| With the rapid growth in the popularity of multimedia applications, such as voice-over-IP and video conferencing, the demand for high bandwidth and Quality-of-Service (QoS) in WLANs is increasing dramatically. There are many inherent QoS limitations in Distributed Coordination Function (DCF) and the system efficiency is considerably affected by various overheads referred to as control frames, backoff, and inter-frame space. To mitigate the impacts of the overheads, a new standard IEEE 802.11e EDCA is specified and the mechanism of burst transmission based on the Contention Free Bursting (CFB) has been specified in the IEEE 802.11e standard. IEEE 802.11e EDCA support QoS by providing differentiated classes of service in the medium access control (MAC) layer. With the CFB scheme, multiple data frames are transmitted in a burst once a station obtains a transmission opportunity (TXOP) after winning the contention for channel access to improve the channel utilization.In this paper, we first study the characteristic of the IEEE 802.11 DCF, the IEEE 802.11e EDCA and the CFB mechanism. And presents Markov chain model for the EDCA and CFB mechanism, analyse the system delay, throughput, packet loss rate. Seconed, in the NS-2 network simulation platform, we simulate the three mechanisms under the complex network topology. Moreover we simulate the system delay, throughput, packet loss rate to verify the correctness of the theory, and prove that the QoS characteristic of EDCA compared to the traditional DCF and CFB can improve the channel utilization. Third, we use NS-2 simulate the impact of the different system parameters on network performance, especially the TXOPlimit (Transmission Opportunity). Form the simulate relust we point out the effect of TXOP. Finally, we present a network topology to achieve the real network which has a large number of nodes and each station can send frame randomly. Then study the new network topology performance and the effect of the CFB mechanism. |
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