那裡。」他又說：「月球正好大到能造成日蝕，小到仍能讓人看到日冕，在天文學上找不出理由解釋此種現象，這真是巧合中的巧合 ！」難道只是巧合嗎？有些科學家並不這麼認為。科學家謝頓(Willian R. Shelton)在《羸得月亮》一書中說：「要使太空船在軌道上運行，必須以每小時18,000哩的速度在100哩的太空中飛行才可以達成平衡；同理，月球要留在現有軌道上，與地球引力取得平衡，也需有精確的速度、重量和高度才行。」問題是：這樣的條件不是自然天體做得到的，那麼，為何如此？做為衛星它太大了太陽系的行星擁有衛星，這是自然現象，但是我們的地球卻擁有一個大得「不自然」的衛星 ─ 月球，也就是說做為一個衛星，月球的體積和其行星 ─ 地球相比實在是太大了。我們來看看下列數據：地球直徑12,756公里，衛星月球直徑3,467公里，是地球的27%。火星直徑6,787公里，有二個衛星，大的直徑有23公里，是火星的0.34%。木星直徑 142,800公里，有13個衛星，最大的一個直徑5,000公里，是木星的3.5%。土星直徑120,000公里，有23個衛星，最大的一個直徑4,500公里，是土星的3.75%。看一看，其它行星的衛星，直徑都沒有超過母星的百分之五，但是我們旳月球卻大到百分之二十七，這樣比較之後，是不是發現月球實在「大得不自然」了。這個資料，又在告訴我們，月球的確不尋常。隕石坑都太淺了科學家告訴我們，月球表面的坑洞是隕石和彗星撞擊形成的。地球上也有些隕石坑，科學家計算出來，若是一顆直陘10哩的隕石，以每秒三萬哩的速度(等於100萬噸黃色炸藥的威力)撞到地球或月球，它所穿透的深度應該是直徑的四到五倍。地球上的隕石坑就是如此，但是月球上的就奇怪了，所有的隕石坑竟然都「很淺」，以月球表面最深的加格林坑(Gagrin Crater)只有4哩，但它的直徑郤有186哩寬！直徑186哩，深度最少應該有700哩，但是事實上加格林坑的深度只是直徑的２%而已，這是科學上的不可能。為什麼如此？大文學家無法圓滿解釋，也不去解釋，因為心裡清楚，一解釋就會推翻所有已知的月球知識。因為，只能用月球表面約四哩深處下有一層，很堅硬的物質結構，無法讓隕石穿透，所以，才使所有的隕石坑都很淺。那麼，那一層很硬的物質結構是什麼？不可能存在的金屬月球隕石坑有極多的熔岩，這不奇怪，奇怪的是這些熔岩含有大量的地球上極稀有的金屬元素，如鈦、鉻、釔等等，這些金屬都很堅硬、耐高溫、抗腐蝕。科學家估計，要熔化這些金屬元素，至少得在2、3千度以上的高溫，可是月球是太空中一顆「死寂的冷星球」，起碼30億年以來就沒有火山活動，因此月球上如何產生如此多需要高溫的金屬元素呢？而且，科學家分析太空人帶回來的 380公斤月球土壤樣品後，發現竟含有純鐵和純鈦，這又是自然界的不可能，因為自然界不會有純鐵礦。這些無法解釋的事實表示了什麼？表示這些金屬不是自然形成的，而是人為提煉的。那麼問題就來了，是誰在什麼時候提煉這些金屬的？地球上看不到的那面月球永遠以同一面對著地球，它旳背面直到太空船上去拍照後，人類才能窺視容顏。以前天文學家認為月球背面應和正面差不多，也有很多隕石坑和熔岩海。但是，太空船照片卻顯示大為不同，月球背面竟然相當崎嶇不平，絕大多數是小隕石坑和山脈，只有很少的熔岩海。此種差異性，科學家無法想出解答，照理論言，月球是太空中自然星體，不管那一面受到太空中的隕石撞擊的機率應該相同，怎會有內外之分呢？月球為何永遠以同一面向著地球？科學家旳說法是說它以每小時 16.56公里的速度自轉，另一方面也在繞著地球公轉，它自轉一周的時間正好和公轉一周的時間相同，所以月球永遠以一面向著地球。