| Summary: | 碩士 === 國立中興大學 === 電機工程學系 === 86 === IEEE 802.11 無線區域網路標準中定義媒體存取控制層(MAC Layer)和實
體層(PHY Layer), 而本篇論文主要是研究並模擬媒體存取控制層, 其中
包括了分散式協調(DCF)和集中式協調(PCF)兩部分. 在分散式協調模式中
主要是用來傳送非同步資料, 各使用者採用避免碰撞之載波偵測多重存取
協定(CSMA/CA)來競爭傳輸媒介的使用權, 其中包括了用來解決隱藏端問
題的 RTS/CTS 機制. 而集中式協調模式主要是用來傳送多媒體以及具有
時效性的資料, 傳輸媒介的使用是由存取點(AP)來控制, 由存取點通知使
用者, 被通知者方可傳送資料. 其中各個不同模擬實驗環境主要是利用
Ptolemy 來建構以研究各種因素如: 功率控制, 訊框錯誤率, 封包長
度...等對系統表現的影響. 實驗結果顯示在 DCF 模式中使用 RTS/CTS
機制能降低訊框碰撞機率以及碰撞長度因而有效提高系統表現. 若考慮訊
框錯誤率, 系統表現會比不考慮此因素時來的差. 另外, 當最大競爭視窗
如果越大, 則系統表現越好. 在 PCF 模式, 由於尚未制定通知法則(
polling scheme), 對此我們設計了一種具有優先權的方法以提高系統的
整體表現. 在 PCF 的使用者之間利用直接傳送的延遲較經由 AP 轉傳來
的低很多. 無論 DCF 或 PCF 模式, 使用功率控制將使系統表現降低. 而
只支援 DCF 模式的 Ad-hoc 無線區域網路因為其無 AP 因而產生較多的
overhead, 造成系統表現較 Infrastructure 無線區域網路來的差.
A wireless communication system is more feasible than a
wired communication system in some special environments where
the wired network is difficult to implement. To satisfy the
needs of wireless data networking, the 802.11, recommended by
IEEE becomes an international recognized standard. The Medium
Access Control (MAC) protocol which supports asynchronous and
time bounded service of data frame is a key part of the
standard.We conduct a performance study of the DCF (asynchronous
data transfer), PCF (time bounded service) and several issues
(like energy efficiency and RTS/CTS problem etc.) that are
proposed in the IEEE 802.11 draft standard. Ptolemy, a
simulation tool, is used to simulate these models. In DCF mode,
the effect of backoff procedure on system throughput and delay
is studied. The simulation results show that using the RTS/CTS
mechanism can reduce the collision frame length and enhance
performance.The larger the maximum contention window size
implies the better the performance. And the performance
decreases while we take the frame error rate into account. We
also purpose a "priority" polling scheduling algorithm in PCF
mode. The simulation results indicate that using the "Priority"
polling algorithm improves the system performance. The using of
power saving not only in DCF but also in PCF, shall reduce the
system performance. On the other hand, owing to that Ad-hoc
wireless LAN which only supports DCF mode without an AP, it
makes the more overhead than Infrastructure wireless LAN. The
Ad-hoc wireless LAN's performance is worse than the
Infrastructure wireless LAN's.
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