October 2012
Intermediate to advanced
160 pages
2h 49m
Chinese
Content preview from 802.11n技術手冊
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鏈結層的基礎
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67
非 HT 混合模式（Non-HT mixed mode）
此模式描述於第 64 頁 的「PHY 層保護：在 HT 混合模式下的 L-SIG 期間」，其
為最常採用的保護形式，因為大多數的網路都需要保護機制，且都有連結著非
802.11n 的設備。在這種模式下，傳輸都透過混合模式標頭隱含地保護著，並且對於
802.11a/g 設備不需要有特別的程序。
802.11n 保護機制也會與定義於 802.11g 的現存 Use_Protection 位元進行互動。當
802.11g 的 Use_Protection 旗標被設定時，任何保護模式的訊框傳輸都會使用 802.11 直
接序列或 802.11b 速率（1、2、5.5 或 11 Mbps）進行傳送。
安全性
首次在 802.11i 標準化的基本安全架構在 802.11n 中並不需要有顯著的改變。其中有三個
關於安全架構的小變更。首先，CCMP 這個基於 AES 的加密協定被擴展用於保護更長
的訊框，如圖 5-1 所示。其次，標準中宣稱重播偵測在一般 MAC 承載與聚合 MAC 承
載的設定上，是基於同等的基礎。第三，也是最重要的，802.11n 中規定 TKIP 已經不再
被允許使用。
是否在 802.11n 排除 TKIP 的使用，這是一個在 802.11 工作小組內的辯論主題。TKIP 最
初被設計為一個權宜之計，用以換取部分安全上的強固性，因此他可以被翻新使用在現
存的 802.11b 設備上。TKIP 的設計不容易擴展至新的協定特性，例如當增加 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.