【作者】 林碧娇；

【导师】 肖明波；

【作者基本信息】 厦门大学 ， 通信与信息系统， 2007， 硕士

【摘要】 随着无线通信技术和移动终端技术的迅猛发展,以IEEE 802.11为基础的无线局域网(WLAN)得到了越来越广泛的应用。与此同时,VoIP以其价格低廉、资源利用率高等优点成为当今发展最快的网络应用之一。在这两种技术的推动下,VoWLAN(Voice over WLAN)逐渐兴起。现在IEEE 802.11b已成为WLAN的事实标准,而WLAN又有传输语音业务的需求,然而IEEE 802.11b使用竞争信道,主要支持非实时的数据业务,不能满足对时延和时延抖动敏感的语音业务的QoS保证。因此,研究IEEE802.11b WLAN接入环境下的语音传输质量保证方案,具有重要的应用推广意义。本论文就是在这样的背景下展开的。本论文首先对无线局域网技术及其IEEE802.11系列标准进行了介绍,重点剖析了IEEE 802.11b DCF机制。然后,描述了VoIP相关技术,重点介绍了语音编码、信令接入等关键技术,并简要介绍了语音质量评价标准。在此基础上,本论文对现有的IEEE 802.11b协议下的传输容量做了简单估算,表明它在传输实时语音时效率低下。于是,我们针对语音业务的特点,提出了两种改进方案:TD-RAM和TD-TDMA,并分析了其容量。然后利用Qualnet仿真软件对VoWLAN系统的丢包率、端到端的平均时延、平均时延抖动三个性能进行了仿真分析,得到了在不同编码标准下的传输容量值,验证了理论分析值,也验证了改进方案确实提高了容量。本论文同时提出要想显著提高容量,应该使用改进MAC协议与降低开销(比如增大语音帧长、压缩包头和物理层使用短帧头模式)相结合的方案。更多还原

【Abstract】 With the rapid development of wireless communications technology and mobile terminal technology, the wireless local area network (WLAN) based on IEEE 802.11 is finding more and more applications. At the same time, with low price and high resource utilization, VoIP becomes one of the network applications experiencing tremendous growth. Driven by both technologies, VoWLAN (Voice over WLAN) is also on the rise. Now that IEEE802.11b has become the de facto standard for the MAC of WLAN,and there is demand of supporting voice services on WLAN, however, IEEE802.11b uses shared competitive channel suitable for non-real-time data traffic, and it is difficult to meet QoS requirements of voice,which is sensitive to the delay and delay jitter. Thus, it is important to study voice transmission with quality provisioning in IEEE802.11b WLAN. Our research is carried out on this background.At first,this thesis introduces wireless LAN technologies and the IEEE802.11 standards, with focuses on the IEEE802.11b DCF mechanism. Then, technologies related to VoIP are presented, with details on the key technologies,such as voice coding and signaling access technologies,and a brief introduction on the speech quality evaluation. Based on the above discussions, we estimate the capacity for existing 802.11b, and show that it is inefficient in supporting real-time voice traffic. In light of the characteristics of voice traffic, we propose two improved schemes on the MAC, namely, TD-RAM and TD-TDMA, and analyze their capacity. Qualnet simulator is used to evaluate the proposed schemes, in terms of packet loss rate, the average end-to-end delay, and average jitter performances. The upper bound capacities under different coding standards are obtained, and the theoretical analysis results are verified. The simulation results show that the improved schemes do increase the system capacity. This thesis also states that, in order to increase capacity significantly, improved MAC schemes should be combined with cost-reducing mechanisms, such as increasing voice frame size, compressing the headers and using short PLCP PPDU format in physical layer.更多还原