太陽系其它行星的衛星都沒有這種情形，為何月球「正好」如此，這又是一種巧合中的巧合嗎？難道除了巧合之外，不能找一些其它的解釋嗎？數百年來的怪異現象月球曾發生過不少無解的現象，數百年來的天文學家不知已看過多少次了。一六七一年，三百多年前的科學家卡西尼就曾發現月球上出現一片雲。一七八六年四月，現代天文學之父威廉赫塞爾發現月球表面似乎有火山爆發，但是科學家認為月球在過去三十億年來已沒有火山活動了，那麼這些「火山」是什麼？一八四三年曾繪製數百張月球地圖的德國天文學家約翰史谷脫，發現原來約有10公里寬的利尼坑正在逐漸變小，如今，利尼坑只是一個小點，周圍全是白色沈積物，科學家不知原因為何？一八八二年四月廿四日，科學家發現月球表面「亞里斯多德區」出現不明移動物體。一九四五年十月十九日，月面「達爾文牆」出現三個明亮光點。一九五四年七月六日晚上，美國明尼蘇達州天文台台長和其助手，觀察到皮克洛米尼坑裡面，出現一道黑線，過不久就消失了。一九五五年九月八日，「泰洛斯坑」邊緣出現二次閃光。一九五六年九月廿九日，日本明治大學的豐田博土觀察到數個黑色物體，似乎排列成DYAX和JWA字形。一九六六年二月四日，蘇俄無人探測船月神九號登陸「雨海」後，拍到二排塔狀結構物，矩離相等，依凡桑德生博士說：「它們能形成很強的日光反射，很像跑道旁的記號。」伊凡諾夫博士從其陰影長度估計，大約有15層樓高，他說：「附近沒有任何高地能使這些岩石滾落到現在位置，並且成幾何形式排列。」另外，月神九號也在「風暴海」邊緣拍到一個神祕洞穴，月球專家威金斯博士因為自己也曾在卡西尼Ａ坑發現一個巨大洞穴，因此他相信這些圓洞是通往月球內部。一九六六年十一月廿日，美國軌道二號探測船在距「寧靜海」46公里的高空上，拍到數個金字塔形結構物，科學家估計高度在15至25公尺高，也是以幾何形式排列，而且顏色比周圍岩石和土壤要淡，顯然不是自然物。一九六七年九月十一日，天文學家組成的蒙特婁小組發現「寧靜海」出現「四周呈紫色的黑雲」。這些奇異現象，不是一般的外行人發現，全是天文學家和太空探測器的報告，意味著：月球上有人類未知的神祕！月面上的不明飛行物一九六八年十一月廿四日，太陽神八號太空船在調查將來的登陸地點時，遇到一個巨大、約l0平方英里的大幽浮，但在繞行第二圈時，就沒有再看到此物。它是什麼？沒人知曉。太陽神十號太空船也在離月面上空五萬呎的地方，突然有一個不明物體飛升，接近他們，這次遭遇拍下了紀錄片。一九六九年七月十九日，太陽神十一號太空船載著三位太空人奔向月球，他們將成為第一批踏上月球的地球人，但是在奔月途中，太空人看到前方有個不尋常物體，起初以為是農神四號火箭推進器，便呼叫太空中心確認一下，誰知太空中心告訴他們，農神四號推進器距他們有六千英里遠。太空人用雙筒望遠鏡看，那個物體呈Ｌ狀，阿姆斯壯說：「像個打開的手提箱。」再用六分儀去看，像個圓筒狀。另一位太空人艾德林說：「我們也看到數個小物體掠過，當時有點振動，然後，又看到這較亮的物體掠過」。七月廿一日，當艾德林進入登月小艇做最後系統檢查時，突然出現二個幽浮，其中一個較大且亮，速度極快，從前方平行飛過後就消失，數秒鐘後又出現，此時兩個物體中間射出光束互相連接，又突然分開，以極快速度上升消失。在太空人要正式降落月球時，控制台呼叫：「那裡是什麼？任務控制台呼叫太陽神十一號。」太陽神十一號竟如此回答：「這些寶貝好巨大，先生……很多……噢，天呀！你無法相信，我告訴你，那裡有其它的太空船在那裡，……在遠處的環形坑邊緣，排列著，……他們在月球上注視著我們……。」