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標題: IEEE 802.11 媒體存取控制層協定之效能分析
Analysis of Traffic Characteristics for IEEE 802.11 MAC Layer Protocol in Ad-Hoc Networks
作者: 蔡宗憲
關鍵字: WLAN;無線區域網路;Ad-hoc mode;Infrastructure mode;MAC;DCF;PCF;NS-2;無基礎架構;有基礎架構;分散式協調機制;集中式協調機制
出版社: 電機工程學系
IEEE 802.11無線區域網路中,可分為無基礎架構區域網路與有基礎架構區域網路。而分散式協調機制 (DCF) 是主要的媒體擷取控制機制。分散式協調機制是利用載波偵測多重擷取/碰撞避免 (CSMA/CA) 的協定來擷取媒體,工作站必須感測傳輸媒介閒置了一段固定時間(DIFS)後才可傳送資料,每個工作站爭取到通道使用權的機會是相同的;集中式協調機制(PCF)者是集中協調者利用輪循的方式來決定哪個工作站有權去傳送資料。我們利用NS-2針對系統效能、時間延遲、封包遺失率、競爭視窗的大小等來做模擬研究,實驗結果顯示在DCF模式中使用 RTS/CTS機制可以降低訊框發生碰撞的機率及碰撞長度,因此提高了系統效能也降低了系統的時間延遲。而在不同大小的競爭視窗中不使用RTS/CTS機制的系統,其效能會嚴重的受到競爭視窗大小的影響,反之使用RTS/CTS機制則不然。而封包越長的其效能也越好,不過其時間延遲者會變的較大。另外工作站越多時其系統效能及時間延遲都變的比較差。

The IEEE 802.11 WLAN specification define two ways to configure a wireless network: Ad-hoc mode and Infrastructure mode. The distributed coordination function (DCF) scheme is the primary medium access control (MAC) scheme. The medium access of the DCF is based on the carrier sense multiple access with collision avoidance (CAMA/CA) protocol. A station have to sense the medium before sending a packet. If the medium is found idle for a specific time (DIFS) then it can transmit. The point coordination function (PCF) is a centralized MAC protocol which is able to support collision free and time bounded services. We used NS-2 to simulate the wireless LAN MAC layer protocol and to evaluate the throughput, delay, packet lose rate, and contention window sizes. The simulation results show that using the RTS/CTS mechanism can effectively reduce the frame collision probability and reduce delay time. Using the RTS/CTS mechanism can also avid the effect that caused by the change of the maximum contention window size. The results also show that for the longer transmission packet size, the throughput goes high but delay time takes longer. For fewer stations, the throughput goes up and delay time goes down.
Appears in Collections:電機工程學系所

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