October 2012
Intermediate to advanced
160 pages
2h 49m
Chinese
Content preview from 802.11n技術手冊
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802.11n-2009 導論
|
5
鏈結層（MAC）強化
十年前，經常在估計無線區域網路的最高速率時，是藉由「宣稱」的實體層速率減半得
之。這是由於有許多的協定負擔存在，例如媒介競爭（Medium Contention）與正面回應
（Positive Acknowledgment），這意謂著在 802.11 的最佳情境下，802.11 MAC 大約能
有 50% 的效率。網路也許宣稱具備 54 Mbps 的資料速率，但你可以期望的最佳速率通
常只有 25 至 27 Mbps。在許多情況下，50% 的效率已經過於高估了。正如許多其他市
場，無線區域網路廠商在生產製造時，有許多可選的性能增強功能（包含基於標準的選
項與廠商定義的選項），然而廠商僅會達到市售產品的最低需求。
802.11 是透過分配網路媒介的存取傳輸以進行競爭的管理，但並非所有的傳送器都
是以同樣基礎所製造的。當速率增加時，所需要的訊框間隔將會造成更高的成本。在
802.11b 網路中，一個 10 微秒（
μ
s）短訊框間隔（Short Interframe Space，SIFS）的機
會成本約為 14 個位元組（bytes）。當資料速率增加為一般 802.11n 的速率時，相同的短
訊框間隔成本將會超過千個位元組（kilobyte）。
802.11n 最重要的部份，就是致力於改善 MAC 的效率。當許多 802.11 設備的使用者關
注在高資料傳輸速率時，所提升的效率即為改善速率的主要目標。在 802.11n 中，用來
增強效率的主要技術為訊框聚合 ...
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