Supported by the National Natural Science Foundation of China under Grant Nos.60472076, 60673178, 60434030 (国家自然科学基金); the National Basic Research Program of China under Grant No.2006CB303007 (国家重点基础研究发展计划(973)); the National High-Tech Research and Development Plan of China under Grant Nos.2006AA01Z218, 2006AA01Z215 (国家高技术研究发展计划(863))
提出一种分析模型,基于信号流图和母函数方法分析了无线局域网中EDCA(enhanced distribution channel access)机制的MAC(medium access control)接入延时.得到了MAC接入延时的概率分布和相关的数字特征,包括接入延时的均值、方差、标准差和变异系数.仿真结果与模型的分析结果吻合良好,验证了模型和计算方法的正确性.进一步分析了各类AC(access category)的发送概率、碰撞概率、挂起概率和延时均值随终端数目变化的规律;分析了MAC接入延时的成因;指出了AIFS(arbitration inter frame spacing)值对MAC接入延时的影响;指出接入延时的变异系数大于1,因此,采用负指数分布作为MAC接入延时的近似分布,会错误地估计系统的服务能力.
A model is proposed to analyze the MAC (medium access control) access delay of EDCA (enhanced distribution channel access) mechanism of wireless LANs (local area networks), which is based on the moment generating function and signal flow graph. The model gives the probability distribution function (PDF) of MAC access delay and several relative characteristic numbers, including the mean value, variance, standard deviation and deviation coefficient of MAC access delay. The results of the analytical model coincide with simulations, which validates the model proposed. A lot of analyses are presented. Firstly, the model presents the relationship among number of nodes and sending probability, collision probability, freezing probability and mean delay of all ACs (access categories). Secondly, the model discloses the composition of MAC access delay. Thirdly, the model presents the effect of different values of AIFS (arbitration inter frame spacing) on MAC access delay. Lastly, the model presents that the deviation coefficient of MAC access delay is larger than 1, so when the exponential distribution is adopted as the distribution of MAC access delay, the capacity of the queue system will be evaluated wrongly.