无线局域网的VoIP QoS研究

【作者】 周婷；

【导师】 黄烟波；

【作者基本信息】 中南大学 ， 系统工程， 2007， 硕士

【摘要】 WLAN技术和VoIP技术的发展，使得利用无线局域网来进行廉价的VoIP(VoWLAN，Voice over WLAN)通话成为可能。但传统的IP网络采用尽力而为的服务方式，同时无线网络的带宽资源有限，很难有效满足语音等实时业务的服务质量要求。影响VoIP服务质量的主要因素有延时、抖动、丢包率以及带宽。目前应用得最广的无线局域网技术是IEEE 802.11b，绝大部分的802.11b设备采用DCF方式竞争信道。DCF平等地对待语音和数据，不对实时业务提供接入优先级，这使得对延时敏感的语音业务的服务质量得不到保障。本文详细分析了DCF下的基本接入机制和RTS／CTS接入机制的工作原理，通过实验仿真得出VoIP不宜采用四次握手方式的RTS／CTS机制的结论。得出可从排队延时、帧间隔和随机退避时间三方面来控制VoWLAN的延时，并由此引出EDCA信道接入方式。分析了EDCA的QoS保障技术，并通过仿真模拟了EDCA对VoIP性能的改善。鉴于MAC层的CRC机制要求分组中不能存在任何错误，否则会引起MAC重传，但实际的语音业务采用小包发送，能够容忍一定程度的语音比特位错误，提出了一种改进的MAC层CRC机制，让CRC不再保护语音数据，减少MAC重传，从而降低VoIP延时和抖动。仿真结果表明，该机制有效改善了延时和抖动。VoWLAN的容量是一个和QoS紧密联系且非常重要的问题。论文从理论上分析了理想信道条件下802.11b的VoIP容量，仿真结果和理论分析值相符。更多还原

【Abstract】 The development of Wireless LAN technology and VoIP technology makes it possible to transmit voice over Wireless LAN. But because of the IP network’s best-effort service and the limited bandwidth of wireless network, QoS requirements of real-time applications such as voice can’t be met. The main factors that affect the performance of VoIP are delay, jitter, packet loss rate as well as bandwidth.Currently, the most widely used WLAN is IEEE 802.11b. Most 802.11b devices adopt DCF to contend for the channel. DCF treats voice and data equally, and doesn’t provide any access priority to the real-time applications. This can’t guarantee the QoS of delay-sensitive voice application. This paper analyzes the principles of basic access scheme and RTS/CTS scheme under DCF in detail, draws a conclusion that the four handshakes RTS/CTS scheme is not suitable for VoIP through simulation, and concludes that the delay of VoWLAN can be reduced by the control to the queueing delay, IFS and the random backoff time. On this basis, another method to contend for the channel-EDCA is presented. After the analysis of EDCA’s QoS support, experiment is performed. The result shows that EDCA greatly improves VoIP’s performance.Considering that CRC of MAC requires error free packets, otherwise MAC retransmission will occur, but voice transmitted in the form of small packets can tolerate some bits error, so a modified CRC scheme is brought forward. The modified CRC doesn’t protect voice data any more, so MAC retransmission is less. This results in reduced delay and jitter of VoIP. Simulation result shows that the modified CRC scheme can improve delay and jitter.Capacity of VoWLAN is related with QoS of VoWLAN, and it is also a very important problem. The thesis analyzes VoIP capacity of 802.11b network theoretically in ideal channel condition, and simulation result matches the theory.更多还原