| Summary: | 碩士 === 國立中興大學 === 電機工程學系 === 93 === 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.
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