目前VoIP技术逐步转向无线组网的方向发展,但是VoIP技术在无线局域网(Wireless Local Area Networks,WLAN)中的应用需要满足特定的要求,并且在基于802.11协议的MAC层PCF与DCF的接入方式上存在缺陷.通过对802.11e协议的MAC层EDCF接入方...目前VoIP技术逐步转向无线组网的方向发展,但是VoIP技术在无线局域网(Wireless Local Area Networks,WLAN)中的应用需要满足特定的要求,并且在基于802.11协议的MAC层PCF与DCF的接入方式上存在缺陷.通过对802.11e协议的MAC层EDCF接入方式以及802.11协议的PCF及DCF接入方式进行对比研究,并使用计算机仿真证明VoIP节点使用802.11e的EDCF能更好的满足语音数据高服务质量的需求,通过结果解决了组建VoIP无线局域网时的VoIP接入站点数量与使用何种接入方式的选择问题.展开更多
Endothelial cells can initiate contraction(constriction)of the vascular smooth muscle cells that surround them.Such endothelium-dependent,acute increases in contractile tone can be due to the withdrawal of the product...Endothelial cells can initiate contraction(constriction)of the vascular smooth muscle cells that surround them.Such endothelium-dependent,acute increases in contractile tone can be due to the withdrawal of the production of nitric oxide,to the production of vasoconstrictor peptides(angiotensin II,endothelin-1),to the formation of oxygen-derived free radicals(superoxide anions)and/or the release of vasoconstrictor metabolites of arachidonic acid.The latter have been termed endothelium-derived contracting factor(EDCF)as they can contribute to moment-to-moment changes in contractile activity of the underlying vascular smooth muscle cells.To judge from animal experiments,EDCF-mediated responses are exacerbated when the production of nitric oxide is impaired as well as by aging,spontaneous hypertension and diabetes.To judge from human studies,they contribute to the blunting of endothelium-dependent vasodilatations in aged subjects and essential hypertensive patients.Since EDCF causes vasoconstriction by activation of the TP-receptors on the vascular smooth muscle cells,selective antagonists at these receptors prevent endothelium-dependent contractions,and curtail the endothelial dysfunction in hypertension and diabetes.展开更多
在M a lli提出的AEDCF[1]机制的基础上,把基于信道条件的P因子引入到EDCF机制中去,提出了一种动态自适应EDCF(DAEDCF)机制。在DAEDCF机制中,退避时间和竞争窗口的更新依赖于P因子,在信道条件好的情况下,减少了时延,提高了信道利用率,在...在M a lli提出的AEDCF[1]机制的基础上,把基于信道条件的P因子引入到EDCF机制中去,提出了一种动态自适应EDCF(DAEDCF)机制。在DAEDCF机制中,退避时间和竞争窗口的更新依赖于P因子,在信道条件好的情况下,减少了时延,提高了信道利用率,在信道条件糟糕时,减小了碰撞几率,提高了系统的性能。通过仿真可以看到,DAEDCF机制在高负载下,进一步提高了各优先级,特别是中、低优先级业务流的性能。和AEDCF机制相比,在信道负载达到80%时,V ideo流的吞吐量提高了2%,平均时延降低了25%,Background流的吞吐量提高了14%,平均时延降低了20%。展开更多
为了在802.11的网络中提供服务质量(QoS)支持,IEEE 802.11 Task Group E提出了EDCF协议.然而EDCF只能提供业务区分服务,并不能提供服务质量(QoS)保证.为了能在重负载下提供QoS保证,在WLAN中加入呼叫接纳控制(CAC)机制是非常必要的.本文...为了在802.11的网络中提供服务质量(QoS)支持,IEEE 802.11 Task Group E提出了EDCF协议.然而EDCF只能提供业务区分服务,并不能提供服务质量(QoS)保证.为了能在重负载下提供QoS保证,在WLAN中加入呼叫接纳控制(CAC)机制是非常必要的.本文首先提出了一个新的3维Markov模型对非饱和状态下EDCF的吞吐量和平均接入时延进行了分析.并在此基础上,提出了一种基于效用函数的CAC策略,它可以使网络的总收益达到最大.最后通过大量仿真验证了所提出的CAC策略的有效性.展开更多
文摘目前VoIP技术逐步转向无线组网的方向发展,但是VoIP技术在无线局域网(Wireless Local Area Networks,WLAN)中的应用需要满足特定的要求,并且在基于802.11协议的MAC层PCF与DCF的接入方式上存在缺陷.通过对802.11e协议的MAC层EDCF接入方式以及802.11协议的PCF及DCF接入方式进行对比研究,并使用计算机仿真证明VoIP节点使用802.11e的EDCF能更好的满足语音数据高服务质量的需求,通过结果解决了组建VoIP无线局域网时的VoIP接入站点数量与使用何种接入方式的选择问题.
文摘Endothelial cells can initiate contraction(constriction)of the vascular smooth muscle cells that surround them.Such endothelium-dependent,acute increases in contractile tone can be due to the withdrawal of the production of nitric oxide,to the production of vasoconstrictor peptides(angiotensin II,endothelin-1),to the formation of oxygen-derived free radicals(superoxide anions)and/or the release of vasoconstrictor metabolites of arachidonic acid.The latter have been termed endothelium-derived contracting factor(EDCF)as they can contribute to moment-to-moment changes in contractile activity of the underlying vascular smooth muscle cells.To judge from animal experiments,EDCF-mediated responses are exacerbated when the production of nitric oxide is impaired as well as by aging,spontaneous hypertension and diabetes.To judge from human studies,they contribute to the blunting of endothelium-dependent vasodilatations in aged subjects and essential hypertensive patients.Since EDCF causes vasoconstriction by activation of the TP-receptors on the vascular smooth muscle cells,selective antagonists at these receptors prevent endothelium-dependent contractions,and curtail the endothelial dysfunction in hypertension and diabetes.
文摘在M a lli提出的AEDCF[1]机制的基础上,把基于信道条件的P因子引入到EDCF机制中去,提出了一种动态自适应EDCF(DAEDCF)机制。在DAEDCF机制中,退避时间和竞争窗口的更新依赖于P因子,在信道条件好的情况下,减少了时延,提高了信道利用率,在信道条件糟糕时,减小了碰撞几率,提高了系统的性能。通过仿真可以看到,DAEDCF机制在高负载下,进一步提高了各优先级,特别是中、低优先级业务流的性能。和AEDCF机制相比,在信道负载达到80%时,V ideo流的吞吐量提高了2%,平均时延降低了25%,Background流的吞吐量提高了14%,平均时延降低了20%。
文摘为了在802.11的网络中提供服务质量(QoS)支持,IEEE 802.11 Task Group E提出了EDCF协议.然而EDCF只能提供业务区分服务,并不能提供服务质量(QoS)保证.为了能在重负载下提供QoS保证,在WLAN中加入呼叫接纳控制(CAC)机制是非常必要的.本文首先提出了一个新的3维Markov模型对非饱和状态下EDCF的吞吐量和平均接入时延进行了分析.并在此基础上,提出了一种基于效用函数的CAC策略,它可以使网络的总收益达到最大.最后通过大量仿真验证了所提出的CAC策略的有效性.