蘇俄科學家阿查查博士說：「根據我們截獲的電訊顯示，在太空船一登陸時，與幽浮接觸之事馬上被報告出來。」一九六九年十一月廿日，太陽神十二號太空人康拉德和比安登月球，發現幽浮。一九七一年八月太陽神十五號，一九七二年四月太陽神十六號，一九七二年十二月太陽神十七號，……等等的太空人也都在登陸月球時．見過幽浮。科學家蓋利曾說過：「幾乎所有太空人都曾見過不明飛行物體。」第六位登月的太空人艾德華說：「現在只有一個問題，就是他們來自何處？」第九位登月的太空人約翰楊格說：「如果你不信，就好像不相信一件確定的事。」一九七九年，美國太空總署前任通訊部主任莫里士．查特連表示「與幽浮相遇」在總署裡是一平常事，並說：「所有太空船都曾在一定距離或極近距離內被幽浮跟蹤過，每當一發生，太空人便和任務中心通話。」數年後，阿姆斯壯透露一些內容：「它真是不可思議……，我們都被警示過，在月球上曾有城市或太空站，是不容置疑的，……我只能說，他們的太空船比我們的還優異，它們真的很大……。」數以千計的月球神祕現象，如神祕閃光、白雲、黑雲、結構物、幽浮等，全都是天文學家和科學家共睹的事實，這些現象一直未有合理解釋，到底是什麼呢？空心的太空船月球一九七０年，俄國科學家柴巴可夫(Alexander Scherbakov)和米凱威新(MihKai Vasin)提出一個令人震驚的「太空船月球」理論，來解釋月球起源。他們認為月球事實上不是地球的自然衛星，而是一顆經過某種智慧生物改造的星體，加以挖掘改造成太空船，其內部載有許多該文明的資料，月球是被有意的置放在地球上空，因此所有的月球神祕發現，全是至今仍生活在月球內部的高等生物的傑作。當然這個說法被科學界嗤之以鼻，因為科學界還沒有找到高等智慧的外星人。但是，不容否認的，確是有許多資料顯示月球應該是「空心」的。最令科學家不解的是，登月太空人放置在月球表面的不少儀器，其中有「月震儀」，專用來測量月球的地殼震動狀況，結果，發現震波只是從震央向月球表層四周擴散出去，而沒有向月球內部擴散的波，這個事實顯示月球內部是空心的，只有一層月殼而已！因為，若是實心的月球，震波也應該朝內部擴散才對，怎麼只在月表擴散呢？架構新月球現在，我們可以來重新架構月球理論了：月球是空心的，月殼分為兩層，外殼是岩石及礦物層，像是自然的星體，由於隕石撞擊月球後，只能穿透這一層，已知隕石坑的深度都不深，最深只有四哩，所以此層厚度最多五哩。月球內殼是堅硬的人造金屬層，厚度不知道，也許只有十哩，成分含有鐵、鈦、鉻等，能耐高溫、高壓、腐蝕，是一種地球人未知的合金。因為太空人安裝在月球，表面的月震儀顯示震波只在月表傳遞，而不深入內部，可見月球的確只有這兩層月殼。既然如此，月球就不是自然界的，它是人造的，造它的「人」經過精細計算，將月球從他們的星系迎到太陽系來，擺在現在的位置，使地面上的人能在夜間看到它，而且和太陽一樣大。所以，月球起源的三種理論都不對。「造月的人」讓月球永遠以一面向著地球，因為這一面有不少控制地球的設備。他們自己住在月球背面的內部，因為月球表面日夜溫差太大，中午最熱是攝氏 127度，夜間最泠是零下 l83度，不適合居住，所以都住在內部。他們巳發展出飛碟，經常飛出外面做些研究或修護儀器，並注意地球人的動靜，有時被地球太空人看到，有時被地面上的望遠鏡觀測到。「造月的人」是那一種外星人？他們來此有多久了？我們目前都還不知道。也許不久，地球人就能知道月球的真相了。我用科學無法解釋的實際月球現象，來重新架構月球的理論，「圓滿的」將月球之謎……解答，有誰能說這樣做是「不科學」呢？

