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標題: 802.11無線區域網路之適應性增強分散式協調機制
Adaptive Enhanced Distributed Coordination Function in 802.11 Wireless LAN
作者: 施金山
關鍵字: WLAN;802.11無線區域網路;ad-hoc networks;infrastructure networks;medium access control;distributed coordination function;point coordination function;Quality of Service;無基礎架構網路;有基礎架構網路;媒介存取控制;分散式協調機制;集中式協調機制;服務品質保證
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有線或無線區域網路聯結,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CSMA/CA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 Network (WLAN) specification defines two ways to configure a wireless network: ad-hoc mode and infrastructure mode. Ad-hoc networks do not use an access coordinator to connect wireless stations to a Distribution System (DS) but infrastructure networks do. 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 the medium is found idle for a specific time then 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.
DCF can not provide any Quality of Service (QoS), so there are many medium access schemes proposed for IEEE 802.11 WLAN to provide QoS. The IEEE 802.11 is currently working on the support of QoS in a new standard, called IEEE 802.11e. A new access method called Hybrid Coordination Function (HCF) is introduced. It is a queue-based differentiation that uses both DCF and PCF enhancements.
One of the main problems of Enhanced DCF is the static reset of the Contention Window (CW) which decreases the throughput performance and increases the collision rate especially at high load conditions. We use a mechanism named Adaptive Enhanced Distributed Coordination Function (AEDCF) that stations can dynamically adjust their CW depending on traffic load, so the collision rate will increase more slowly. We use Network Simulator Version 2 (NS2) to simulate AEDCF and EDCF, and results show that AEDCF outperforms the basic EDCF especially at high load conditions.
Appears in Collections:電機工程學系所

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