崑山科技大學
資訊管理系 四年制大學部
無線網路管理報告作業十
學號:4981Q022 學生:王雨喬
指導老師：曾生元老師
為何 802.11n 可以在原本的工作頻率提供較大的資料?
(1).多輸入多輸出(Multiple-input multiple-output, MIMO)，多輸入多輸出系統採用天 線陣列，利用空間多工技術來提高所使用頻寬的效率。這系統從多個天線發送或 在多個天線處接收，採用空間多工，系統能在 一個頻率上同時傳輸一個以上的 空間數據串流[2]。所以從外表來看，802.11n 無線網路基地台的天線數比以前的 802.11abg 天線多，考慮價格性 能比，最佳是 3x3 的天線陣列，而 3x3 的天線陣 列支援 2 個空間的串流，傳輸效率直接升成 2 倍，如圖一。
(2).支援 40MHz 的頻寬，原本 802.11abg 只使用 20MHz 的頻寬，到了 802.11n 可使 用 40MHz 的頻寬，傳輸效率直接升成 2 倍以上。
(3).管理用(overhead)網路封包縮減，以及縮短保護間隔(Short Guard Interval)或中框 空間的縮減(Reduced Interframe Space)，如此大概可以提升傳輸效率百分之十。
802.11n 向下相容，啟動 802.11n 後原 802.11abg 仍可使用。
何謂 RSSI、SNR?數值大好，還是數值小好?
RSSI－線電波訊號強度值，強度規格則由廠商自行定義。
SNR－信號雜訊比，信號和噪聲信號功率的對比值。
數值越大越好，信號越強。
算式：EIRP (mW)(dBm)= Tx (mW)+ Gain (dBi)- Loss (dB) X dBm = 10 log(y/1mW)
1. An AP transmitter emits 40 mW of power through a cable that is "adding" 3 dB loss. The Yagi antenna that is being used has 13.5 dBi gain.What is the EIRP?
ANS: 40mW = 16dBm, 16 + 13.5 - 3 = 26.5 dBm
2. An AP transmitter emits 20 mW of power through a cable that is "adding" 4 dB loss per 100 feet. The cable is 20 feet long.The omnidirectional antenna that is being used is 5.2 dBi gain. What is the EIRP?
ANS: 20mW = 13dBm, 13 + 5.2 -4/5 =17.4 dBm
3. An AP transmitter emits 100 mW of power to an antenna directly connected to it.
The antenna that is being used is an 8.5 dBi patch antenna. What is the EIRP?
ANS: 100mW = 20 dBm, 20 + 8.5 = 28.5 dBm
4. You have been asked not to exceed 20 dBm EIRP on a 3.0 dBi omnidirectional antenna. Which power level should you set your AP to knowing that you use 50 feet of 6 dB/100 feet loss cable?
ANS: 20 = Tx + 3 - (50/100)*6, Tx = 20 dBm
5. You have been asked not to exceed 17 dBm EIRP on a 13.5 dBi Yagi antenna.
Which power level should you set your AP to knowing that you will use 150 feet of 6 fB/100 feet loss cable and that the cable connectors add an extra 0.5 dB loss?
ANS: 17 = Tx + 13.5 - 9 - 0.5(額外損耗), Tx = 13 dBm
6. You have been asked not to exceed 17 dBm EIRP on a 5.2 patch antenna. How much length of 2.8 dB loss per 100 feet cable should you use, knowing that the AP power level is statically set to 40 mW?
ANS: 40mW = 16dBm, 17 = 16 + 5.2 - Loss, Loss = 4.2, 4.2 / 2.8 = 1.5, 1.5 * 100 =150 feet
Cisco AP 可以運作在哪幾種模式?
1. local *AP 預設模式,每 180 秒 scan 一次 2. H-REAP
3. monitor *不做廣播 SSID,只收訊 1.1 秒 scan 一次 4. Rogue Detector *抓非法
5. Sniffer *監聽用 6. Brige
1. Convert 20 mW to its dBm equivalent = 13 dBm 2. Convert 40 mW to its dBm equivalent = 16 dBm 3. Convert 2 W to its dBm equivalent = 33 dBm
4. Convert 23 dBm to its milliwatts(mW) equivalent = 200 mW 5. Convert -13 dBm to its milliwatts(mW) equivalent =
0.050118723362727228500155418688495 mW 6. A station receives 0.000001 mW RSSI from an AP.
The noise level is around 0.00000025 mW. Convert these values to dBm and determine the SNR level.
Is the SNR level acceptable?
先轉成 dBm 在直接減
【(RSSI) -60dBm】 - 【(noise) -66dBm】 = (SNR) 6dBm 不可以接收~
7. How many dBd is a 7.24 dBi antenna? = 5.1 dBd 8. How many dBd is a 13.56 dBi antenna? = 11.42 dBd 9. How many dBi is a 13.56 dBd antenna? = 15.7 dBi 10. How many dBd is a 18.86 dBd antenna? = 21 dBi
11. What is the dBd gain of 21 dBi dish antenna(碟狀天線)? =18.86 dBd 12. Which antenna has more gain:2.14 dBi or 3.28 dBd? 3.28 dBd
13. Which antenna has more gain:3.41 dBi or 4.18 dBm? none
