| Based on the performance research of IEEE802.11 MAC protocol, some optimization mechanisms are proposed in this dissertation.At the beginning, the basic knowledge of IEEE802.11 MAC protocol, especially the MAC frame structure and the basic theory of DCF protocol are introduced. After this, some research work is done to get the performance of CSMA/CA+BEB based DCF. This research work tells the relation between system throughput and some parameters such as the number of competing nodes, the frame length, the minimum contention window and the frame transmitting probability. The most attraction is that the system throughput scales much with different network scales.So the next research work is to estimate the number of competing nodes in a dynamic network. S-DCF is firstly proposed to achieve this estimation result, which is easily to be established but also accurate enough. The main idea of S-DCF is to count the number of different source MAC addresses contained in the header of successfully transmitted MAC frames. Based on this estimation result, two different optimization mechanisms are proposed to improve the performance of IEEE802.11DCF. The first is to adjust the minimum contention window to improve the system throughput. The second is to directly give the station an optimized constant contention window according to the estimated number, which is also called optimal-DCF/CCW algorithm.In this paper, most of the research work is on the optimal-DCF/CCW. Firstly, the feasibility of using constant contention window in IEEE802.11DCF is analyzed. And then give a deep research of the system performance of DCF/CCW, such as the system throughput and the system access delay. From this analysis we finally obtain the equation of how to calculate the optimized constant contention window. In fact, the optimal-DCF/CCW algorithm is a combination of S-DCF and DCF/CCW. From the OPNET simulation results, we can see that the performance of optimal-DCF/CCW is much better than the legacy IEEE802.11DCF. |
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