| Summary: | 碩士 === 中華大學 === 電機工程學系(所) === 98 === Abstract
This dissertation is proposing an improved mechanism to better eliminate the predicament of network congestion occurs in Wireless LAN (WLAN). As the number of workstations in a crowded network increases, the probabilities of collision increase as well. From the IEEE 802.11 DCF algorithm, queuing of station transmission is scheduled based on the CSMA/CA and Binary Exponential Back-off scheme. Although Back-off reduce number of collisions in the network, its effectiveness is restrained because each individual workstation is unaware of the current network congest state and places transmitting data onto the channel indiscriminately, which causes a higher collision probabilities.
According to the paper, a Distributed Contention Control (DCC) method is suggested to correct the problem with collision avoidance in the IEEE 802.11 DCF system. Before a workstation transmits, DCC mechanism evaluates the current Slot usage rate. A low Slot usage rate corresponds to an intense channel contention. To avoid collision, DCC would Defer Access to the transmission channel for a period of time and rearrange the transmission moment for the workstations. Even though this method is capable of decreasing collision possibility and thus alleviating congestion, it causes an unstable condition in channel access for the workstations. This is due to that DCC can only detect the “presence” of data transmission. A DCC system would generate unnecessary waiting by stalling transmission even if a merely small number of workstations exist.
Take this concern into consideration, a mechanism as an improvement to the DCC scheme is proposed in our paper, the Modifier DCC (M-DCC) method. We narrow the requirement to include the condition, “when multiple workstations are competing for access,” so that only when this condition is true, will access be deferred. In addition, a Markov Chain mathematical model is used to analyze and compare between DCF, DCC, and the M-DCC method. Through MATLAB evaluation, it is verified that the M-DCC method can not only enhance DCC efficiency, effectively diminish congestion level, reduce the collision probabilities, but also boost data throughput.
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