Analyzing And Optimizing Of Stability For WLAN Access Mechanism

The WLAN (WireLess Local Area Networks) has been applied in campus, shopping malls, railway stations, airports, offices, and other daily places. As the advantage of WLAN has become increasingly obvious, it has become a standard specification in smart phone, PAD and other digital equipments. The application of the WLAN has been extended from social application to specialized fields such as intelligent transportation, industrial control, vehicular network and so on. Everyone enjoys the convenience and benefits brought by the WLAN.The most widely used protocol standard for WLAN is IEEE 802.11, which defines the physical layer and the MAC layer to provide reliable data transmission over unreliable media. The MAC protocol is built on CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) with virtual carrier sensing binding DCF (Distributed Coordination Function) mechanism, and binary exponential back off algorithm to achieve access to different nodes control. According to the current researches and the actual situation, for the complex networks and specific network, the reliability of wireless links suffers a lot and the erratic performance of the IEEE 802.11’s DCF protocol makes it difficult to meet the requirements of network reliability.In order to solve the reliability and stability problems of data transmission in the WLAN, the paper made an effort to put forward a new strategy from MAC layer and application layer. First, we conducted further research on the protocol of MAC layer, and propose an improved MAC protocol. Then a network access mode based on TMD principle idea at the application level is realized, which solved the channel competition problem of the excessive accesses, and the problem that the ACK signal cannot be received timely. The main tasks of the paper are as follows:(1) Aiming at modifying strategy for IEEE802.11DCF contention window, this thesis propose a new contention window back off algorithm (HBDCF). As the algorithm adopts the TCP congestion control mechanism, when the transmission fails, the increasing of contention window is exponential at first, and then becomes linear after reaching the threshold. While the number of sending successful reaches the function value, the decrease of contention window is linear at first and then becomes exponential. The DCF protocol can effectively decrease the probability of collisions, reduce network delay, and increase the saturation throughput of the system and so on.(2) In order to verify the performance of HBDCF Algorithm, this thesis establishes a two-dimensions Markov chain models to conduct theoretical analysis on the protocol, and simulates the collision probability, throughput and network delay of HBDCF protocol by OPNET simulation tool.(3) This thesis proposes a network access mechanism of TDM mode based on network application layer. And the paper realizes the algorithm on RT5350 hardware platforms, using OpenWrt open source embedded operating. The algorithm is used in the recruitment of students test system, and the performance of the system was tested, which verified the reliability and effectiveness of the system.