經典員工心理測試題

(Reference: http://mypaper.pchome.com.tw/news/candys333/3/1311018730/20081104180917)

中國人專用心理測驗(美國蘭德公司製)


測試看看囉~~~^_^


前言︰
本心理測試是由中國現代心理研究所以著名的美國蘭德公司（戰略研究所）擬製的一套經典心理測試題為藍本，根據中國人心理特點加以適當改造後形成的心理測試題，目前已被一些著名大公司，如聯想、長虹、海爾等公司作為對員工心理測試的重要輔助試卷，據說效果很好。

現下已經有人建議將來作為對公務員的必選輔助心理測試推展使用。
快來測試一下，很準的﹗

注意︰每題只能選擇一個答案，應為你第一印象的答案，
把相應答案的分值加在一起即為你的得分。


1、你更喜歡吃那種水果？
A、草莓2分B、蘋果3分 C、西瓜 5分 D、鳳梨10分 E、橘子 15分

2、你平時休閑經常去的地方
A、郊外2分 B、電影院 3分 C、公園 5分 D、商場10分
E、酒吧15分 F、練歌房20分

3、你認為容易吸引你的人是？
A、有才氣的人2分 B、倚賴你的人3分 C、優雅的人5分
D、善良的人 10分 E、性情豪放的人15分


4、如果你可以成為一種動物，你希望自己是哪種？
A、貓2分 B 、馬 3分 C、大象 5分 D、猴子 10分
E、狗15分 F、獅子 20分

5、天氣很熱，你更願意選擇什麼模式解暑？
A、游泳 5分 B、喝冷凍飲料 10分 C 、開空調 15
分

6、如果必須與一個你討厭的動物或昆蟲在一起生活，你
能容忍哪一個？
A、蛇2分 B 、豬 5分 C、老鼠 10分 D、蒼蠅15
分

7、你喜歡看哪類電影、電視劇？
A、懸疑推理類2分 B、童話神話類 3分 C、自然科學類
5分 D、倫理道德類 10分 E、戰爭槍戰類 15分


8、以下哪個是你身邊必帶的物品？
A、打火機 2分 B、口紅 2分 C、記事本 3分 D、紙巾 5
分 E、手機 10分

9、你出行時喜歡坐什麼交通工具？
A、火車 2分 B、單車 3分 C、汽車 5分 D、飛機 10分
E、步行 15分

10、以下顏色你更喜歡哪種？
A、紫2分 B 、黑 3分 C、藍 5分 D、白 8分 E、黃 12
分 F、紅 15分

11、下列運動中挑選一個你最喜歡的（不一定擅
長）？
A、瑜珈2分 B、單車 3分 C、桌球 5分 D、拳擊 8分
E、足球 10 F、蹦極 15分

12、如果你擁有一座別墅，你認為它應當建立在那裡？
A、湖邊 2分 B、草原3分 C、海邊 5分 D、森林 10分
E、城中區15分

13、你更喜歡以下哪種天氣現象？
A、雪 2分 B 、風 3分 C、雨 5分 D、霧 10分 E、雷
電 15分

14、你希望自己的視窗在一座30層大樓的第幾層？
A、七層2分 B、一層 3分 C、二十三層5分 D、十八層
10分 E、三十層 15分

15、你認為自己更喜歡在以下哪一個城市中生活？
A、麗江 1分 B、拉薩 3分 C、昆明 5分 D、西安 8分
E、杭州 10分 F、北京 15分

180分以上︰
意志力強，頭腦冷靜，有較強的領導欲，事業心強，不達目的不罷休。
外表和善，內心自傲，對有利於自己的人際關係比較看重，有時顯得性格急噪，咄咄逼人，得理不饒人，不利於自己時頑強抗爭，不輕易認輸。
思惟理性，對愛情和婚姻的看法很現實，對金錢的慾望一般。

140分至179分︰
聰明，性格活潑，人緣好，善於交朋友，心機較深。
事業心強，渴望成功。
思惟較理性，崇尚愛情，但當愛情與婚姻發生衝突時會選擇有利於自己的婚姻。
金錢慾望強烈。

100分至139分︰
愛幻想，思惟較感性，以是否與自己投緣為標準來選擇朋友。
性格顯得較孤傲，有時較急噪，有時優柔寡斷。
事業心較強，喜歡有創造性的工作，不喜歡按常規辦事。
性格倔強，言語犀利，不善於妥協。
崇尚浪漫的愛情，但想法往往不切合實際。
金錢慾望一般。

70分至99分︰
好奇心強，喜歡冒險，人緣較好。
事業心一般，對待工作，隨遇而安，善於妥協。
善於發現有趣的事情，但耐心較差，敢於冒險，但有時較膽小。
渴望浪漫的愛情，但對婚姻的要求比較現實。不善理財。

