October 2012
Intermediate to advanced
160 pages
2h 49m
Chinese
Content preview from 802.11n技術手冊
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第六章
淨空頻道評估（Clear-Channel Assessment，CCA）
因為 802.11 為一個先聽後說（Listen-before-talk）協定，所以確定媒介是否處於閒置狀
態是一個重要的特性，以進行適當的協定作業。在嘗試傳輸前，一個 802.11n 設備會先
進行
淨空頻道評估
（
CCA
），並且僅會在 CCA 顯示頻道為閒置狀態時進行傳輸。如果
該頻道為忙碌狀態，設備就必須根據 802.11 協定規則遞延傳輸。CCA 規則的建立是依
據選擇何種等級的狀況，CCA 機制必須偵測到一個傳輸的存在，通常是以 bBm 的方式
來呈現，並且宣稱若任何訊號強度大於此閾值，將會導致 CCA 判讀為忙碌。如同以往
的實體層，CCA 是基於
訊號偵測
（
Signal Detection
）與
能量偵測
（
Energy Detection
）。
訊號偵測意即一個設備接收並解碼一個 802.11 傳輸，而能量偵測則意謂一個設備接收到
一個傳輸，其能量遠高於
雜訊基準
（
Noise Floor
），以必免受到其他傳輸的干擾。雜訊
基準是在一個區域內的周圍無線訊號，傳輸的訊號強度必須在它之上，方能被接收。一
個粗略的譬喻，雜訊基準就像是在派對裡的嘟囔對話，只有在高於背景噪音的情形下，
接收者方能偵測到傳輸的存在。在無線網路的環境下，雜訊基準隨著媒介的使用率而增
長。在 2.4 GHz 頻段中，雜訊基準的增長是隨著藍牙、無線電話、微波爐、甚至是其他
的 802.11 AP。
當一個 802.11n 網路設置為 20 MHz 頻道時，偵測媒介是否忙碌的規則是相當簡單的： ...
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