Research And Implementation Of DCF Mechanism In MAC Layer About IEEE 802.11 Protocol

With the recent development of wireless technology and Internet, there arise increasing demands for portable computers or workstations. And WLAN (Wireless Local Area Network) is used widely to meet these demands because of its high flexibility and robustness. There are many existing standards, such as IEEE 802.11, HiperLAN and Bluetooth etc. But IEEE802.11 is most widely used. IEEE802.11 Wireless LAN standard defines a distributed random access MAC protocol: DCF (Distributed Coordination Function). DCF includes two medium access mechanism, that is, CSMA/CA and access mechanism with RTS/CTS. But some problems which lie in these two mechanisms lead to inefficient utilization of wireless network bandwidth and degrade the performance of 802.11. The thesis is focused on DCF mechanisms provided in IEEE802.11. In this thesis the author introduces briefly the background, concepts, advantages, application future and many standards of WLAN, and the DCF mechanisms provided in IEEE802.11. The author also introduced in detail the technique of transmission, basic network topology and protocol system. The author analyzed with emphasis the effectiveness of the IEEE802.11 RTS/CTS handshake protocol and the stability of the binary exponential backoff algorithm. In wireless network, the hidden terminal may happen frequently and lead to data packet corruption. The RTS/CTS handshake protocol is mainly designed for solving such a problem. However, it has an underlying assumption that all hidden nodes are within the transmission range of receivers. From our study, we realize that such an assumption may not hold when the transmitter-receiver distance exceeds a certain value. Based on IEEE802.11, we proposed a simple MAC layer scheme-Conservative CTS Reply to solve this problem.In DCF the other important scheme is binary backoff algorithm. From our study, we know that the way in which DCF use random backoff time to resolve the channel contention leads to inefficient utilization of wireless scarce resource. In particular, in a highly loaded network, the portion of channel bandwidth wasted due to collisions is significantly high. In this thesis, we present a different mechanism which uses two "virtual" stage of contention resolution to achieve better and more stable performance than DCF.The simulation results verify that our improvement on DCF is effective. Finally we prospect the future of the DCF in IEEE802.11.