40分至69分︰
性情溫良，重友誼，性格塌實穩重，但有時也比較狡黠。
事業心一般，對本職工作能認真對待，但對自己專業以外事物沒有太大興趣，喜歡有規律的工作和生活，不喜歡冒險，家庭理念強，比較善於理財。


40分以下︰
散漫，愛玩，富於幻想。
聰明機靈，待人熱情，愛交朋友，但對朋友沒有嚴格的選擇標準。
事業心較差，更善於享受生活，意志力和耐心都較差，我行我素。
有較好的異性緣，但對愛情不夠堅持認真，容易妥協。
沒有財產理念。

[www.connect802.com] Receive Sensitivity

A fundamental specification of an 802.11 card is its receive sensitivity. The receive sensitivity is the minimum power level at which a signal can be reliably received. For example, a NIC manufacturer may indicate that their particular card has a receive sensitivity of –96 dBm at 1Mb/sec. If the actual RF energy present at that card were less than –96 dBm, then the card would no longer be able to differentiate between signal and noise. The NIC would not detect the incoming packet at all, and the packet would be lost. But how do vendors measure receive sensitivity and what are the implications of their methods for assessing an 802.11 card's performance?

We asked a major vendor of 802.11 hardware how they measured receive sensitivity in their cards. They told us that to measure receive sensitivity, the WLAN card is placed into an RF-shielded room. This guarantees that the test signal will be the only RF transmission in the room, and no background noise in the environment will interfere with the test. The test receiver is placed on a rotating turntable so that measurements can be taken (and then averaged) for all possible horizontal orientations of the receiving antenna. The vendor then transmits packets at weaker and weaker power levels. As the power level decreases, the bit error rate as measured by the card increases. The receive sensitivity of the card will be the minimum power level at which the bit error rate remained below a certain threshold. Therefore, a lower receive sensitivity value (-93 dBm) is better than a higher one (-85 dBm), since it means that the card was able to “reliably receive” data at lower power levels.

Of course, different data rates, having more and less complex encoding and modulation methods, and being more and less resistant to corruption, will result in different receive sensitivities. As data rate increases, receive sensitivity decreases. To put it another way, the higher the data rate, the stronger the signal strength must be for the packet to be reliably received. This is why 802.11 cards drop to lower data rates when interference is present or when they are at the edges of their coverage range. For example, an 802.11b card might have specifications like this:

Receive sensitivity -95 dBm at 1 Mbps
Receive sensitivity -91 dBm at 2 Mbps
Receive sensitivity -89 dBm at 5.5 Mbps
Receive sensitivity -85 dBm at 11 Mbps

While receive sensitivity might seem like a reliable way of comparing two vendors’ cards, we know of no organization that certifies the veracity of the vendor’s results. Therefore, there is the potential for vendors to manipulate the thresholds of their tests to influence their chipset’s receive sensitivity numbers. For example, a vendor that uses a BER threshold of one error in every 1,000,000,000 bits) will end up with lower receive sensitivities than a vendor that uses a BER threshold of one error in every 100,000,000 even though the second vendor’s card may actually be better at receiving bits. Fortunately, some vendors make their BER threshold available in their card's documentation.

[www.connect802.com] RSSI Measurement and dB-Milliwatts (dBm)

Most 802.11 analysis tools and vendors' client management utilities provide a representation of signal strength. Four units of measurement are used to represent RF signal strength in 802.11. These are: mW (milliwatts), dBm ("db"-milliwatts), RSSI (Received Signal Strength Indicator), and a percentage measurement. All of these measurements are related to each other, some more closely than others, and it's possible to convert from one unit to another.

The first two units to consider are the mW and the dBm (pronounced "dee-bee-em" or spoken as "dee-bee milliwatts"). Although these are not the most common units in 802.11, we discuss them first because they are the most basic. Just like a pound is a basic unit for measuring weight, a watt is a basic unit for measuring energy (and, in keeping with metric conventions, a mW is one one-thousandth of a watt). It turns out that measuring RF energy in mW units is not always convenient. This is due, in part, to the fact that signal strength does not fade in a linear manner, but inversely as the square of the distance. This means that if you are a particular distance from an access point and you measure the signal level, then you move twice as far away, the signal will have decreased by a factor of four. This relationship can be characterized as logarithmic, and one can say that "RF power drops off logarithmically."

