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標題: 802.11無線區域網路之直接集中式協調機制
Direct Point Coordination Function in 802.11 Wireless LAN
作者: 黃柏承
Huang, Bo-Cheng
關鍵字: WLAN;802.11無線區域網路;ad-hoc networks;medium access control;point coordination function;distributed coordination function;infrastructure networks;無基礎架構網路;集中式協調機制;分散式協調機制;媒介存取控制;有基礎架構網路
出版社: 電機工程學系
802.11無線區域網路可大致分為無基礎架構網路與有基礎架構網路,無基礎架構網路指的是基本服務區內沒有擷取點與其他的區域網路聯結,反之則為有基礎架構的網路。在802.11無線區域網路中的工作站必須經由802.11 媒介存取控制協定來取得無線傳輸媒介的傳送權以傳送資料,802.11 媒介存取控制協定包含有分散式協調機制與集中式協調機制。分散式協調機制就是載波感測多重存取及衝撞避免機制,工作站必須感測傳輸媒介閒置了一段特定時間後才能傳送資料,每個工作站都有相同的機會競爭到傳輸媒介的傳送權。而集中式協調機制則是由一個稱為集中協調者的裝置經由輪詢的方式決定那個工作站何時有權利傳送資料。

The IEEE 802.11 Wireless Local Area Network (WLAN) specification defines two ways to configure a wireless network: ad-hoc mode and infrastructure mode. Ad-hoc networks do not connect to other LAN's wireless stations while infrastructure networks have an access point connecting to other networks. Wireless stations in IEEE 802.11 WLAN should use IEEE 802.11 Medium Access Control (MAC) protocol to access wireless medium to transfer data. IEEE 802.11 MAC protocol incorporates two access methods: Distributed Coordination Function (DCF) and Point Coordination Function (PCF). DCF is based on the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocol. In this mode, a station must sense the medium before sending a packet. If a station(STA) finds the medium idle for a specific time, it can transmit a packet. PCF uses a Point Coordinator (PC) to determine which STA currently has the right to transmit. The operation is essentially that of polling, with the PC performing the role of the polling master.
PCF provides STA to transmit delay-limited data. However, PCF doesn't transmit packets by the optimum path. Packets which are forwarded by non-optimal path lead to reduced performance. We use a mechanism called Direct Point Coordination Function (DPCF) that PC can choose if packet need to be forwarded or not. So, the performance will improve greatly. We use Network Simulator Version 2 (NS2) to simulate DPCF and PCF. The results show that DPCF outperforms PCF at high load conditions.
Appears in Collections:電機工程學系所

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