| Summary: | 碩士 === 義守大學 === 資訊管理學系碩士班 === 94 === he popularity of wireless local area network (WLANs), especially of IEEE 802.11 standard [1], has generated much interests on improvement and modeling of the protocol. The distributed coordination function (DCF) of IEEE 802.11 standards adopts the binary exponential backoff (BEB) mechanism based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). However, it is well known that, as the number of active stations increases, the performance of IEEE 802.11 MAC in term of delay and throughput degrades dramatically. To explore the inherent problems, it is important to find out the relation between the collision probability, the number of competing stations, and the dynamic change of contention window size (CW). In this thesis, we first evaluate several simple but effective schemes (BEB, DIDD, EIED) by resetting the window size in the backoff mechanism to improve the performance of the contention based IEEE 802.11 DCF. Simulation results show that those schemes, with correct window size resetting, out-performs the BEB scheme employed in the IEEE DCF. Secondly, a simple but effective method [Appendix B] based on a closed queuing network model is established to analyze the number of estimation stations in an IEEE 802.11 network. By using this method, each individual station can estimate the number of competing stations based on a simple run-time measurement.
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