The "dBm" is a logarithmic measurement of signal strength. Since it is logarithmic, just like the power of the RF signal, as the RF signal's strength changes (logarithmically), the dBm value changes linearly. To put it more generally, if you measure a quantity that changes logarithmically (RF power) with a linear unit (mW), the unit will change logarithmically, which is inconvenient. If you measure a quantity that changes logarithmically with a logarithmic unit (dBm), the unit will change linearly, which is more convenient.

dBm values can be exactly and directly converted to and from mW values. Just like miles and kilometers can be converted directly, so can mW and dBm. The formulas to convert are:

dBm = log(mW) * 10
mW = 10^(dBm/10)

We use dBm because it's much easier to say, and write, "-96dBm" than have to say "0.000 000 000 25 mW". That's a lot of zeroes! You should realize that convenience and ease-of-understanding are two fundamental reasons why the dBm metric is used for RF signal strength, rather than mW.

The IEEE 802.11 standard defines a mechanism by which RF energy is to be measured by the circuitry on a wireless NIC. In 802.11b, g, and a, this numeric value is an integer with an allowable range of 0-255 (a 1-byte value) called the Received Signal Strength Indicator (RSSI). Notice that nothing has been said here about measurement of RF energy in dBm or mW. RSSI is an arbitrary integer value, defined in the 802.11 standard and intended for use internally by the physical and data link layers (the hardware in the card and its drivers). For example, when an adapter wants to transmit a packet it must be able to detect whether or not the channel is clear (i.e.: nobody else is transmitting). If the RSSI is below some very low threshold then the chipset decides that the channel is clear. 802.11 does not require that a particular RSSI value correspond to any particular mW value, so each vendor makes this decision on its own. This means that you probably can't compare "signal strength" values between two vendors' chipsets, because those values are based on the RSSI, and different vendors' chipsets associate different power levels with different RSSI values.

To circumvent the complexities (and potential inaccuracies) of using RSSI as a basis for reporting dBm signal strength, it is common to see signal strength represented as a percentage. The percentage represents the RSSI for a particular packet divided by the maximum RSSI value (multiplied by 100 to derive a percentage). If all vendors used that formula for converting RSSI to signal strength percentage, then percentage for signal strength would provide a reasonable cross-vendor metric for use in network analysis and site survey work. However, if vendors do not consistently use the formula above, then we once again end up in a scenario where it's impossible to compare numbers from different vendors. For example, a vendor might hypothetically use a logarithmic function to map RSSI to signal strength, which would cause the signal strength to stay at high values longer as RSSI decreased, and then to drop off very rapidly as RSSI approached zero. Frankly, we don't know the exact details, on a model-number-by-model-number basis, of how each of the many NIC manufacturers map RSSI to signal strength percentage, so its difficult to draw concrete conclusions on this matter.

[www.connect802.com] Reciprocity Theorem

There is a basic principle of antennae that is so unexpected (to the uninitiated student) that some people refuse to believe it's true the first time they hear it. The principle is called the Reciprocity Theorem. Its consequences are that, if we are using the same input and output gain, then regardless of differences in our antenna gain, if one I can hear you, you can hear me. This month, we'll explore this concept and its implications.

Consider the case where an AP has a 12 dBi omni antenna attached and a client has a 2 dBi omni antenna on a PCMCIA card. Both the AP and the client are using 15 dBm of transmit power. It might not surprise you that the AP's high-gain antenna can push a signal a long way out to the client, but you might guess that the client's low-gain antenna couldn't get a signal back to the AP. You'd be wrong. Antenna reciprocity basically means that the exact same qualities that make an antenna good at transmitting a signal also make it good at receiving a signal. To put it another way, the Rayleigh-Helmholtz reciprocity theorem states:
If an electromagnetic force of some particular magnitude is applied to the terminals of antenna "A" and the received current is measured at some other antenna "B" then an equal current (in both amplitude and phase) will be obtained at the terminals of antenna "A" if the same electromagnetic force is applied to the terminals of antenna "B".

As an analogy for an RF antenna, imagine a paddle sticking up out of the smooth surface of a lake. Another paddle is sticking up at the opposite end of the lake. One paddle begins to oscillate back and forth, creating waves that push on the other paddle, causing it to move. In our analogy, the paddles are antennas and the waves are RF waves. To carry the analogy further, imagine that one paddle is much bigger than the other--it represents our high-gain antenna. When the big paddle oscillates, it makes much bigger waves, causing the smaller antenna to move more even though it's got a smaller surface area. When the little paddle oscillates, on the other hand, the big paddle's increased surface area causes it to move more as well! The analogy fails somewhat because, in reality the increased mass of the big paddle would give it enough inertia that it wouldn't really move more, but for the sake of the analogy, the antennas are massless.

Antenna reciprocity arises from a property of physics equations called "time-symmetry". Time symmetry means that it doesn't matter whether time runs forwards or backwards, the physics equations should work out the same. Time symmetry is one of the touchstones of new physics theories. Any theory that violates time symmetry is called into serious question. To understand the significance of time symmetry, consider a pool table with a white ball near one end and a black ball in the center. The white pool ball is accelerated by the force of impact with the cue stick and travels towards the center of the pool table. In the center, the white ball strikes the black ball in a straight, center-to-center impact. The inertia of the white ball is transferred to the black ball and it is now accelerated away from the white ball in a straight line, leaving the white ball stationary at the point of impact. If you were to make a movie of the two balls striking and then played the movie backwards, it would show exactly the same thing except now it would be the black ball that starred in the opening scene of the movie. If the mass, velocity, and other characteristics of the Amazing Pool Ball Adventure movie were represented through mathematical equations, the equations would not be time dependent. Time could run forward or backward and the results would be identical.

To some readers, the reciprocity theorem may be new. The implications of antenna reciprocity are far reaching and, if this is the first time you've encountered the concept, the implications may be too hard to accept without proof. In fact, not only is reciprocity demonstrable in the lab and in real-world installations, but the physicists of the world can provide mathematical proof that the theorem holds true. If the antennae and the space between them are replaced with a network of linear, passive, bilateral impedances, then the current through the network can be calculated in accordance with standard practices in electronics theory. Whether on paper or in practice, given the same input power on both ends, "If you can hear me, then I can hear you!"

Next month, we'll discuss some of the real-world implications of antenna reciprocity

[www.connect802.com] How To Get More Range: Output Power, Antenna Gain, or Receive Sensitivity?

This month's "Essential Wi-Fi" addresses the issue of increasing range by increasing output power. The specific example given is of switching to higher gain antennas. In that article, we discuss how simply increasing output power may not result in the increase in usable range that you expect. It turns out that the usable range of an AP is dependent on several factors. In this column, we examine those factors and discuss the best way to increase the range of an AP in an indoor environment.

We all know of vendors who sell access points with extremely high power output. For example, one vendor sells an AP that outputs a full watt of power (the FCC maximum) with no external amplifiers. Another vendor sells a PCMCIA card that outputs 250 mW with no external amplification (most PCMCIA cards are around 30 mW). Clients using these devices and expecting dramatic range increases may be disappointed. An 802.11 link is two-way; it doesn't do any good for the one device to be able to blast a signal a long way if the other device can't get a signal back. For example, a 100 mW AP might be able to blast its Beacon packets through several walls. A regular PCMCIA card with 30 mW of output power could receive those Beacons and the user would see the AP in his or her list of available wireless networks. But when the user tried to connect to the AP, the user's card wouldn't have enough power to get back to the AP, and the connection would always fail.

This example points out a fundamental limitation of high-output APs. It might seem at first like a high-powered AP is a great, cost-effective way of increasing the range of your network. An casual site survey, which only measured receive signal strength, would even confirm that the AP's coverage had increased (if all you intend to do is receive packets, then the site survey is right). But as soon as clients actually tried to connect to the network, the flaw would become obvious. In general, there's not much point in having an AP that is much more powerful than the clients that it will serve. All of the AP's extra transmit power will go into pushing signal into areas where the clients can't get a signal back to the AP, and that essentially wastes that coverage, and all the money you spent on a high-powered AP. Typical wireless client cards have an output power between 15 and 30 mW, so we at Connect802 usually design networks with APs that have approximately this output power.

Increased output power is useful if you know that all of the devices in the network will be using the same power output. For example, if you've got two wireless bridges at the end of a point-to-point link, you could increase the usable range of the link by putting an equal amplifier on both of the bridges. This circumstance is difficult to guarantee in the more typical one-AP, many clients scenario, so it's usually better to design that type of network with APs that only put out as much power as the lowest-powered client that you expect to use the network. Another option is to design the network so that clients using maximum power (say, 30 mW) will get maximum data rates (say, 54 Mbps) and clients using less power will still be able to connect, but only at lower data rates.

Increased power output in the AP doesn't result in increased range if clients are not able to get a signal back to the AP, but there's a second parameter in this equation that we have so far neglected: receive sensitivity. Receive sensitivity determines the weakest signal that a device can reliably receive. If an AP combines increased power output with a proportionally better receive sensitivity, then it can not only transmit a more powerful signal to the client, but it can also receive the client's weaker signal. For example, consider a client that transmits at 15 mW and has a receive sensitivity of -72 dBm. Now consider an AP that has a transmit power of 30 mW. The AP is transmitting 3 dB "louder" than the client, so its receive sensitivity must be 3 dB better than the client's (-72 dBm minus 3 dB = -75 dBm) to offset the client's weaker transmit power. If this relationship holds true, then the combination of increased transmit power and increased receive sensitivity will result in better range. To put it simply, just making the AP talk louder doesn't work, but by increasing the AP's receive sensitivity, we've made it both talk louder and listen harder.

Unlike transmit power, the receive sensitivity of an radio is not directly adjustable; it's a fundamental property of the engineering of the radio. But the receive sensitivity of an 802.11 radio differs depending on the data rate that the radio is using, with higher data rates requiring more signal power. Typically, an 802.11 network is designed towards a certain minimum desired data rate, and then radios are purchased with an output power and a receive sensitivity that allows them to achieve that data rate in the required coverage area.

At this point, we have discussed the relationship between usable range, transmit power, and receive sensitivity. Essentially, we are considering two link budgets: one from the client to the AP and one from the AP to the client. These link budgets give a maximum range from the AP to the client and from the client to the AP. The usable range of the AP is limited by the smaller of these two ranges. If the AP-to-client range is already greater than or equal to the client-to-AP range, then increasing the AP's transmit power won't result in more usable range because increasing transmit power only increases the AP-to-client side of the link. Increasing transmit power with a corresponding increase in receive sensitivity will result in more usable range because increasing receive sensitivity also increases the client-to-AP side of the link.

Increasing transmit power can increase usable range in another way that is somewhat independent of receive sensitivity. Each 802.11 data rate requires a certain minimum signal-to-noise ratio. If the signal is too weak to achieve the required signal-to-noise ratio for a given data rate, increased transmit power is the only answer. Increasing receive sensitivity or antenna gain doesn't work because both of those options will amplify the noise as well as the incoming signal, leaving the signal-to-noise ratio the same. In summary, if range is limited by interference, you must at least increase transmit power to the point where the clients receive the minimum signal-to-noise ratio for the desired data rate. At that point, range may still be limited by the AP's receive sensitivity or the client's transmit power or receive sensitivity.

Increasing antenna gain is another way of increasing range. Increasing antenna gain differs from increasing transmit power because of the principle of antenna reciprocity. Put one way, antenna reciprocity states that the same qualities that increase an antenna's gain when it transmits also increase its gain when it receives. This means that an increase in antenna gain on either the client or the AP increases BOTH the client-to-AP side of the link AND the AP-to-client side of the link.

Putting a higher-gain antenna on the access point can be a cheap way of increasing usable range for all clients of that AP, regardless of the clients' transmit powers and receive sensitivities. The tradeoff is that increasing antenna gain changes the antenna's coverage pattern, which may limit the maximum antenna gain that can be achieved. More than about 6-10 dB of antenna gain usually results in coverage area that is small enough to preclude any link except one with fixed endpoints at pre-determined locations.

What conclusions can we draw from this analysis? Putting a higher-gain antenna on the AP increases performance for all clients of the AP, but may not offer enough extra power to significantly increase coverage, especially in indoor environments. Increasing the AP's power output might seem like a cheap way of increasing range, but in reality, a complex link budget relationship exists between transmit power and receive sensitivity on the client and AP that dictates the usable range for each client. Simply dropping a 1000 mW into the center of a building is unlikely to provide the results that the AP's vendor would promise or that naive WLAN administrator might expect. Buying an AP with a very good receive sensitivity (for example, an 802.11g AP with receive sensitivity of -75 dBm or lower at 54 Mbps, -95 dBm or better at 6 Mbps) is a cost-effective way of maximizing the effectiveness of the clients' power output.